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Newton's laws on an incline

  1. Jul 13, 2008 #1
    1. The problem statement, all variables and given/known data
    A man pushes on a piano with mass 180 kg so that it slides at constant velocity down a ramp that is inclined at 11deg above the horizontal floor. Neglect any friction acting on the piano. Calculate the magnitude of the force applied by the man if he pushes (a) parallel to the incline and (b) parallel to the floor


    2. Relevant equations
    Trig (SOHCAHTOA)
    F= ma


    3. The attempt at a solution
    At first I thought this would be zero because the acceleration is 0. But looking at the back of the book the answer is NOT zero, so I attempted to figure out a way to solve it..

    I got the first part right, but can't figure out the second part (b)

    f=ma
    f=(180kg)(9.8m/s^2)
    f=1764N

    Sin (11) (1764N) = 336.6 N parallel to the ramp (answer checked out in the back of the book)

    Now, for the second part... I know that there is zero acceleration so the sum of all forces must be zero. So I know from the first part the force is 336.6, so the opposite must also be 336.6. I also know that it makes a 11 deg angle with the horizontal component (what I am looking for) due to it being a similar triangle. So:

    Cos (11) (336.6N)= 330.4N. (which is apparently wrong, my book says it should work out to 343N. How so?
     
  2. jcsd
  3. Jul 13, 2008 #2

    PhanthomJay

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    Science Advisor
    Homework Helper
    Gold Member

    You're looking at the wrong triangle. The force parallel to the incline is the component of the applied horizontal force.
     
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