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?

ACCUMULATED SOLUTION

(a) *F*_{g}*d* = (15 kg)(-9.8 m/s^{2})(0.66
m) = -97 J

Correct!

Since *mg* and *d* are in opposite directions
then one of them is negative. If +*y* is up then the gravitational
force is -*mg*.

This is the answer to part (a).

The applied force is:

(A)*F*_{app}
= *mg*

(B)*F*_{app}
= *-mg*

(C)*F*_{app}
= 0