Problem 7-43 Work, grav. PE - Part 2 - B

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

FBD A with forces, directions and weight indicated.

(A)  \(F_gd = (15 \;kg)(-9.8\; m/s^2)(0.66\; m) = -97\; J\)


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)  \(F_{app} = 0\)