Uniform motion of a body in a circle. Circular motion
Since the linear speed uniformly changes direction, then the movement along the circle cannot be called uniform, it is uniformly accelerated.
Angular velocity
Pick a point on the circle 1 . Let's build a radius. For a unit of time, the point will move to the point 2 . In this case, the radius describes the angle. The angular velocity is numerically equal to the angle of rotation of the radius per unit time.
Period and frequency
Rotation period T is the time it takes the body to make one revolution.
RPM is the number of revolutions per second.
The frequency and period are related by the relation
Relationship with angular velocity
Line speed
Each point on the circle moves at some speed. This speed is called linear. The direction of the linear velocity vector always coincides with the tangent to the circle. For example, sparks from under a grinder move, repeating the direction of instantaneous speed.
Consider a point on a circle that makes one revolution, the time that is spent - this is the period T. The path traveled by a point is the circumference of a circle.
centripetal acceleration
When moving along a circle, the acceleration vector is always perpendicular to the velocity vector, directed to the center of the circle.
Using the previous formulas, we can derive the following relations
Points lying on the same straight line emanating from the center of the circle (for example, these can be points that lie on the wheel spoke) will have the same angular velocities, period and frequency. That is, they will rotate in the same way, but with different linear speeds. The farther the point is from the center, the faster it will move.
The law of addition of velocities is also valid for rotational motion. If the motion of a body or frame of reference is not uniform, then the law applies to instantaneous velocities. For example, the speed of a person walking along the edge of a rotating carousel is equal to the vector sum of the linear speed of rotation of the edge of the carousel and the speed of the person.
The Earth participates in two main rotational movements: daily (around its axis) and orbital (around the Sun). The period of rotation of the Earth around the Sun is 1 year or 365 days. The Earth rotates around its axis from west to east, the period of this rotation is 1 day or 24 hours. Latitude is the angle between the plane of the equator and the direction from the center of the Earth to a point on its surface.
According to Newton's second law, the cause of any acceleration is a force. If a moving body experiences centripetal acceleration, then the nature of the forces that cause this acceleration may be different. For example, if a body moves in a circle on a rope tied to it, then the acting force is the elastic force.
If a body lying on a disk rotates along with the disk around its axis, then such a force is the force of friction. If the force ceases to act, then the body will continue to move in a straight line
Consider the movement of a point on a circle from A to B. The linear velocity is equal to v A And v B respectively. Acceleration is the change in speed per unit of time. Let's find the difference of vectors.
Circular motion is the simplest case of curvilinear motion of a body. When a body moves around a certain point, along with the displacement vector, it is convenient to introduce the angular displacement ∆ φ (the angle of rotation relative to the center of the circle), measured in radians.
Knowing the angular displacement, it is possible to calculate the length of the circular arc (path) that the body has passed.
∆ l = R ∆ φ
If the angle of rotation is small, then ∆ l ≈ ∆ s .
Let's illustrate what has been said:
Angular velocity
With curvilinear motion, the concept of angular velocity ω is introduced, that is, the rate of change in the angle of rotation.
Definition. Angular velocity
The angular velocity at a given point of the trajectory is the limit of the ratio of the angular displacement ∆ φ to the time interval ∆ t during which it occurred. ∆t → 0 .
ω = ∆ φ ∆ t , ∆ t → 0 .
The unit of measure for angular velocity is radians per second (r a d s).
There is a relationship between the angular and linear velocities of the body when moving in a circle. Formula for finding the angular velocity:
With uniform motion in a circle, the speeds v and ω remain unchanged. Only the direction of the linear velocity vector changes.
In this case, a uniform movement along a circle on the body is affected by centripetal, or normal acceleration, directed along the radius of the circle to its center.
a n = ∆ v → ∆ t , ∆ t → 0
The centripetal acceleration module can be calculated by the formula:
a n = v 2 R = ω 2 R
Let us prove these relations.
Let's consider how the vector v → changes over a small period of time ∆ t . ∆ v → = v B → - v A → .
At points A and B, the velocity vector is directed tangentially to the circle, while the velocity modules at both points are the same.
By definition of acceleration:
a → = ∆ v → ∆ t , ∆ t → 0
Let's look at the picture:
Triangles OAB and BCD are similar. It follows from this that O A A B = B C C D .
