Just A Moment

Think back to your childhood days at the playground, playing on the swings and the climbing frame. If you were anything like me, the see-saw will also feature heavily. I love that weightless feel when you get thrown into the air (although the pain in your bum when you come back down on the seat isn’t as great!).

What you need for a good game on the see-saw is a couple of fairly evenly matched people in terms of mass. It’s difficult to play with your mum, for example, as you are constantly up in the air, unable to make the see-saw go down. And the opposite is true with your little brother or sister, you’re stuck on the ground and the only way to carry on is to push on the floor as hard as you can, but you can’t escape the floor for long!

This is one of our first encounters with moments, a slightly strange name for a turning force. You can tell when you’re dealing with moments because there will be a fixed point and a straight bar that rests on the fixed point, called the pivot. Moments are sometimes also called torque forces, particularly when cars are involved.

The important word to look out for when dealing with a moment problem is  equilibrium. This means that the system is not moving, so we know instantly that the forces are balanced. Which gives us:

So looking a bit closer at moments, we’ll do a little experiment. Grab something heavy (a bottle of water or a can of soup will do) and stand up. Hold it down at your side, this should be easy, you can stay like that for ages. Then hold it out in front of you with your arm straight, this is a lot harder. Same can, it now seems to weigh more. This is because moments come into play, in this case, the moment around your shoulder.

So we can see that a moment is related to the weight of the object (which is normally given in Newtons) and the distance from the pivot. In fact, this relationship is very simple.

M = F x d

Moment  = Force x distance

Just one last moment of your time (groan). It’s important to remember that the distance you’re looking for is the shortest possible distance from the point where the force acts to the pivot, and this is always the perpendicular distance, so your force arrow and your distance measurement are at right angles to each other.