AP Physics C: Mechanics : Understanding and Finding Center of Mass

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

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Example Questions

Example Question #52 : Mechanics Exam

Doing which of the following would allow you to find the center of mass of an object? 

Possible Answers:

Sliding it along a flat surface

Recording its shape

Hanging it from a fixed point

Spinning it 

Correct answer:

Spinning it 

Explanation:

Center of mass can be found by spinning an object. It will naturally spin around its center of mass, due to the concept of even distribution of mass in relation to the center of mass. Shape and mass are important factors in this property, but the most improtant factor is the mass distribution.

Example Question #53 : Mechanics Exam

If the fulcrum of a balanced scale is shifted to the left, what type of adjustment must be made to rebalance the scale?

Possible Answers:

If the scale was initially balanced, moving the fulcrum will not change this

Apply more mass to the new position of the fulcrum

Apply the same amount of mass to both ends

Apply more mass to the left end

Apply more mass to the right end

Correct answer:

Apply more mass to the left end

Explanation:

Changing the position of the fulcrum by moving it to the left means the center of mass will be to the right of the new position. Therefore, the scale will tip right. Adding more mass to the left end will rebalance the scale. None of the other options make sense. Adding more mass to the new fulcrum position will not change the balance of the scale because that mass is a negligible distance from the new fulcrum position and does nothing to change the masses on either side.

Example Question #54 : Mechanics Exam

If two masses,  and  are placed on a seesaw of length , where must the fulcrum be placed such that the seesaw remains level?

Possible Answers:

Correct answer:

Explanation:

This question asks us to find the center of mass for this system. We know that the center of mass resides a distance  from the first mass such that:

 

In this case:

Plug in known values and solve.

Example Question #4 : Understanding And Finding Center Of Mass

Three  point masses are at the points ,  and


and a  point mass is at the point .

How far from the origin is the center of mass of the system?

Possible Answers:

Correct answer:

Explanation:

To find the center of mass, we have to take the weighted average of the x coordinates and the y coordinates.

 Measures:                                        Measures

                             

 

First, we take the weighted measurement of the x-axis:

We can see that the result of the x-axis contribution is equal to .

Now, let's look at the y-axis contribution:

This equals to 

Now that we have the x and y components, we take the root of squares to get the final answer:

This will give us 

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