A student pulls a box of books on a smooth horizontal floor with a force of 100 N in a
direction 37.0 degrees above the horizontal. If the mass of the box and the books is 40.0 kg, what is the normal force on the box?
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The normal force is the force applied by the floor to the box due to the force the box applies on the floor (Newton's third law).
The force the box applies to the floor is the weight of the box:
This solution helps with a problem regarding static and kinetic friction.