posted by Rosie .
A man pushes a 50N box across a level floor at a constant speed of 2.0 m/s for 10 s. Whats the average power output by the man.
P = F*V = 50 * 2 m/s = 100 Joules / s.
Think of Newton's first law:
Without the influence of external forces, every object continues its stationary state or motion at a uniform velocity.
If there is no friction between the box and the floor (think of a slippery floor like on ice), then the man did no work over the 10 seconds, so the average power is zero.
If there is friction, with the coefficient of kinetic friction equal to μ, then the frictional resistance (force) is μmg, and the work done per second is μmg (N)*2 (m/s) = 100 μ g j/s