EXAMPLE 1 (a) How much work is done in lifting a 1.3-kg book off the floor to put it on a desk that is 0.7 m high? Use the fact that the acceleration due to gravity is g = 9.8 m/s2. (b) How much work is done in lifting a 25-lb weight 4 ft off the ground? SOLUTION (a) The force exerted is equal and opposite to that exerted by gravity, so the force is F = m J'S = mg = (1.3) (9.8) = N dt 2 and then the work done is W = Fd = (0.7 ) = (b) Here the force is given as F = 25 lb, so the work done is W = Fd = 25 . 4 = ft-lb. Notice that in part (b), unlike part (a), we did not have to multiply by g because we were given the weight (which is a force) and not the mass of the object