If the value of the angle ∆ φ is small, the distance A B = ∆ s ≈ v · ∆ t . Taking into account that O A \u003d R and C D \u003d ∆ v for the similar triangles considered above, we get:
R v ∆ t = v ∆ v or ∆ v ∆ t = v 2 R
When ∆ φ → 0 , the direction of the vector ∆ v → = v B → - v A → approaches the direction to the center of the circle. Assuming that ∆ t → 0 , we get:
a → = a n → = ∆ v → ∆ t ; ∆t → 0 ; a n → = v 2 R .
With uniform motion along a circle, the acceleration module remains constant, and the direction of the vector changes with time, while maintaining orientation to the center of the circle. That is why this acceleration is called centripetal: the vector at any time is directed towards the center of the circle.
The record of centripetal acceleration in vector form is as follows:
a n → = - ω 2 R → .
Here R → is the radius vector of a point on a circle with origin at its center.
In the general case, acceleration when moving along a circle consists of two components - normal and tangential.
Consider the case when the body moves along the circle non-uniformly. Let us introduce the concept of tangential (tangential) acceleration. Its direction coincides with the direction of the linear velocity of the body and at each point of the circle is directed tangentially to it.
a τ = ∆ v τ ∆ t ; ∆t → 0
Here ∆ v τ \u003d v 2 - v 1 is the change in the velocity module over the interval ∆ t
The direction of full acceleration is determined by the vector sum of normal and tangential accelerations.
Circular motion in a plane can be described using two coordinates: x and y. At each moment of time, the speed of the body can be decomposed into components v x and v y .
If the motion is uniform, the values v x and v y as well as the corresponding coordinates will change in time according to a harmonic law with a period T = 2 π R v = 2 π ω
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1. Uniform movement in a circle
2. Angular speed of rotational movement.
3.Period of rotation.
4.Frequency of rotation.
5. Relationship between linear velocity and angular velocity.
6. Centripetal acceleration.
7. Equally variable movement in a circle.
8. Angular acceleration in uniform motion in a circle.
9. Tangential acceleration.
10. The law of uniformly accelerated motion in a circle.
11. Average angular velocity in uniformly accelerated motion in a circle.
12. Formulas that establish the relationship between angular velocity, angular acceleration and the angle of rotation in uniformly accelerated motion in a circle.
1.Uniform circular motion- movement, in which a material point passes equal segments of a circular arc in equal time intervals, i.e. a point moves along a circle with a constant modulo speed. In this case, the speed is equal to the ratio of the arc of the circle passed by the point to the time of movement, i.e.
and is called the linear speed of motion in a circle.
As in curvilinear motion, the velocity vector is directed tangentially to the circle in the direction of motion (Fig.25).
2. Angular velocity in uniform circular motion is the ratio of the angle of rotation of the radius to the time of rotation:
In uniform circular motion, the angular velocity is constant. In the SI system, angular velocity is measured in (rad/s). One radian - rad is a central angle that subtends an arc of a circle with a length equal to the radius. A full angle contains a radian, i.e. in one revolution, the radius rotates by an angle of radians.
3. Rotation period- the time interval T, during which the material point makes one complete revolution. In the SI system, the period is measured in seconds.
4. Rotation frequency is the number of revolutions per second. In the SI system, the frequency is measured in hertz (1Hz = 1). One hertz is the frequency at which one revolution is made in one second. It is easy to imagine that
If in time t the point makes n revolutions around the circle, then .
Knowing the period and frequency of rotation, the angular velocity can be calculated by the formula:
5 Relationship between linear velocity and angular velocity. The length of the arc of a circle is where the central angle, expressed in radians, subtending the arc is the radius of the circle. Now we write the linear velocity in the form
It is often convenient to use formulas: or Angular velocity is often called the cyclic frequency, and the frequency is called the linear frequency.
6. centripetal acceleration. In uniform motion along a circle, the speed modulus remains unchanged, and its direction is constantly changing (Fig. 26). This means that a body moving uniformly in a circle experiences an acceleration that is directed towards the center and is called centripetal acceleration.
