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Homework Help: Dot Product

  1. Feb 24, 2008 #1
    1. The problem statement, all variables and given/known data

    A box with a weight of 215 N rests on a ramp. There is a frictional force of 27 N exerted on the box directed up the ramp. If the ramp is inclined at 30 degrees, what force must be applied at an angle of 40 degrees to the ramp to maintain equilibrium?

    Textbook Answer: 80.5 N and 105 N

    2. Relevant equations

    u (dot) v = |u||v|cosx

    3. The attempt at a solution

    F = 215(cos30) = 186.2 N
    F = 27(cos20) = 25.37 N
    F = 215(cos40) = 164.7 N
    F = 242(cos40) = 185.38 N
    F = 242(cos70) = 82.77 N

    None of them are right and I don't know how else to solve this problem.
  2. jcsd
  3. Feb 24, 2008 #2


    User Avatar
    Science Advisor

    What reason do you have to think that the answer will be given by a single dot product? Draw a picture of the situation and think about it.
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