A girl on roller skates accelerates at a rate of 2 m/sec/sec with a force of 100 N. What is her mass?

By the newtons Second Law:

F = ma

Solving for m:

m = F / a

m =  100 N / 2 m/s²

none, its a machine

the motion of a pendulum is a classic example of mechanical energy conservation. a pendulum consists of a mass (known as a bob ) attached by a string to a pivot point. as the pendulum moves it sweeps out a circular arc, moving back and forth in a periodic fashion.