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Homework Help: Applying newton's laws

  1. Sep 19, 2005 #1
    A person pushes a 13.0 kg lawn mower at constant speed with a force of 84.0 N directed along the handle, which is at an angle of = 49.0° to the horizontal.

    (a) Calculate the horizontal retarding force on the mower

    (b) Calculate the normal force exerted vertically upward on the mower by the ground.

    (c) Calculate the force the person must exert on the lawn mower to accelerate it from rest to 1.3 m/s in 2.0 seconds (assuming the same retarding force).

    Here's what I have so far...

    a) FPx = 84 cos 49 = 55.11 N
    FPy = 84 sin 49 = 63.396 N
    ∑ F = ma
    N – mg = ma
    N = mg
    = 13.0(9.8)
    = 127.4 N

    b) Ffr = (coefficient of friction)FN
    = (0.30)(127.4)
    = 38.22 N
    c) I have no idea where to even start for this question!

    Please help!!!
  2. jcsd
  3. Sep 19, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    All you need to answer this part is to realize that the net horizontal force must be zero. The retarding force must balance the horizontal component of the applied force.

    For this part, all you need to find is the normal force. (Where did you get the coefficient of friction?) This time use the fact that the net vertical force must be zero. (Hint: There are three vertical forces.)
    To get an acceleration, the net horizontal force must be greater than zero. Apply Newton's 2nd law.
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