When you pull a sled through the snow, why does only part of your force do work?
Only part of the force you apply to your sled will be transferred to kinetic energy.
This operation is not 100% efficient . You apply energy from your muscles to the sled. Friction between the runners and the snow will cause some of this energy to be converted to sound and thermal energy. After these deductions, the sled will transfer the rest of the applied energy to kinetic energy, which will cause the sled to move in the direction you are pulling.