1. Not finding help here? Sign up for a free 30min 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!

Potential energy and a skateboard, only have until 11:30

  1. Feb 25, 2005 #1
    Potential energy and a skateboard, only have until 11:30!!!!

    A 54.5 kg. skateboarder starts out with a speed of 1.95 m/s. He does +80.0 J of work on himself by pushing with his feet against the ground. In addition, friction does -265 J of work on him. In both cases, the forces doing the work are nonconservative. The final speed of the skateboarder is 6.10 m/s

    Calculate the change (PE = PEf - PE0) in the gravitational potential energy

    (b) How much has the vertical height of the skater changed?



    ok so potential energy is PE=MGH buti don't see a vertical height here!

    Please help!!
     
  2. jcsd
  3. Feb 25, 2005 #2
    right...
    You can define the height to be zero anwhere you like. I would suggest letting the height that he starts at be zero.

    Set up your Conservation of Energy equations (ME_i = ME_f, where ME = KE +PE) using variables for any unknowns.
     
  4. Feb 25, 2005 #3

    xanthym

    User Avatar
    Science Advisor

    {Initial Total Energy} = (1/2)*m*v0^2 + PE0 = (1/2)*(54.5)*(1.95)^2 + PE0 =
    = 103.62 + PE0
    {Final Total Energy} = (1/2)*m*vf^2 + PEf = (1/2)*(54.5)*(6.10)^2 + PEf =
    = 1013.97 + PEf
    {Delta Total Energy} = {1013.97 + PEf} - {103.62 + PE0} =
    = 910.35 + {Delta PE} =
    = {Work Performed On Subject} = (80) + (-265) = (-185)
    {Delta PE} = (-185) - (910.35)

    A) {Delta PE} = (-1095.35 J)

    m*g*{Delta H} = (-1095.35)
    {Delta H} = (-1095.35)/{(54.5)*(9.8)}

    B) {Delta H} = (-2.051 m)


    ~~
     
    Last edited: Feb 25, 2005
  5. Feb 25, 2005 #4
    here's what i did. plug all the numbers into Wnc= (1/2mvf^2-1/2mv0^2)+(mghf-mgh0) and you get Wnc= a number + a number *(h0-hf)
    then you go and find Wnc first since it already told you in the question= +80-265 and you get -185. so
    -185=a number + a number *(h0-hf) and you can now find out what (h0-hf) equals to. which is close to 2. this is the answer to b (i got 2.15 cuz our variables are different on webassign)

    answer to a. since you know what h0-hf is.
    change in PE= mghf-mgh0
    plug h0=hf+(your answer to B)
    and you get
    mghf-mghf-mg*(your answer to B)
    mghf cancels out. -mg*(your answer to B) is the answer for A (negative number)

    hope that helps......LOL
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Similar Discussions: Potential energy and a skateboard, only have until 11:30
Loading...