AP Physics C: Mechanics : Mechanics Exam

Study concepts, example questions & explanations for AP Physics C: Mechanics

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Example Questions

Example Question #2 : Energy

If a roller coaster car is traveling at  when it is  above the ground, how fast is it going when  above the ground?

Possible Answers:

Correct answer:

Explanation:

This is a classic conservation of energy problem. We know that potential energy and kinetic energy both have to conserve. So we use the following equation:

What this equations says is, the sum of kinetic and potential energy is same at varying heights and velocities.

We can simplify this equation by cancelling out all the m terms.

We know all the terms except for , which is the final speed we are trying to solve for , which is ,  is  and  is .

If we plug in all the numbers and solve for , we get .

Example Question #1 : Interpreting Work, Energy, And Power Diagrams

An object with a mass of  is moving at  in a straight line on a fricitonless surface. After a force of  acting in the direction of its motion is applied to it for , what is the object's speed in meters per second?

Possible Answers:

Correct answer:

Explanation:

Begin by using the following equation relating the initial and final kinetic energy and the work done on the object:

Then, plug in the given variables and solve for the final speed.

Simplify terms.

Isolate the final velocity and solve.

Example Question #1 : Interpreting Work, Energy, And Power Diagrams

 projectile is launched straight upwards at an initial velocity of . What is the maximum height that this projectile reaches in meters?

Round to the nearest meter, and assume the projectile encounters no air resistance.

Possible Answers:

Correct answer:

Explanation:

You can use the motion equation and find the maximum, but it may be faster to use energy equations. Set the initial kinetic energy equal to the gravitational potential energy at the maximum height and solve for the height.

Mass cancels.

Isolate the height and solve.

Round to .

Example Question #131 : Mechanics Exam

A set of cars on a roller coaster with a combined mass of  is at the top of its initial hill and will drop  down the hill before the track starts to rise again. What will the coaster's speed be at the moment the track starts to rise again?

Round to the nearest meter per second. You may also assume the track does not create friction.

Possible Answers:

Correct answer:

Explanation:

Remember that gravitational potential energy is not affected by the path downward (or upward)—whether it is straight, curved, or winding—only by how big the drop is. Once that is taken into account, you can simply set the initial gravitational potential energy and final kinetic energy equal to each other as if the coaster were falling straight down and solve for the final velocity.

The mass cancels.

Isolate the velocity and solve.

Example Question #2 : Interpreting Work, Energy, And Power Diagrams

A plane weighing 1500kg dives 40m with its engine off before it goes into a circular pattern with a radius of 200m while maintaining its speed at the end of its dive. How much centripetal force, in Newtons, is acting on the plane?

Possible Answers:

Correct answer:

Explanation:

First, find the gravitational potential energy of the drop. Then, set it equal to the kinetic energy at the end of the drop and solve for the velocity.

The mass cancels.

Isolate the velocity and solve.

This gives you the last term you need to solve for the centripetal force.

Example Question #133 : Mechanics Exam

If the maximum speed of an object attached to the end of a elastic has a 1:1 ratio (a meter per second for each meter) with how much the elastic is stretched or compressed from its starting position, which of the following is true?

Possible Answers:

The spring constant has a 1:2 ratio with the mass of the object in kilograms

The spring constant has a 1:1 ratio with the mass of the object in kilograms

The spring constant has a 2:1 ratio with the mass of the object in kilograms

None of these statements are true

The spring constant has a 4:1 ratio with the mass of the object in kilograms

Correct answer:

The spring constant has a 1:1 ratio with the mass of the object in kilograms

Explanation:

Set the elastic potential and kinetic energy equations equal to each other:

You are given the fact that in this case, . This allows you to simplify the equality.

This shows us that there is a 1:1 ratio between the spring constant of the elastic and the mass of the object.

Example Question #1 : Interpreting Work, Energy, And Power Diagrams

A pumpkin is being launched out of an air cannon. For safety reasons, the pumpkin cannot be more than  off the ground during flight, and this particular cannon always launches pumpkins at  meters per second—any more power and the pumpkin could be blasted apart; any less and the pumpkin may not leave the launch tube.

What is the maximum possible angle of launch in degrees?

Round to the nearest whole degree.

Possible Answers:

Correct answer:

Explanation:

We know the maximum height of the pumpkin, which tells us the maximum energy of the launch. Calculate the final gravitational potential energy.

Now set this value equal to a kinetic energy equation that uses the vertical component of the velocity at launch.

Use the vertical velocity component to determine the launch angle.

 

Example Question #1 : Understanding Conservation Of Momentum

In an inelastic collision, which combination of quantities is conserved?

Possible Answers:

Mass, kinetic energy, and momentum

Mass and momentum

Mass and potential energy

Kinetic energy and potential energy

Momentum and kinetic energy

Correct answer:

Mass and momentum

Explanation:

In a perfectly inelastic collision, the two colliding objects stick together; the two colliding objects deform, but mass is still conserved. Momentum is conserved during collisions of any sort, including inelastic collisions.

Kinetic energy is reduced during an inelastic collision, and is only conserved in elastic collisions. During inelastic collisions, some kinetic energy is lost to the environment in the form of heat or sound.

The problem does not give any information regarding position, and thus we cannot comment on any changes or lack of changes in potential energy.

Example Question #2 : Understanding Conservation Of Momentum

In an elastic collision, what combination of quantities is conserved?

Possible Answers:

Momentum, kinetic energy, and potential energy

Momentum, kinetic energy, and potential energy

Mass and momentum

Mass, momentum, and kinetic energy

Potential energy and kinetic energy

Correct answer:

Mass, momentum, and kinetic energy

Explanation:

The primary difference between elastic and inelastic collisions is the conservation of kinetic energy. Kinetic energy is conserved in elastic collisions, but is not conserved in inelastic collisions. Momentum is always conserved, regardless of collision type. Mass is conserved regardless of collision type as well, but the mass may be deformed by an inelastic collision, resulting in the two original masses being stuck together.

There is no description in the problem reagrding position, so we cannot comment on potential energy.

Example Question #131 : Mechanics Exam

A 200kg car is traveling west at . It collides with a 150kg car that was at rest. Following the collision, the second car moves with a velocity of  west. Assuming that the collision is elastic, what is the velocity of the first car after the collision?

Possible Answers:

Correct answer:

Explanation:

The collision is assumed to be elastic, so both momentum and kinetic energy are conserved. Use the law of conservation of momentum:

Momentum is the product of velocity and mass:

We can expand the summation for the initial and final conditions:

Use the given values to fill in the equation and solve for :

Since the final velocity is positive, we know that the car is still traveling toward the west.

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