Exploring Periods in a Pendulum Lab with Logger Pro

[8parks11]

I'm doing a Pedelum Periods lab to measure the period in different amplitude,length, and bob mass. There is a program called loggerpro that most people might know and some might not Anyways the photogate measures the period but the question is


1) Why is Logger Pro set up to report the time between every other blocking of the Photogate? why not the time between every block?


Please I need help in this one problem..
My first answer was random spread and my second guess was that it would be because every swing had the same constant period.

Can someone explain me the correct path (or answer?)

 

[Office_Shredder]

I'm not sure what exactly you're doing, but I think I can guess.

You have a pendulum swinging. At some point on its path, it blocks a light source, and a program records that this source has been blocked. You want to know why, in measuring the period, it only takes into account every other block.

The period is the time it takes for the pendulum to return to its starting conditions. This means both initial position, AND initial velocity (otherwise it hasn't begun to repeat its cycle yet). Take the pendulum by hand, and start moving it. You can see the first time it blocks the gate, the velocity is moving in the opposite direction

 

[8parks11]

I think that's pretty much correct. Thanks

 

[andrevdh]

Another way of looking at the problem is that the oscillating motion creates a sine graph of displacement versus time. To obtain the period one need to measure the time between two points that have the same phase on the graph. This means measuring the time lapse between two points in the motion where the displacement and the direction of motion is the same. Since it is generally not possible to get the position of the photogate exactly at the equilibrium of the motion we take the elapsed time between crossings in the same direction, otherwise one would get a sort of half period (a little under or over) value.