1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Mechanical Energy

  1. Oct 24, 2008 #1


    User Avatar

    1. A 1100 kg car at rest at the top of a hill accelerates down the road, reaching a speed of 20 m/s after descending a height of 120 m. What was its total change in mechanical energy (KE plus PEG) as of that moment in Megajoules?

    2.KE = 1/2 mv^2 and PEg = mgh

    3. I used the above formulas to figure out KE = 2.20E5 and PEg = 1.3E6. When I add both together, I get 1.51E6 and convert it to MJ = 1.51MJ. When I put in the answer, this is incorrect. Am I missing something?
  2. jcsd
  3. Oct 24, 2008 #2


    User Avatar
    Homework Helper

    The KE at the bottom of the hill already includes the m*g*h from the descent. You probably want to just use the KE as the answer.
  4. Oct 24, 2008 #3
    It starts high and ends low, so it LOSES potential energy. It starts at rest and finishes in motion, so it gains kinetic energy. (I think)
  5. Oct 24, 2008 #4
    kraus is right: so how much mechanical energy does the car lose?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook