6-44. In a biceps curl, a weightlifter lifts a mass
of 15 kg a vertical distance of 66 cm.
(a) How much work is done by gravity on the mass?
(b) Neglecting any accelerations, how much work
is done by the weightlifter on the mass?
(c) By how much does the gravitational potential
energy of the mass increase?
The work done by gravity on the mass is:
(15 kg)(9.8 m/s2
)(0.66 m) = 97 J
(15 kg)(-9.8 m/s2)(0.66 m) = -97 J
(C) No work is
done since the net force acting on the mass is zero.