# Work/Energy Question!

1. Jan 25, 2006

### neoking77

A pendulum bob is on a 2.2m long string. It is released from position A. Find its speed at position B.

W = 1/2m(V^2-Vi^2)
V = root 2W/m

the trouble is I cannot get W because i do not know the displacement. also mass is not given.

2. Jan 25, 2006

### finchie_88

The way that I would do this is to work out the time period of the oscillation using t = 2π√(l/g), and then use this to work out the velocity at point B by using v = w√(A² - x²). To work out A, use trig on point A, then use trig on point B to work out x.

I think that'll give you the right answer.

3. Jan 25, 2006

### andrevdh

The pendulum experiences two forces, its weight and the tension in the string. Since the tension in the string is always perpendicular to its displacement it does no work on the pendulum. The weight is a conservative force, therefore energy will be conserved and
$$U_A+T_A\ =\ U_B+T_B$$
where $U$ is the potential energy of the system (earth-pendulum) and $T$ is the kinetic energy of the pendulum.