Let a path equal to the arc of a circle pass over a period of time. We move the vector , leaving it parallel to itself, so that its beginning coincides with the beginning of the vector at point B. The modulus of change in speed is , and the modulus of centripetal acceleration is
In Fig. 26, triangles AOB and DVS are isosceles and the angles at the vertices O and B are equal, as are the angles with mutually perpendicular sides AO and OB. This means that triangles AOB and DVS are similar. Therefore, if that is, the time interval takes on arbitrarily small values, then the arc can be approximately considered equal to the chord AB, i.e. . Therefore, we can write Considering that VD= , ОА=R we obtain Multiplying both parts of the last equality by , we will further obtain the expression for the module of centripetal acceleration in uniform motion in a circle: . Given that we get two frequently used formulas:
So, in uniform motion along a circle, the centripetal acceleration is constant in absolute value.
It is easy to figure out that in the limit at , angle . This means that the angles at the base of the DS of the ICE triangle tend to the value , and the velocity change vector becomes perpendicular to the velocity vector , i.e. directed along the radius towards the center of the circle.
7. Uniform circular motion- movement in a circle, in which for equal intervals of time the angular velocity changes by the same amount.
8. Angular acceleration in uniform circular motion is the ratio of the change in the angular velocity to the time interval during which this change occurred, i.e.
where the initial value of the angular velocity, the final value of the angular velocity, angular acceleration, in the SI system is measured in. From the last equality we obtain formulas for calculating the angular velocity
And if .
Multiplying both parts of these equalities by and taking into account that , is the tangential acceleration, i.e. acceleration directed tangentially to the circle, we obtain formulas for calculating the linear velocity:
And if .
9. Tangential acceleration is numerically equal to the change in velocity per unit time and is directed along the tangent to the circle. If >0, >0, then the motion is uniformly accelerated. If<0 и <0 – движение.
10. Law of uniformly accelerated motion in a circle. The path traveled along the circle in time in uniformly accelerated motion is calculated by the formula:
Substituting here , , reducing by , we obtain the law of uniformly accelerated motion in a circle:
Or if .
If the movement is uniformly slowed down, i.e.<0, то
11.Full acceleration in uniformly accelerated circular motion. In uniformly accelerated motion in a circle, the centripetal acceleration increases with time, because due to tangential acceleration, the linear speed increases. Very often centripetal acceleration is called normal and denoted as . Since the total acceleration at the moment is determined by the Pythagorean theorem (Fig. 27).
12. Average angular velocity in uniformly accelerated motion in a circle. The average linear speed in uniformly accelerated motion in a circle is equal to . Substituting here and and reducing by we get
If , then .
12. Formulas that establish the relationship between angular velocity, angular acceleration and the angle of rotation in uniformly accelerated motion in a circle.
Substituting into the formula the quantities , , , ,
and reducing by , we get
Lecture - 4. Dynamics.
1. Dynamics
2. Interaction of bodies.
3. Inertia. The principle of inertia.
4. Newton's first law.
5. Free material point.
6. Inertial frame of reference.
7. Non-inertial frame of reference.
8. Galileo's principle of relativity.
9. Galilean transformations.
11. Addition of forces.
13. Density of substances.
14. Center of mass.
15. Newton's second law.
16. Unit of measurement of force.
17. Newton's third law
1. Dynamics there is a branch of mechanics that studies mechanical motion, depending on the forces that cause a change in this motion.
2.Body interactions. Bodies can interact both with direct contact and at a distance through a special type of matter called the physical field.
For example, all bodies are attracted to each other and this attraction is carried out by means of a gravitational field, and the forces of attraction are called gravitational.
Bodies that carry an electric charge interact through an electric field. Electric currents interact through a magnetic field. These forces are called electromagnetic.
Elementary particles interact through nuclear fields and these forces are called nuclear.
3.Inertia. In the IV century. BC e. The Greek philosopher Aristotle argued that the cause of the movement of a body is a force acting from another body or bodies. At the same time, according to the movement of Aristotle, a constant force imparts a constant speed to the body, and with the termination of the force, the movement stops.
In the 16th century Italian physicist Galileo Galilei, conducting experiments with bodies rolling down an inclined plane and with falling bodies, showed that a constant force (in this case, the weight of the body) imparts acceleration to the body.
