posted by Anonymous .
A boy pulls a sled of mass 5.0 kg with a rope that makes an 60.0° angle with respect to the
horizontal surface of a frozen pond. The boy pulls on the rope with a force of 10.0 N; and the
sled moves with constant velocity. What is the coefficient of friction between the sled and the
Constant velocity means that the forces add up to zero.
Do a force balance in the direction of motion. It says:
10.0 cos 60 = friction force = (M g - 10.0 sin 60)*Uk
Solve for Uk, the coefficient of (kinetic) friction.
The reason for the -10.0 sin 60 term is that the upward pull on the rope reduces the normal force of the sled onto the ice. That term gets subtracted from the sled's weight.