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Homework Help: Any Hints?

  1. Sep 29, 2004 #1
    Any Hints!?!?

    I'm stuck on these 2 problems.. Any hints would be helpful!

    1. What is the minimum work needed to push a 950-kg car 810m up along a 9 degree incline? a) ignore friction, b) assume the effective coefficient of a friction retarding the car is .25.

    This is what I did, but I don't think it's correct... :
    A) W= Fdcost = mdcost = (950)(810cos9) = 7.6 * 10^5 J.
    B) Wfr= Ffr*x*cos(180+9) = .25 * 810 *cos189 = -200 J

    2. A 65 kg trampoline artist jumps vertically upward from the top of a platform with a speed of 5 m/s. A) how fast is he going as he lands o nteh trampoline, 3 m below? b) if the trampoline behaves liek a sprign with spring stiffness constant 6.2 * 10 ^4 N/m how far does he depress it?
    I got the a) part using .5mv^2 + mgy1 = .5mv^2 + mgy2
    and found v= 9.15 m/s
    b) I'm not sure which equation to use... I tried using .5mvi^2 + .5kx1^2 = .5mvf^2 + .5kx2^2 But I wasn't quite sure what to plug in, and when I tried all the different possibilities I did not get the corect answer.
  2. jcsd
  3. Sep 29, 2004 #2
    Nobody!?! Please help! I have a quiz this friday! I really need to know how to do these problems!!!
  4. Sep 29, 2004 #3


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    Homework Helper

    Fused do a Free body diagram for the body on the incline.
  5. Sep 30, 2004 #4
    I did that, but I don't know how to put the information into formulas

  6. Sep 30, 2004 #5


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    Homework Helper

    Use Newton's 2nd Law

    [tex] \sum^{n}_{i=1} \vec{F}_{i} = m \vec{a} [/tex]
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