So, on the basis of experiments, Galileo showed that the force is the cause of the acceleration of bodies. Let us present Galileo's reasoning. Let a very smooth ball roll on a smooth horizontal plane. If nothing interferes with the ball, then it can roll indefinitely. If, on the way of the ball, a thin layer of sand is poured, then it will stop very soon, because. the friction force of the sand acted on it.
So Galileo came to the formulation of the principle of inertia, according to which a material body maintains a state of rest or uniform rectilinear motion if external forces do not act on it. Often this property of matter is called inertia, and the movement of a body without external influences is called inertia.
4. Newton's first law. In 1687, based on Galileo's principle of inertia, Newton formulated the first law of dynamics - Newton's first law:
A material point (body) is in a state of rest or uniform rectilinear motion if no other bodies act on it, or the forces acting from other bodies are balanced, i.e. compensated.
5.Free material point- a material point, which is not affected by other bodies. Sometimes they say - an isolated material point.
6. Inertial Reference System (ISO)- a reference system, relative to which an isolated material point moves in a straight line and uniformly, or is at rest.
Any frame of reference that moves uniformly and rectilinearly relative to the ISO is inertial,
Here is one more formulation of Newton's first law: There are frames of reference, relative to which a free material point moves in a straight line and uniformly, or is at rest. Such frames of reference are called inertial. Often Newton's first law is called the law of inertia.
Newton's first law can also be given the following formulation: any material body resists a change in its speed. This property of matter is called inertia.
We encounter the manifestation of this law every day in urban transport. When the bus picks up speed sharply, we are pressed against the back of the seat. When the bus slows down, then our body skids in the direction of the bus.
7. Non-inertial frame of reference - a frame of reference that moves non-uniformly relative to the ISO.
A body that, relative to ISO, is at rest or in uniform rectilinear motion. Relative to a non-inertial frame of reference, it moves non-uniformly.
Any rotating frame of reference is a non-inertial frame of reference, since in this system, the body experiences centripetal acceleration.
There are no bodies in nature and technology that could serve as ISO. For example, the Earth rotates around its axis and any body on its surface experiences centripetal acceleration. However, for fairly short periods of time, the reference system associated with the Earth's surface can be considered, in some approximation, the ISO.
8.Galileo's principle of relativity. ISO can be salt you like a lot. Therefore, the question arises: how do the same mechanical phenomena look in different ISOs? Is it possible, using mechanical phenomena, to detect the movement of the IFR in which they are observed.
The answer to these questions is given by the principle of relativity of classical mechanics, discovered by Galileo.
The meaning of the principle of relativity of classical mechanics is the statement: all mechanical phenomena proceed in exactly the same way in all inertial frames of reference.
This principle can also be formulated as follows: all laws of classical mechanics are expressed by the same mathematical formulas. In other words, no mechanical experiments will help us detect the movement of the ISO. This means that trying to detect the movement of the ISO is meaningless.
We encountered the manifestation of the principle of relativity while traveling in trains. At the moment when our train stops at the station, and the train that was standing on the neighboring track slowly starts moving, then in the first moments it seems to us that our train is moving. But it also happens the other way around, when our train is gradually picking up speed, it seems to us that the neighboring train started moving.
In the above example, the principle of relativity manifests itself within small time intervals. With an increase in speed, we begin to feel shocks and rocking of the car, i.e., our frame of reference becomes non-inertial.
So, the attempt to detect the movement of the ISO is meaningless. Therefore, it is absolutely indifferent which IFR is considered fixed and which one is moving.
9. Galilean transformations. Let two IFRs and move relative to each other with a speed . In accordance with the principle of relativity, we can assume that the IFR K is motionless, and the IFR moves relatively at a speed of . For simplicity, we assume that the corresponding coordinate axes of the systems and are parallel, and the axes and coincide. Let the systems coincide at the start time and the motion occurs along the axes and , i.e. (Fig.28)
11. Addition of forces. If two forces are applied to a particle, then the resulting force is equal to their vector, i.e. diagonals of a parallelogram built on vectors and (Fig. 29).
The same rule when decomposing a given force into two components of the force. To do this, on the vector of a given force, as on a diagonal, a parallelogram is built, the sides of which coincide with the direction of the components of the forces applied to the given particle.
