Abox of mass 50 kg is pushed hard enough to set it in motion across a flat surface. then a 99-n pushing force is needed to keep the box moving at a constant velocity. what is the coefficient of kinetic friction between the box and the floor? 0.23 0.18 0.20

The box is kept in motion at constant velocity by a force of F=99 N. Constant velocity means there is no acceleration, so the resultant of the forces acting on the box is zero. Apart from the force F pushing the box, there is only another force acting on it in the horizontal direction: the frictional force  which acts in the opposite direction of the motion, so in the opposite direction of F.
Therefore, since the resultant of the two forces must be zero,

so


The frictional force can be rewritten as

where , . Re-arranging, we can solve this equation to find , the coefficient of dynamic friction:

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