AP Physics 1 : Circular, Rotational, and Harmonic Motion

Study concepts, example questions & explanations for AP Physics 1

varsity tutors app store varsity tutors android store

Example Questions

Example Question #41 : Centripetal Force And Acceleration

A  model train completes a circle of radius  in . Determine the angular momentum.

Possible Answers:

None of these

Correct answer:

Explanation:

One circle is equal to , thus the linear velocity is:

The definition of angular momentum:

Plugging in values:

Example Question #131 : Circular, Rotational, And Harmonic Motion

An object has a mass M, velocity V, and moves in a circle with radius R.

What happens to the centripetal acceleration on the object when the mass is doubled?

Possible Answers:

The centripetal acceleration remains the same.

The centripetal acceleration remains the same.

The centripetal acceleration is halved.

The centripetal acceleration is doubled.

The centripetal acceleration is quadrupled.

Correct answer:

The centripetal acceleration remains the same.

Explanation:

The equation for centripetal acceleration is:

Where  is the centripetal acceleration on an object,  is the velocity of an object, and  is the radius in which the object moves in a circle.

We can see that mass does not play a role in the centripetal acceleration of an object, so no matter what happens to the mass, the centripetal acceleration remains the same.

Example Question #44 : Centripetal Force And Acceleration

An object has a mass M, velocity V, and moves in a circle with radius R.

What happens to the centripetal acceleration on the object when the radius is halved?

Possible Answers:

The centripetal acceleration of the object is quadrupled.

The centripetal acceleration of the object remains the same.

The centripetal acceleration of the object goes to zero.

The centripetal acceleration of the object is doubled.

The centripetal acceleration of the object is halved.

Correct answer:

The centripetal acceleration of the object is doubled.

Explanation:

The equation for centripetal acceleration is:

Where  is the centripetal acceleration on an object,  is the velocity of an object, and  is the radius in which the object moves in a circle.

The radius has an inverse relationship with centripetal acceleration, so when the radius is halved, the centripetal acceleration is doubled.

Example Question #42 : Centripetal Force And Acceleration

An object has a mass M, velocity V, and moves in a circle with radius R.

What happens to the centripetal acceleration on the object when the velocity is doubled?

Possible Answers:

The centripetal acceleration of the object is quadrupled.

The centripetal acceleration of the object remains the same.

The centripetal acceleration of the object is halved.

The centripetal acceleration of the object goes to zero.

The centripetal acceleration of the object is doubled.

Correct answer:

The centripetal acceleration of the object is quadrupled.

Explanation:

The equation for centripetal acceleration is:

Where  is the centripetal acceleration on an object,  is the velocity of an object, and  is the radius in which the object moves in a circle.

The velocity has an quadratic relationship with centripetal acceleration, so when the velocity is doubled, the centripetal acceleration is quadrupled.

Example Question #51 : Centripetal Force And Acceleration

It takes a disc player  seconds to get up to its speed of rotations per second, what is the angular acceleration of the disc player in radians?

Possible Answers:

Correct answer:

Explanation:

Similar to linear acceleration, angular acceleration is related to the angular velocity by the following

from here we simply plug in our values of  and  to get . Next to convert to radians we take the relations that pi is equal to  rotation and multiply our  by  to arrive at our final answer of 

Example Question #52 : Centripetal Force And Acceleration

You decide to ride an attraction at the fair; it is a circular, flat, ride that spins very fast; so fast, that it's nearly impossible to fall to the ground. Riders "stick" to the walls when the ride hits its top speeds.

Let's say the attraction spins in a clockwise manner. While riding the attraction, toward which direction is the centripetal (net) force pointing?

Possible Answers:

Outward

Inward

Against motion (counter clockwise)

With motion (clockwise)

Correct answer:

Inward

Explanation:

Most of the success for this ride experience can be credited to Newton's Third Law. It states that if object 1 exerts  onto object 2, then object 2 must be exerting a force . They are equal and opposite forces, called an action reaction pair. This concept applies when you're typing on your cell phone. when you touch down on your screen or button, your finger doesn't go through the phone, right? The phone is sending back an equal force to you which prevents your finger from going through the screen.

