Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Calculating potential energy

  1. Mar 24, 2010 #1
    A 800 kg roller-coaster car is initially at the top of a rise, at point A. It then moves 135 ft, at an angle of 40.0° below the horizontal, to a lower point B.
    (a) Choose the car at point B to be the zero configuration for gravitational potential energy of the roller coaster-Earth system. Find the potential energy of the system when the car is at points A and B, and the change in potential energy as the coaster moves.

    GPE = mgy cos theta

    This is my attempt to solve, however I keep getting that my answer is wrong.
    (800kg)(9.8)(41.15m)x cos 40 =247,138.19J

    Can someone please let me know what is incorrect in my process?
  2. jcsd
  3. Mar 24, 2010 #2


    User Avatar
    Science Advisor

    Suppose that the angle is zero degrees below the horizontal. What would be the height change?

    Now put zero into your formula instead of 40.

    Why is it different? Just becos.
  4. Mar 24, 2010 #3
    The height changes by 9.6m...

    If I do (800kg)(9.8)(41.15m) = 322,616J
    .. but that is not the correct answer either.
  5. Mar 24, 2010 #4


    User Avatar
    Science Advisor

    0 degrees from horizontal is horizontal....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook