AP Physics 1 : Impulse and Momentum

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

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Example Question #261 : Linear Motion And Momentum

A blue rubber ball weighing  is rolling with a velocity of  when it hits a still red rubber ball with a weight of . After this elastic collision, what are the speeds and directions of the blue and red balls respectively?

Possible Answers:

 to the left,  to the right

At rest,  to the right

 to the right,  to the left

 to the left,  to the right

At rest,  to the right

Correct answer:

 to the left,  to the right

Explanation:

Because we are solving for two velocities (two unknowns), we need two equations. We can use the conservation of linear momentum:

Because we know that the collision is elastic, we know that kinetic energy is conserved:

The red ball starts at rest so

The above equations can then be simplified and one can solve for and .

Negative implies the ball is moving to the left.

Example Question #267 : Linear Motion And Momentum

Jennifer has a mass of . She is riding her skateboard east at . She collides with Tommy, who is riding in the opposite direction at . After they collide, they continue in Tommy's direction at . Find the mass of Tommy.

Possible Answers:

Correct answer:

Explanation:

Using conservation of momentum

Solving for 

Plugging in values:

(A negative value is assumed for Tommy's direction for simplicity, a negative could have been used for Jennifer's instead, it will yield the same result.

Example Question #268 : Linear Motion And Momentum

 ship traveling at  collides with a  ship traveling at. The two ships are  apart prior to collision, and stick together afterward. What is the final speed of the combined ships?

Possible Answers:

Correct answer:

Explanation:

A coordinate system isn't immediately given, so the first step is to establish an x- and y-direction. Let the first ship travel in the positive y-direction with the second traveling  from the y-axis.

The linear momentum for the first ship is given by 

and for the second ship by 

Conservation of linear momentum says that . Thus the final velocity can be expressed as

Further,

The final speed is then 

Example Question #269 : Linear Motion And Momentum

A  ball is launched across a flat, frictionless surface at a constant velocity of in the rightward direction. While traveling at , it collides in a perfectly inelastic manner with a stationary  ball and the two balls continue traveling in the rightward direction, stuck to one another. What is the velocity of the  ball unit? 

Possible Answers:

Correct answer:

Explanation:

This question tests your understanding of the concept of conservation of momentum. More specifically, this tests your understanding of perfectly inelastic collisions, or collisions in which the two colliding particles stick together and move together as a system. 

By conservation of momentum, the initial total momentum of a system is equal to the final total momentum of the system. The formula for the momentum of an object is equal to the mass of the object multiplied by the velocity of the object. When applying the conservation of momentum formula to perfectly inelastic collisions, you consider the two objects as a single mass after they collide, since they stick together and travel together. Thus, this problem can be solved as follows:

The final velocity of the  ball unit is therefore (sign is positive because initially the  ball was traveling to the right, and its velocity was positive, and when the  ball unit formed and moved, it continued moving in the rightward, positive direction.

 

Example Question #81 : Impulse And Momentum

A  block is launched in the positive direction across a frictionless surface at a constant velocity of . It collides elastically with an  block that then travels in the positive direction at a velocity of . What is the sign and magnitude of the velocity of the block after the collision? Assume that no energy is lost to friction during the collision or while the blocks travel. 

Possible Answers:

Correct answer:

Explanation:

This question tests your understanding of the conservation of momentum, which states that momentum (mass multiplied by velocity) can neither be created nor destroyed in a system. In this elastic collision, we are presented with two blocks (one initially moving, and one initially stationary), that collide such that each block is moving separately afterwards and neither loses energy to friction or other forces. You can set up the equation as follows:

Therefore, the final velocity of the  block is  in the positive direction.

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