"Centripetal Force" Circular Motion PPT (Centripetal Force in the First Lesson)

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"Centripetal Force" Circular Motion PPT (Centripetal Force in the First Lesson)

Part 1: Essential knowledge and foundation of literacy

1. Centripetal force

[Thinking] A small ball tied to a thin thread on a smooth table makes a uniform circular motion. In what direction does the resultant force it experiences?

Tip: The resultant force is along the thin line and points towards the center of the circle.

1. Examples show that: an object in uniform circular motion receives a resultant force directed toward _____.

2. Direction: Always point along the radius toward _____.

3.Expression:

(1)Fn=_____

(2)Fn=_____.

4. Effect force: Centripetal force is named according to the _________ of force. Any force that produces centripetal acceleration is centripetal force.

2. Variable speed circular motion and general curved motion

1. The resultant force of variable speed circular motion

(1) The component force Ft tangent to the circumference produces tangential acceleration at, tangential acceleration

Degree describes how quickly ___________ changes.

(2) The component Fn pointing toward the center of the circle produces centripetal acceleration an, centripetal acceleration

Describe the speed of _______________.

2. General curved motion:

(1) Treatment method:_______. A general curve can be divided into many short segments, which can be regarded as a small arc.

(2) Use the method of circular motion to study the motion of the particle on this small arc.

(3) Among the following descriptions, which ones do you think are scientific facts?

①The centripetal force of uniform circular motion is constant force

②The resultant force of uniform circular motion is centripetal force

③The centripetal force of variable speed circular motion does not point to the center of the circle

④ Centripetal force is a force with certain specific properties

⑤ Centripetal force can be acted by gravity or elastic force

Centripetal Force PPT, Part 2: Key Competencies·Quality Formation

One centripetal force

1. The direction of centripetal force in uniform circular motion:

The direction changes all the time, always pointing to the center of the circle, perpendicular to the direction of the linear velocity.

2. Characteristics of centripetal force: Since the direction of centripetal force is always perpendicular to the direction of motion of the object, centripetal force does not change the magnitude of the linear velocity, but only changes the direction of the linear velocity.

3. The source of centripetal force: In uniform circular motion, the centripetal force is equal to the net external force of the object, which is often equivalent to three situations: the net force acts as the centripetal force, a certain force acts as the centripetal force, and the component of a certain force acts as the centripetal force.

【Think·Discussion】

Situation: The airplane makes uniform circular motion in the horizontal plane in the air; the ball makes uniform circular motion on the inner wall of the smooth funnel.

discuss:

(1) What forces act on the airplane and the ball during their movement? (Model construction)

Tip: Gravity and support.

(2) What is the direction and effect of the resultant force of these forces? (Physical concepts)

Tip: The resultant force of these forces points toward the center of the circle and acts as a centripetal force to provide centripetal force.

[Typical example demonstration]

Picture A shows the "Flying Chair" game facility in an amusement park. Its basic device is to fix the upper end of the rope on the edge of the turntable, and the lower end of the rope is connected to the seat. People sit on the seat and fly in the air as the turntable rotates. Spin. If the person and the seat are regarded as a particle, it can be simplified to the physical model as shown in Figure B, where P is a turntable in the horizontal plane that can rotate around the vertical axis OO′, assuming the rope length l=10 m, The mass of the particle is m = 60 kg. When the turntable is stationary, the distance between the particle and the axis of rotation is d = 4.0 m. The turntable gradually accelerates and rotates. After a period of time, the particle and the turntable make a uniform circular motion. At this time, the rope and the vertical clamp The angle θ=37°, regardless of the air resistance and the weight of the rope, and the rope is not elongated, sin37°=0.6, cos 37°=0.8, g is 10 m/s2, when the particle and the turntable make uniform circular motion:

【Problem Solving Exploration】

(1) When a particle makes uniform circular motion in the horizontal plane, what is the resultant force it experiences in the vertical direction?

Tip: The net force in the vertical direction is zero.

(2) How to determine the radius of the circular motion of the particle?

Tip: The distance between the particle and the vertical axis OO' is the radius of the particle's circular motion.

[Law Method] Basic steps for analyzing uniform circular motion problems

(1) Clarify the research object, conduct force analysis on the research object, and draw a force diagram.

(2) Determine the orbital plane, center and radius of the object in circular motion.

(3) Decompose the external force on the object into the tangent direction and the radial direction through the orthogonal decomposition of the force.

2. Variable speed circular motion and general curved motion

1. The effect of the resultant force of variable speed circular motion:

(1) The component force Ft that is tangent to the circumference: produces tangential acceleration, which changes the linear velocity.

(2) Component force Fn directed toward the center of the circle: produces centripetal acceleration, which changes the direction of the linear velocity.

2. Comparison between uniform circular motion and variable speed circular motion:

3. General curved motion:

(1) The motion trajectory is neither a straight line nor a circular curved motion.

(2) Processing method: In general curved motion, the curve can be divided into many short segments, and the movement of the particle in each segment can be regarded as part of the circular motion.

【Think·Discussion】

When a high-speed rail train enters a curve as shown in the figure at a constant speed, how to study its motion patterns?

(scientific thinking)

Tip: When a high-speed rail train enters a certain section of a curve at a constant speed, it can be regarded as part of a circular motion.

[Typical example demonstration]

As shown in the figure, block P is placed on a horizontal turntable and moves with the turntable, and is relatively stationary with the turntable. In the figure, c points along the radius to the center of the circle, and a and c are perpendicular. The following statement is correct: ()

A. When the turntable rotates at a constant speed, the direction of friction force on P is direction b.

B. When the turntable accelerates, the friction force on P may be in the direction of c

C. When the turntable accelerates and rotates, the direction of the friction force on P may be direction a.

D. When the turntable slows down, the friction force on P may be in the direction of d.

3. Critical Problems of Uniform Circular Motion

1. Common types:

(1) The tension of the rope reaches the maximum or is zero.

(2) The static friction force reaches its maximum when the object starts to slide.

(3) The pressure is zero when the object breaks away from the contact surface.

2. Key to solving the problem:

(1) In the problem of circular motion, when words such as "exactly", "maximum", "at least" and "value range" appear, it means that there is a critical point in the motion process.

(2) Analyze the force in the critical state and list Newton's second law equation under critical conditions.

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Update Time: 2024-07-29

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