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: Energy Transformation Efficiency Problem

  1. Nov 26, 2015 #1
    1. The problem statement, all variables and given/known data
    A ball that weights 1kg is thrown in the air, the hand being 1m above the ground. The velocity the ball was thrown at was 14m/s and it's maximum height was 6.5m above the ground. What is the efficiency for this energy transformation?

    2. Relevant equations
    Do I just get the mechanical energy of when the ball is in the hand (input) and the mechanical energy of when the ball reaches its maximum height (output) and use the standard efficiency equation (My attempt at the solution)? Or do I just act as if the hand is reference point and find the kinetic energy, then the gravitational potential energy (Using 5.5m because hand is 1m above ground) and find the efficiency that way

    3. The attempt at a solution
    EmBottom = m*g*h + 1/2 (mv^2)
    = 1kg*9.8N/kg*1m + 1/2 (1kg(14m/s)^2)
    = 107.8 J
    EmTop = m*g*h (Just gravitational potential because the velocity at the top is 0, making the kinetic energy 0)
    = 1kg*9.8*6.5m
    = 63.7J

    Efficiency = (output/input) * 100
    = (63.7/107.8) * 100
    = 59%
  2. jcsd
  3. Nov 26, 2015 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The 1m initial height is certainly arbitrary, and is not involved in any transformation. What is transformed is KE into GPE, so only count that.
  4. Nov 27, 2015 #3
    Damn, was a test question, hopefully i got partial marks lol. Thanks for the help.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted