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Homework Help: Help:Equilibrium & Angular Momentum

  1. Nov 11, 2004 #1
    1) A plank of mass 2.5 kg and length 2 meters is resting horizontally on two wedges. The first is at one of the ends and the other is a distance 0.3 meters from the second end. If a mass of 0.2 kg is placed on this second edge, what is the force (in Newtons) acting on the second wedge.

    Here's what I did:
    T1=F1(0) = 0
    T(mass)=-(0.2*9.81*2) = -3.924
    T(plank)=-(2.5*9.81*(2/2)) = -24.525
    sum of torque = F1(0) + F2(1.7) - 3.924 - 24.525 = 0
    so, F2=16.73 Newtons
    but the correct answer is 1.71 Newtons. Where did I go wrong?

    2) The mass of the earth is 6.0E24 kg and its radius is 3950 miles. Assuming that the earth is a uniform sphere, its angular momentum (in Joule. Secs) is aE+33 where a is?

    I = (2/5) MR^2 ---> (2/5)(6.0E24)(6355550meters)^2 = 9.694E37

    L = Iw ---> (9.694E37)((2pi)/86400) = 7.0499E33
    which makes a=7.050
    but the correct answer is 6.970. (I did not round anything either.) What did I do wrong? Please help. Thanks!
    Last edited: Nov 11, 2004
  2. jcsd
  3. Nov 12, 2004 #2
    Is there anyone that can help me with these two problems? :frown:
  4. Nov 12, 2004 #3
    All your workings looks fine. I don't know why you couldn't get the answer. Are you sure you were looking at the correct answer?
  5. Nov 12, 2004 #4
    I'm sure the answers that I got were wrong because I worked out other similar problems (same problem but with different numbers) with the same method and still got it wrong.
  6. Nov 12, 2004 #5

    Doc Al

    User Avatar

    Staff: Mentor

    I see nothing wrong with your method or your answers.
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