Car Coasting Down a Circular Hill: Which Free-Body Diagram is Correct?

[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

 

[Doc Al]

Since the car is at a constant speed, there's no acceleration.

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

 

[BrownBoi7]

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

 

[Doc Al]

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

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

 

[lendav_rott]

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

 

[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?

 

[Doc Al]

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

Good!

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

 

[adjacent]

This question is very similar to centripetal motion.

Spoiler

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)

 

[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.

 

[Doc Al]

W = mg ---acting downwards
N = Normal Force ---acting upwards opposite to W

All true.

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?

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.