Measuring the velocity of a pendulum using the width of the bob I did an experiment on pendulum motion and in the method it says use the following equation to measure the velocity of the pendulum:
$$v=\frac{W}{t}$$
where $W$ is the width of the pendulum bob and t is the time.
Why is this? Is it correct?
Edit: the time is for the bob to cross the photogate.
 A: Note that $W=\Delta x$ and that $t$ is the time required for the bob to pass to lowest position (I presume you have a photogate set up?). So by dividing the width of the bob and the time it takes to cross the sensor, you have the average velocity, $v=\Delta x/\Delta t$, at the point of the sensor (presumably at the trough).
I have had students get less than 1% error on their lab using this approximation, so I would say yes it is correct.
A: Typically you see this in a case where the bob occludes light beam or some such. 
So what time is it that is measured? There are several options but the likely one here is the length of time during which the beam in blocked.
Now how far does the bob move in that time? Well, it moves from the point where it first blocks the beam to the point when it stops blocking the beam, which is to say the width on the bob in the direction of motion.
If these two things are true than the (average) velocity of the bob while the sensor is blocked is, by definition the distance traveled of the time it takes to travel it, no? More over, if the width of the bob represents a small angular range in the swing you can take this to be the instantaneous veolocity with very little error.
