Swing...
Well, my favorite pendulum is the playground swing. Go out on a sunny day and measure the chain and then time the period (how many times does the swinging student come to the front in a minute?). Then, try shortening the chain if you can (throw it over the bar if you've got a ladder or someone tall enough to unwrap it when you are done). Does the period change (yes, it's faster). Put a heavy student (well, don't hurt anyone's feelings) and put a light student in the swing. Does that change the period? No. Have one student swing really high and one sort of low (barely swinging). Again, the period doesn't change. Have two swingers next to each swing at the same time. Does one student "get ahead" in swinging compared to the other? It's because the "faster" swing has a shorter chain, maybe even just a bit shorter.
Do a little research on why pumping legs work. It has something to do with the change in the center of gravity of the swinger and, by pushing the seat forward (ie. the chain takes a bend at the swinger's hands), it increases the period of the swing. That might be more than you are looking for.
While you are on the playground, swing student-made pendulums both vertically and horizontally. Play with different string lengths.
Oh, and this is always cool (or "cold"). Get a bucket of water (not too heavy as you don't want to hurt the kids) and tie a rope to it and demonstrate swinging it around. The water stays inside. Not really a pendulum, but it's similar.
For English, find poems or songs about swinging (the only one that comes to mind is Swing Low Sweet Chariot, but that's not what we're looking for!!). Write a poem about swinging or about why Sally's swing goes "faster" than Bobby's. Write about the feeling of flying that you feel at the top of the swing (right when you go from going up to going back down).
Also, have the kids brainstorm on fun pendulums (think about state fair and Six Flags style rides) that they like. I can think of the Pirate Ship ride (you'd never catch me on one) but perhaps they have some other ideas. Or, the students can write swinging chants (sort of like jump rope rhymes) to say to the rhythm of the swings.
