Refraction occurs when a wave
a.
enters a new medium at an angle.
b.
hits a surface through which it cannot pass.
c.
moves around a barrier.
d.
interferes with another wave.

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.

we need to know the time interval during which the force acts. impulse of force = momentum change