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Car-Hill-Free body

  1. Feb 17, 2014 #1
    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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  3. Feb 17, 2014 #2

    Doc Al

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    Staff: Mentor

    Careful. Acceleration means a change in velocity, not necessarily speed.
     
  4. Feb 17, 2014 #3
    That makes sense. I still haven't figured my answer yet. Should I still stick with B?
     
  5. Feb 18, 2014 #4

    Doc Al

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    You tell me. Is the car accelerating? (Is its velocity changing as it goes over the hill?)
     
  6. Feb 18, 2014 #5
    The car is constantly changing direction, along with it changes something else, why do you think that is?
     
  7. Feb 18, 2014 #6
    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?
     
  8. Feb 18, 2014 #7

    Doc Al

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    Good!

    What's the direction of the acceleration (and thus the net force)?
     
  9. Feb 18, 2014 #8

    adjacent

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    Gold Member

    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)
     
  10. Feb 18, 2014 #9
    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.
     
  11. Feb 18, 2014 #10

    Doc Al

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    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.
     
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