Question

A pendulum of mass 18 kg is released from rest at some height, as shown by

Answer 1

By the work-energy theorem, the total work done on the mass as it swings is

W = ∆K = 1/2 (18 kg) (17 m/s)² = 153 J

No work is done by the tension in the string, since it's directed perpendicular to the mass at every point in the arc. Similarly, the component of the mass's weight mg pointing perpendicular to the arc also performs no work.

If we ignore friction/drag for the moment, the only remaining force is the parallel component of weight, which performs mgh = (176.4 N) h of work, where h is the vertical distance between points A and B.

Now, if w is the amount of work done by friction/air resistance, then

(176.4 N) h - w = 153 J

If you know the starting height h, then you can solve for w.