How Do You Calculate the Pulling Force on a Crate for Zero Net Work?

[rocky811]

Hello! I need a little help here. I have been trying to figure out this problem, but I just can't and I don't know where I am going wrong.

A 116 -kg crate is being pulled across a horizontal floor by a force P that makes an angle of 35.2 ° above the horizontal. The coefficient of kinetic friction is 0.212 . What should be the magnitude of P, so that the net work done by it and the kinetic frictional force is zero?

now I know what W=FdCos(theta) and that fs=uk*F...but I just don't know where to go from there and I don't know what the end of the question means..."so that hte net work done by it and the kinetic frictional force is zero?"...does that mean I just set the net work and frictional force equal and solve for the force?

 

[vsage]

Any moving object on a surface with friction will have kinetic friction.

Ah I am starting to see now. The question intends to ask "what should P be if the work done by you and the work done by kinetic friction cancel each other out?" So yeah you're right - almost. You can't set work equal to force because they are not of the same unit. Set your horizontal component of P equal to the kinetic friction experienced because if both forces cancel each other out, no work can be done in moving the box even if it's moving at a constant speed.

 

[rocky811]

Thanks! That's exactly what someone said when I asked them about it today and it worked.