# 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

#### Attached Files:

• ###### car.JPG
File size:
3.7 KB
Views:
1,195
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

### adjacent

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.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted