4.2

## UNSW Sydney

Skip to 0 minutes and 10 secondsHere are some cardboard shapes. We're going to locate their centers of gravity. There's a square and a triangle, and a combination of the two, all made from some stiff cardboard. We'll need the wooden beam again, and the plumb bob. Here's what we'll do. Using a drawing pin, we'll suspend the cardboard from the beam by a corner. It's important that the card is free to swing. Now we will suspend the plumb line from the drawing pin, too. We used it in week three. And we'll mark where the plumb line crosses the lowest part of the card. We'll repeat for another the corner and somewhere near the middle of a side.

Skip to 1 minute and 7 secondsNow we'll take the cardboard off the pin and draw a line from each hole to the corresponding mark, like this.

Skip to 1 minute and 21 secondsLook at what we've got. The center of gravity is somewhere near where the lines cross. Sometimes there's a small triangle where the lines cross. It depends on how precise you've been.

Skip to 1 minute and 37 secondsAnd finally, we'll see what happens if the cardboard isn't free to rotate. We'll push the pin in until it grips the cardboard. As you can see, the cardboard will stick at various angles-- no good at all. That's the suspension method. Next, we'll use a balancing method.

# Experiment: Finding centres of gravity by suspension

These simple, quick experiments will give us plenty to analyse later.

### Talking points

• In the video, did you see the green shape with a hole in it (0:15 seconds)? Shapes like this will re-appear later. What would you do if the centre of gravity ended up in the hole?

If you attempt the experiment, take a photo and upload it to our Through Engineers’ Eyes Padlet wall. You can include a link to your photo in the comments for this step (click on your post on the Padlet wall and then copy the web address).