If several forces are applied to the particle, then the resulting force is equal to the geometric sum of all forces:
12.Weight. Experience has shown that the ratio of the modulus of force to the modulus of acceleration, which this force imparts to a body, is a constant value for a given body and is called the mass of the body:
From the last equality it follows that the greater the mass of the body, the greater force must be applied to change its speed. Therefore, the greater the mass of the body, the more inert it is, i.e. mass is a measure of the inertia of bodies. The mass defined in this way is called the inertial mass.
In the SI system, mass is measured in kilograms (kg). One kilogram is the mass of distilled water in the volume of one cubic decimeter taken at a temperature
13. Matter density- the mass of a substance contained in a unit volume or the ratio of the mass of a body to its volume
Density is measured in () in the SI system. Knowing the density of the body and its volume, you can calculate its mass using the formula. Knowing the density and mass of the body, its volume is calculated by the formula.
14.Center of mass- a point of the body that has the property that if the direction of the force passes through this point, the body moves translationally. If the direction of action does not pass through the center of mass, then the body moves while simultaneously rotating around its center of mass.
15. Newton's second law. In ISO, the sum of forces acting on a body is equal to the product of the body's mass and the acceleration imparted to it by this force
16.Force unit. In the SI system, force is measured in newtons. One newton (n) is the force that, acting on a body with a mass of one kilogram, imparts an acceleration to it. That's why .
17. Newton's third law. The forces with which two bodies act on each other are equal in magnitude, opposite in direction and act along one straight line connecting these bodies.
Alexandrova Zinaida Vasilievna, teacher of physics and computer science
Educational institution: MBOU secondary school No. 5, Pechenga, Murmansk region
Item: physics
Class : Grade 9
Lesson topic : Movement of a body in a circle with a constant modulo speed
The purpose of the lesson:
give an idea of curvilinear motion, introduce the concepts of frequency, period, angular velocity, centripetal acceleration and centripetal force.
Lesson objectives:
Educational:
Repeat the types of mechanical motion, introduce new concepts: circular motion, centripetal acceleration, period, frequency;
To reveal in practice the connection of the period, frequency and centripetal acceleration with the radius of circulation;
Use educational laboratory equipment to solve practical problems.
Educational :
Develop the ability to apply theoretical knowledge to solve specific problems;
Develop a culture of logical thinking;
Develop interest in the subject; cognitive activity in setting up and conducting an experiment.
Educational :
To form a worldview in the process of studying physics and to argue their conclusions, to cultivate independence, accuracy;
To cultivate a communicative and informational culture of students
Lesson equipment:
computer, projector, screen, presentation for the lessonMovement of a body in a circle, printout of cards with tasks;
tennis ball, badminton shuttlecock, toy car, ball on a string, tripod;
sets for the experiment: stopwatch, tripod with a clutch and a foot, a ball on a thread, a ruler.
Form of organization of training: frontal, individual, group.
Lesson type: study and primary consolidation of knowledge.
Educational and methodological support: Physics. Grade 9 Textbook. Peryshkin A.V., Gutnik E.M. 14th ed., ster. - M.: Bustard, 2012
Lesson Implementation Time : 45 minutes
1. Editor in which the multimedia resource is made:MSPowerPoint
2. Type of multimedia resource: a visual presentation of educational material using triggers, embedded video and an interactive test.
Lesson plan
Organizing time. Motivation for learning activities.
Updating of basic knowledge.
Learning new material.
Conversation on questions;
Problem solving;
Implementation of research practical work.
Summing up the lesson.
During the classes
Lesson stages
Temporary implementation
Organizing time. Motivation for learning activities.
slide 1. ( Checking readiness for the lesson, announcing the topic and objectives of the lesson.)
Teacher. Today in the lesson you will learn what acceleration is when a body moves uniformly in a circle and how to determine it.
2 minutes
Updating of basic knowledge.
Slide 2.
Fphysical dictation:
Change in body position in space over time.(Movement)
A physical quantity measured in meters.(Move)
Physical vector quantity characterizing the speed of movement.(Speed)
The basic unit of length in physics.(Meter)
A physical quantity whose units are year, day, hour.(Time)
A physical vector quantity that can be measured using an accelerometer instrument.(Acceleration)
Trajectory length. (Path)
Acceleration units(m/s 2 ).
(Conducting a dictation with subsequent verification, self-assessment of work by students)
5 minutes
Learning new material.