6 n3l

The same thing applies on the attraction. Your body is pushing outward on the walls of the ride. If no centripetal force was present, your body would project on a tangent as depicted in the diagram below. However, centripetal force is present. This is the force that is countering your body; it is pushing you towards the center. Now as we know, centripetal force isn't ever drawn on a diagram because it is a "net" force, or a collection of forces. But what we do know is that during circular motion, centripetal force (and acceleration) point towards the center.

Grav v 7

 

During the ride, your body is trying to follow the tangental paths shown, but the reciprocal force, , is keeping your body stationary on the wall. The wall is following a circular path, which makes you follow the circular path. This action reaction pair is what allows the attraction to work. Centripetal force points inwards.

Example Question #1 : Torque

A bolt connecting the main and rear frame of a mountain bike requires a torque of  to tighten. If you are capable of applying  of force to a wrench in any given direction, what is the minimum length of the wrench that will result in the required torque?

Possible Answers:

Correct answer:

Explanation:

The minimum length of the wrench will assume that the maximum force is applied at an angle of . Therefore, we can use the simplified expression for torque:

Here,  is the length of the wrench.

Rearranging for length and plugging in our values, we get:

Example Question #1 : Torque

A uniform rod of length 50cm and mass 0.2kg is placed on a fulcrum at a distance of 40cm from the left end of the rod. At what distance from the left end of the rod should a 0.6kg mass be hung to balance the rod?

Possible Answers:

50cm

45cm

42cm

The rod cannot be balanced with this mass

48cm

Correct answer:

45cm

Explanation:

The counterclockwise and clockwise torques about the pivot point must be equal for the rod to balance. Taking the fulcrum as the pivot point, the counterclockwise torque is due to the rod’s weight, gravitational force acting downwards at the center of the rod. If we use the pivot as our reference, then the center of the rod is 15cm from the reference.

Set this equal to the clockwise torque due to the additional mass, a distance r to the right of the pivot.

.

Solving for r gives r = 0.05m to the right of the pivot, so 40 + 5 cm from the left end of the rod.

Example Question #1 : Torque

A 2kg mass is suspended on a rope that wraps around a frictionless pulley attached to the ceiling with a mass of 0.01kg and a radius of 0.25m. The other end of the rope is attached to a massless suspended platform, upon which 0.5kg weights may be placed. While the system is initially at equilibrium, the rope is later cut above the weight, and the platform subsequently raised by pulling on the rope.

Screen_shot_2013-10-09_at_10.32.21_pm

What is the torque on the pulley when the system is motionless?

Possible Answers:

9.8N*m

19.6N*m

0N*m

10N*m

Correct answer:

0N*m

Explanation:

The net torque on the pulley is zero. Remember that , assuming the force acts perpendicular to the radius. Because the pulley is symmetrical in this problem (meaning the r is the same) and the tension throughout the entire rope is the same (meaning F is the same), we know that the counterclockwise torque cancels out the clockwise torque, thus, the net torque is zero.

In the image below, T1 (due to the platform with the 4 0.5kg weights) = T2 (the 2kg mass).

Screen_shot_2013-10-09_at_10.35.51_pm

Example Question #1 : Torque

Two students are balancing on a 10m seesaw. The seesaw is designed so that each side of the seesaw is 5m long. The student on the left weighs 60kg and is standing three meters away from the center. The student on the right weighs 45kg. The seesaw is parallel to the ground. Assume the board that makes the seesaw is massless.

What distance from the center should the student on the right be if they want the seesaw to stay parallel to the ground?

Possible Answers:

Correct answer:

Explanation:

Torque is defined by the equation . Since both students will exert a downward force perpendicular to the length of the seesaw, . In our case, force is the force of gravity, given below, and is the distance from the center of the seesaw.

Since the torque must be zero in order for the seesaw to stay parallel (not move), the lighter student on the right must make his torque on the right equal to the torque of the student on the left. We can determine the required distance by setting their torques equal to each other.

Learning Tools by Varsity Tutors