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Homework Help: I have a few problems dealing with work and kinetic energy

  1. Oct 29, 2008 #1
    1. A 0.267 kg ball is thrown straight up from 1.81 m above the ground. Its initial vertical speed is 8.00 m/s. A short time later, it hits the ground. Calculate the total work done by the force of gravity during that time.

    I know that I will probably need to use the PEg=KE formula, but I am very lost as to how to set this problem up and any help would be appreciated.

    __
    2.A car (m = 600.0 kg) accelerates uniformly from rest up an inclined road which rises uniformly, to a height, h = 33.0 m. Find the average power the engine must deliver to reach a speed of 20.5 m/s at the top of the hill in 18.3 s(NEGLECT frictional losses: air and rolling, ...)

    W=KEf-KEi

    W=1/2mv^2-1/2mv^2
    W=1/2(600)(205)-0
    mgh=600(9.8)(33)
    W=6150+194040
    W=200190/18.3
    W=10939.34
    I feel like I'm very close to solving this problem but something didn't turn out right..

    ____

    3. A 96.0 kg diver falls from rest into a swimming pool from a height of 3.70 m. It takes 1.54 s for the diver to stop after entering the water. Find the magnitude of the average force exerted on the diver during that time.

    I am pretty sure I should start out with PEg=KE but am not sure where to go from there.


    Help on any of these problems would be VERY MUCH appreciated! Thanks guys and this seems like a great physics forum.
     
  2. jcsd
  3. Oct 29, 2008 #2
    1) This problem is relatively easy if you use W=F(d). Find the displacement of the ball using basic kinematics. Remember your signs!

    3) There are several ways of doing this problem, but I would recommend that you look at it conceptually i.e gravity exerts a constant force of mg on the diver on his downard path, while after he is in the water... the water exerts a force that brings him from Vo(the velocity at which he enters the water) to Vf(rest) in the time given... Now find the avg force exerted
     
    Last edited: Oct 29, 2008
  4. Oct 29, 2008 #3
    Thanks, I got the 3rd one right now! 1st I don't really understand how I can find the F for W=Fd. I could figure it out if they gave me time but they didn't..so how can I go about finding the F?
     
  5. Oct 29, 2008 #4
    The question asks about the gravitational force. F = the gravitational force =....
     
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