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Homework Help: General Physics 1

  1. Mar 1, 2005 #1
    General Physics 1....

    A crate of mass 30.0kg rests on level surface. The coefficient of kinetic friction between the crate and surface is .400 . What force applied at an angle of 30.0 degrees below the horizontal (i.e. pushing down) is required to keep the mass moving at constant speed?......................on this problem i came up with the equation Fn= Uk[mgcos(30)]......with Uk=.400......Fn=102N......as the answer......iam i doing this right.....need help......please.
  2. jcsd
  3. Mar 1, 2005 #2
    You made some erroneous assumtions. The normal force of the crate is not just [itex]mgcos\theta[/itex], if the angle is that of an applied force. This method assumes that the box is on an incline (in which case that would be true). This is a level surface, so the normal force is caused by the box's full weight (mg) and the force you apply ([itex]F_y=F\cos\theta[/itex]) by pushing or pulling.

    Force of friction: [itex]f=\mu N[/itex] with [itex]N=mg+F\sin30[/itex] and so [itex]f=\mu\left(mg + F\sin30\right)[/itex]

    To move at a constant speed, [itex]F\cos30[/itex] must equal the force of the friction trying to keep it from moving:
    [tex]F_{x net} = F\cos30 - F_{friction} = ma =0[/tex]

    That should help.

    PS, as an answer, I got close to [itex]180N[/itex] - I'll hold off on the exact answer I got until you've done the problem. :)
    Last edited: Mar 1, 2005
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