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: Average rate of energy transfer & rate of energy dissipation.

  1. Nov 11, 2007 #1
    1. The problem statement, all variables and given/known data
    1.) A 70kg boy runs as fast as he can up a flight of stairs which has vertical height of 10metres. He achieves this in 8 seconds. Calculate the average rate of energy transfer into gravitational potential energy that must have occured.

    2.) A700kg car travelling at 20ms-1 breaks to rest in 5 seconds. Calculate the rate of energy dissipation the car must achieve.

    Hello, i am not certain how to calculate these.
     
  2. jcsd
  3. Nov 11, 2007 #2
  4. Nov 11, 2007 #3
    Hello,

    i thought maybe mgh and mgh/t?

    70x9.8x10?

    700x9.8x(20)/5
     
    Last edited: Nov 11, 2007
  5. Nov 11, 2007 #4
    E=mgh only works for vertical changes. You will use it in the first one, but not in the second one.

    If I understand the question, you almost have the first one. However, it asks for the rate, which implies a time. So, you have to put the answer in the form J/s.

    The second one requires a different formula, one which uses kinetic energy. See if you can find this formula; theres a handy little sticky with some of the more common formulas at the top of this page.
     
  6. Nov 11, 2007 #5
    Hello,

    1/2mv^2/t

    ?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook