My first thoughts would be the sea…but then most peoples would be too and that doesn’t necessarily win competitions. Whatever you decide on if it is a moving object (wind, sea, car) I’d suggest leaving the shutter speed to a level that can convey the power. Other interpretations could be power of people so with the current unsettled climate a parade with obvious aggression in faces might work. You could look at arcing electricity – dangerous, I know but there are means. One of those plasma globes might give you a sense of power without having to stick your finger in a socket to show it.
You could approach a gym – if there are posers with arms like legs they’ll only be to happy to pose for a picture. Just a few thoughts. Now, where’s the competition (didn’t think I was telling you everything, did you??)
Just go out to Finglas one night, leave a scooter in the middle of the road with running engine and the key in it, hide in a nearby bush and wait… you need a telephoto lens and good reflexes ;)