# Car-Hill-Free body

1. Feb 17, 2014

### BrownBoi7

Car--Hill--Free body

A car coasts at a constant speed over a circular hill. Which of the free-body diagrams in the figure attached is correct? Explain.

My attempt:
I am thinking B. Since the car is at a constant speed, there's no acceleration. So there is no additional force acting downwards besides it's weight.

Which would you choose? Explain please.

Thanks

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2. Feb 17, 2014

### Staff: Mentor

Careful. Acceleration means a change in velocity, not necessarily speed.

3. Feb 17, 2014

### BrownBoi7

That makes sense. I still haven't figured my answer yet. Should I still stick with B?

4. Feb 18, 2014

### Staff: Mentor

You tell me. Is the car accelerating? (Is its velocity changing as it goes over the hill?)

5. Feb 18, 2014

### lendav_rott

The car is constantly changing direction, along with it changes something else, why do you think that is?

6. Feb 18, 2014

### BrownBoi7

Yes the car is constantly changing direction. Velocity is not constant in which case. This means there is some acceleration.

Should I go with 3?

7. Feb 18, 2014

### Staff: Mentor

Good!

What's the direction of the acceleration (and thus the net force)?

8. Feb 18, 2014

This question is very similar to centripetal motion.
There is a net force towards the center even if the speed is constant.The direction changes towards center.So the force is acting towards center

EDIT:(Changed to a spoiler)

9. Feb 18, 2014

### BrownBoi7

W = mg ---acting downwards
N = Normal Force ---acting upwards opposite to W
A= acceleration --- acting downwards. Looking at the figure, it's clear the car is going downhill. So we can treat it as a circular motion problem?

Option A would best describe the scenario.

10. Feb 18, 2014

### Staff: Mentor

All true.

The key is not that the car is going downhill, but that it is going over a circular hill. Yes, this is a circular motion problem. So what's the direction of the centripetal acceleration? Use that to figure out the direction of the net force and thus the correct choice.