Slide 3.
Teacher. We quite often observe such a movement of a body in which its trajectory is a circle. Moving along the circle, for example, the point of the wheel rim during its rotation, the points of the rotating parts of machine tools, the end of the clock hand.
Experience demonstrations 1. The fall of a tennis ball, the flight of a badminton shuttlecock, the movement of a toy car, the vibrations of a ball on a thread fixed in a tripod. What do these movements have in common and how do they differ in appearance?(Student answers)
Teacher. Rectilinear motion is a motion whose trajectory is a straight line, curvilinear is a curve. Give examples of rectilinear and curvilinear motion that you have encountered in your life.(Student answers)
The motion of a body in a circle isa special case of curvilinear motion.
Any curve can be represented as a sum of arcs of circlesdifferent (or the same) radius.
Curvilinear motion is a motion that occurs along arcs of circles.
Let us introduce some characteristics of curvilinear motion.
slide 4. (watch video " speed.avi" link on slide)
Curvilinear motion with a constant modulo speed. Movement with acceleration, tk. speed changes direction.
slide 5 . (watch video “Dependence of centripetal acceleration on radius and speed. avi » from the link on the slide)
slide 6. The direction of the velocity and acceleration vectors.
(working with slide materials and analysis of drawings, rational use of animation effects embedded in drawing elements, Fig 1.)
Fig.1.
Slide 7.
When a body moves uniformly along a circle, the acceleration vector is always perpendicular to the velocity vector, which is directed tangentially to the circle.
A body moves in a circle, provided that that the linear velocity vector is perpendicular to the centripetal acceleration vector.
slide 8. (working with illustrations and slide materials)
centripetal acceleration - the acceleration with which the body moves in a circle with a constant modulo speed is always directed along the radius of the circle to the center.
a c =
slide 9.
When moving in a circle, the body will return to its original point after a certain period of time. Circular motion is periodic.
Period of circulation - this is a period of timeT , during which the body (point) makes one revolution around the circumference.
Period unit -second
Speed is the number of complete revolutions per unit of time.
[ ] = with -1 = Hz
Frequency unit
Student message 1. A period is a quantity that is often found in nature, science and technology. The earth rotates around its axis, the average period of this rotation is 24 hours; a complete revolution of the Earth around the Sun takes about 365.26 days; the helicopter propeller has an average rotation period from 0.15 to 0.3 s; the period of blood circulation in a person is approximately 21 - 22 s.
Student message 2. The frequency is measured with special instruments - tachometers.
The rotational speed of technical devices: the gas turbine rotor rotates at a frequency of 200 to 300 1/s; A bullet fired from a Kalashnikov assault rifle rotates at a frequency of 3000 1/s.
slide 10. Relationship between period and frequency:
If in time t the body has made N complete revolutions, then the period of revolution is equal to:
Period and frequency are reciprocal quantities: frequency is inversely proportional to period, and period is inversely proportional to frequency
Slide 11. The speed of rotation of the body is characterized by the angular velocity.
Angular velocity(cyclic frequency) -
number of revolutions per unit of time, expressed in radians.
Angular velocity - the angle of rotation by which a point rotates in timet.
Angular velocity is measured in rad/s.
slide 12. (watch video "Path and displacement in curvilinear motion.avi" link on slide)
slide 13 . Kinematics of circular motion.
Teacher. With uniform motion in a circle, the modulus of its velocity does not change. But speed is a vector quantity, and it is characterized not only by a numerical value, but also by a direction. With uniform motion in a circle, the direction of the velocity vector changes all the time. Therefore, such uniform motion is accelerated.
Line speed: ;
Linear and angular speeds are related by the relation:
Centripetal acceleration: ;
Angular speed: ;
slide 14. (working with illustrations on the slide)
The direction of the velocity vector.Linear (instantaneous velocity) is always directed tangentially to the trajectory drawn to its point where the considered physical body is currently located.
The velocity vector is directed tangentially to the described circle.
The uniform motion of a body in a circle is a motion with acceleration. With a uniform motion of the body around the circle, the quantities υ and ω remain unchanged. In this case, when moving, only the direction of the vector changes.
slide 15. Centripetal force.
The force that holds a rotating body on a circle and is directed towards the center of rotation is called the centripetal force.
