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Work and kinetic energy

  1. Mar 21, 2009 #1
    1. The problem statement, all variables and given/known data
    1.)A mass is on a frictionless horizontal surface and attached to a horizontal spring of
    spring constant 200 N/m. If the mass is released from rest when the spring is
    compressed by 10 cm, what will its kinetic energy be when the spring is stretched by
    7 cm?

    2)A child on a sled starts at the top of a 1.6 m high hill and slides along a straight
    incline to the bottom at a constant velocity. If the mass of the child and sled is 40 kg,
    what is the work done by friction during his slide down the hill?

    2. Relevant equations

    1.) 1/2mv^2=1/2KX^2
    2.) W=1/2mvf^2-1/2mvi^2 or W=FD

    3. The attempt at a solution
    for the first one i tried plugging in the numbers to 1/2mv^2=1/2Kx^2 and tried to ifnd the kinetic energy that way but it didnt work what went wrong?

    The second one i tired using the work energy theorem but but it said constant velocity so that woould cuase the both teh final velocity and initil velocity to be the same and since the mass is the same wouldnt the W be 0? i also tried listing the forces out but then since it doens have an angle i got stuck... what did i do wrong i am confused
  2. jcsd
  3. Mar 21, 2009 #2


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    Homework Helper

    Hi silentsaber

    for a) are you comparing the difference in energy in the spring at 10cm compressed and 7cm stretched with the kinetic energy?

    for b) try drawing the slope and forces for an arbitrary angle, to be constant velocity, the frictional drag must equal the gravtational force, so there is no net acceleration.... see how the fits with your W = F.d equation
    Last edited: Mar 21, 2009
  4. Mar 21, 2009 #3
    For number one why do you think all of the energy will be kinetic at the position is asks you to evaluate?

    For number two you're only considering the effects of kinetic energy, which is constant throughout the problem -- What can be said about the other types of energy?
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