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Homework Help: What is the magnitue of the average Force from the wall stopping it?

  1. Feb 24, 2004 #1
    What am I doing wrong???

    Question:

    A 41 g bullet, with horizontal velocity of 496 m/s, stops 13cm within a soid wall. What is the magnitue of the average Force from the wall stopping it?

    This is how i tried to solve it

    Vi = 496, Vf=0, and Xf-Xi=.13m

    So Vf^2=Vi^2+2a(Xf-Xi)

    Which = 0 = 246016+2a(.13)

    solve for acceleration you get -946215...

    F=m(a)...so i did (-946215)(41) which does not equal the answer? Am i doing anything wrong?


    2nd Question:

    A girl whose weight is 264 N slides down a 5.8 playground slide that makes an angle of 20degree with the horizontal. The coefficient of kinetic friction is 0.10. If she starts at the top with a speed of 0.457 m/s, what is her speed at the bottom?

    This is how i tried to slove it

    W= change in U + change in K + change in Thermal

    Thermal = 143.98 (i wont show my calculations cause i am 100% sure this is righ)
    so..
    F(d)= -mgh +(1/2)mv^2 - (1.2)m(.457^2) +143.89

    After you plug in all your variables, you have two unkowns,velocity and W.. Provided i did the setup right, how do i figure out the velocity of i dont know the magnitude of her force?
     
  2. jcsd
  3. Feb 24, 2004 #2

    jamesrc

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    #1: Check your units (41g = .041 kg)

    #2. Work-energy theorem:

    [tex] \Delta K + \Delta U = W_{nc} [/tex]
    [tex] K_i = \frac{mv_i^2}{2} [/tex]
    [tex] K_f = \frac{mv_f^2}{2} [/tex]
    [tex] U_f = 0 [/tex] (your choice of datum)
    [tex] U_i = mg(d\sin \theta) [/tex]
    [tex] W_{nc} = -fd [/tex]

    where
    d = 5.8 m; length of the slide
    [tex] \theta = 20^\circ [/tex]; angle of slide
    f = μ N; friction force
    μ = 0.10; coefficient of friction
    [tex] N = mg\cos\theta [/tex]; normal force
     
  4. Feb 24, 2004 #3
    inst the worky - enegy theorem change in K + change in U + change in Thermal (since there is friction?) = W
     
  5. Feb 24, 2004 #4

    jamesrc

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    OK,

    How are you computing the "change in thermal energy" and what is your value for W?
     
  6. Feb 25, 2004 #5

    turin

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    Re: What am I doing wrong???

    How do you know that the acceleration is constant?




    I wouldn't do it that way. I would use the kinematic equation that you are trying to use for the first question.
     
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