To obtain a formula for calculating the magnitude of the centripetal force, one must use Newton's second law, which is applicable to any curvilinear motion.
Substituting into the formula value of centripetal accelerationa c = , we get the formula for the centripetal force:
F=
From the first formula it can be seen that at the same speed, the smaller the radius of the circle, the greater the centripetal force. So, at the turns of the road on a moving body (train, car, bicycle), the greater the force should act towards the center of curvature, the steeper the turn, i.e., the smaller the radius of curvature.
The centripetal force depends on the linear speed: with increasing speed, it increases. It is well known to all skaters, skiers and cyclists: the faster you move, the harder it is to make a turn. Drivers know very well how dangerous it is to turn a car sharply at high speed.
slide 16.
Summary table of physical quantities characterizing curvilinear motion(analysis of dependencies between quantities and formulas)
Slides 17, 18, 19. Examples of circular motion.
Roundabouts on the roads. The movement of satellites around the earth.
slide 20. Attractions, carousels.
Student message 3. In the Middle Ages, jousting tournaments were called carousels (the word then had a masculine gender). Later, in the 18th century, to prepare for tournaments, instead of fighting with real opponents, they began to use a rotating platform, the prototype of a modern entertainment carousel, which then appeared at city fairs.
In Russia, the first carousel was built on June 16, 1766 in front of the Winter Palace. The carousel consisted of four quadrilles: Slavic, Roman, Indian, Turkish. The second time the carousel was built in the same place, in the same year on July 11th. A detailed description of these carousels is given in the newspaper St. Petersburg Vedomosti of 1766.
Carousel, common in courtyards in Soviet times. The carousel can be driven both by an engine (usually electric), and by the forces of the spinners themselves, who, before sitting on the carousel, spin it. Such carousels, which need to be spun by the riders themselves, are often installed on children's playgrounds.
In addition to attractions, carousels are often referred to as other mechanisms that have similar behavior - for example, in automated lines for bottling drinks, packaging bulk materials or printing products.
In a figurative sense, a carousel is a series of rapidly changing objects or events.
18 min
Consolidation of new material. Application of knowledge and skills in a new situation.
Teacher. Today in this lesson we got acquainted with the description of curvilinear motion, with new concepts and new physical quantities.
Conversation on:
What is a period? What is frequency? How are these quantities related? In what units are they measured? How can they be identified?
What is angular velocity? In what units is it measured? How can it be calculated?
What is called angular velocity? What is the unit of angular velocity?
How are the angular and linear velocities of a body's motion related?
What is the direction of centripetal acceleration? What formula is used to calculate it?
Slide 21.
Exercise 1. Fill in the table by solving problems according to the initial data (Fig. 2), then we will check the answers. (Students work independently with the table, it is necessary to prepare a printout of the table for each student in advance)
Fig.2
slide 22. Task 2.(orally)
Pay attention to the animation effects of the picture. Compare the characteristics of the uniform motion of the blue and red balls. (Working with the illustration on the slide).
slide 23. Task 3.(orally)
The wheels of the presented modes of transport make an equal number of revolutions in the same time. Compare their centripetal accelerations.(Working with slide materials)
(Work in a group, conducting an experiment, there is a printout of instructions for conducting an experiment on each table)
Equipment: a stopwatch, a ruler, a ball attached to a thread, a tripod with a clutch and a foot.
Target: researchdependence of period, frequency and acceleration on the radius of rotation.
Work plan
Measuretime t is 10 full revolutions of rotational motion and radius R of rotation of a ball fixed on a thread in a tripod.
Calculateperiod T and frequency, speed of rotation, centripetal acceleration Write the results in the form of a problem.
Changeradius of rotation (length of the thread), repeat the experiment 1 more time, trying to maintain the same speed,putting in the effort.
Make a conclusionabout the dependence of the period, frequency and acceleration on the radius of rotation (the smaller the radius of rotation, the shorter the period of revolution and the greater the value of frequency).
Slides 24-29.
Frontal work with an interactive test.
It is necessary to choose one answer out of three possible, if the correct answer was chosen, then it remains on the slide, and the green indicator starts flashing, incorrect answers disappear.
The body moves in a circle with a constant modulo speed. How will its centripetal acceleration change when the radius of the circle decreases by 3 times?
