# AP Physics 1 : Specific Forces

## Example Questions

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### Example Question #1 : Other Forces

A  car travels around a circular racetrack at a velocity of . The radial distance between the center of the racetrack and the center of the car at any given point of its trip is . What is the centripetal force on the car?

Explanation:

This question tests your understanding of the concept of centripetal force. Centripetal force is a force that maintains a body's circular travel along a curved surface and points inward towards the center of the curve from each point that the body travels along. Centripetal force is calculated as follows:

Therefore the centripetal force on the car is .

### Example Question #2 : Other Forces

Nicola is attempting to create a single tiered mobile as shown. She has a wooden dowel (), and two distinct bird ornaments (, ) which she wishes to attach on either end of the dowel.

If Nicola places ornament ,  to the left of the dowel's midpoint, and ornament ,  to the right of the dowel's midpoint, where should she attach the mobile string in order for it to balance?

Note: figure not drawn to scale.

from the left end of the dowel

from the left end of the dowel

from the right end of the dowel

from the right end of the dowel

from the left end of the dowel

Explanation:

Let's use the midpoint as our point of reference. With that said,  is  from the center, and  is  from the center. However, each of these are on different sides of the midpoint. When we set up our center of gravity equation, we must determine a (+) and (-) side in order to denote under which side our mobile string will fall. It may be helpful to look at the mobile as a number line, with the left being negative, and the right side positive, but you can really use which ever is more comfortable.

Our main equation is this:

Where  is our weight in newtons and  is our distance in meters. Typically, we would want to convert everything to SI units, so let's go ahead and do that (ex: )

Now let's plug in our numbers, remember about our negative/positive sides!

This should give us

Now, because this number is negative, we know it's to the left of our midpoint (if you chose to set up your -/+ sides opposite of how we did it, your answer will be positive). Regardless of the outcome, this measurement is meant to be taken from object A to the midpoint. Well, do any of our answers have either (+) or (-) ? No. But if we read them carefully, we can determine that the answer is  from the left of the dowel by using simple subtraction.

One side is

Think about it logically too (use your pencil). The heavier end of a pencil usually has a bulky eraser on it (just like the bulkier object on the left). Try holding the pencil at the tip's end, and then gradually try balancing it in the same manner as you move closer to the eraser.

ALSO NOTE: the question states the weight of the dowel as well. But look we didn't even need it. Sometimes the AP exam will give you some aspects in the question to distract you.

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