In the centrifuge of the washing machine, the laundry during the spin cycle moves in a circle with a constant modulo speed in the horizontal plane. What is the direction of its acceleration vector?
The skater moves at a speed of 10 m/s in a circle with a radius of 20 m. Determine his centripetal acceleration.
Where is the acceleration of the body directed when it moves along a circle with a constant speed in absolute value?
A material point moves along a circle with a constant modulo speed. How will the modulus of its centripetal acceleration change if the speed of the point is tripled?
A car wheel makes 20 revolutions in 10 seconds. Determine the period of rotation of the wheel?
slide 30. Problem solving(independent work if there is time in the lesson)
Option 1.
With what period must a carousel with a radius of 6.4 m rotate in order for the centripetal acceleration of a person on the carousel to be 10 m / s 2 ?
In the circus arena, a horse gallops at such a speed that it runs 2 circles in 1 minute. The radius of the arena is 6.5 m. Determine the period and frequency of rotation, speed and centripetal acceleration.
Option 2.
Carousel rotation frequency 0.05 s -1 . A person spinning on a carousel is at a distance of 4 m from the axis of rotation. Determine the centripetal acceleration of the person, the period of revolution and the angular velocity of the carousel.
The rim point of a bicycle wheel makes one revolution in 2 s. The wheel radius is 35 cm. What is the centripetal acceleration of the wheel rim point?
18 min
Summing up the lesson.
Grading. Reflection.
Slide 31 .
D/z: p. 18-19, Exercise 18 (2.4).
http:// www. stmary. ws/ high school/ physics/ home/ laboratory/ labGraphic. gif
Among the various types of curvilinear motion, of particular interest is uniform motion of a body in a circle. This is the simplest form of curvilinear motion. At the same time, any complex curvilinear motion of a body in a sufficiently small section of its trajectory can be approximately considered as uniform motion along a circle.
Such a movement is made by points of rotating wheels, turbine rotors, artificial satellites rotating in orbits, etc. With uniform motion in a circle, the numerical value of the speed remains constant. However, the direction of the velocity during such a movement is constantly changing.
The speed of the body at any point of the curvilinear trajectory is directed tangentially to the trajectory at this point. This can be seen by observing the work of a disc-shaped grindstone: pressing the end of a steel rod to a rotating stone, you can see hot particles coming off the stone. These particles fly at the same speed that they had at the moment of separation from the stone. The direction of the sparks always coincides with the tangent to the circle at the point where the rod touches the stone. Sprays from the wheels of a skidding car also move tangentially to the circle.
Thus, the instantaneous velocity of the body at different points of the curvilinear trajectory has different directions, while the modulus of velocity can either be the same everywhere or change from point to point. But even if the modulus of speed does not change, it still cannot be considered constant. After all, speed is a vector quantity, and for vector quantities, the modulus and direction are equally important. That's why curvilinear motion is always accelerated, even if the modulus of speed is constant.
Curvilinear motion can change the speed modulus and its direction. Curvilinear motion, in which the modulus of speed remains constant, is called uniform curvilinear motion. Acceleration during such movement is associated only with a change in the direction of the velocity vector.
Both the modulus and the direction of acceleration must depend on the shape of the curved trajectory. However, it is not necessary to consider each of its myriad forms. Representing each section as a separate circle with a certain radius, the problem of finding acceleration in a curvilinear uniform motion will be reduced to finding acceleration in a body moving uniformly along a circle.
Uniform motion in a circle is characterized by a period and frequency of circulation.
The time it takes for a body to make one revolution is called circulation period.
With uniform motion in a circle, the period of revolution is determined by dividing the distance traveled, i.e., the circumference of the circle by the speed of movement:
The reciprocal of a period is called circulation frequency, denoted by the letter ν . Number of revolutions per unit time ν called circulation frequency:
Due to the continuous change in the direction of speed, a body moving in a circle has an acceleration that characterizes the speed of change in its direction, the numerical value of the speed in this case does not change.
With a uniform motion of a body along a circle, the acceleration at any point in it is always directed perpendicular to the speed of movement along the radius of the circle to its center and is called centripetal acceleration.
To find its value, consider the ratio of the change in the velocity vector to the time interval during which this change occurred. Since the angle is very small, we have
