3.2

UNSW Sydney

Skip to 0 minutes and 9 secondsWe've made a balance. We'll use it to weigh things. And we'll use it to explore rotational equilibrium. It's easy to make. Take a rectangle of stiff cardboard, and draw diagonal lines like this. Make holes on these lines in each lower corner. The distances from the corners should be as similar as possible. Draw a vertical centre line too, as accurately as you can. Make a hole for a drawing pin, or thumb tack, exactly on the centre line, and exactly half way up from the bottom of the card. And pin the card to a support, such as the end of the support beam from last week, making sure that it is free to rotate.

Skip to 1 minute and 5 secondsBend two hooks from identical paper clips, and install them like this. Save a few more of the identical paper clips to use later. Next, install a plumb line, like this. A plumb line is a weight on the end of a length of string.

Skip to 1 minute and 27 secondsFor the experiments, we'll also need some identical weights. The steel washers from week two are good because you can easily hang them on the hooks.

Skip to 1 minute and 39 secondsHere's the first experiment. It's simple. Weigh the paper clips to find out how many equal one washer. To do this, hang a washer on one of the hooks, and see how many paper clips you need to add to the other hook to make the card hang square. You can check whether it's square with the plumb line and the centre line. For better precision, you could use two or more washers. When you're done, record your results.

Skip to 2 minutes and 17 secondsNow for the second experiment. We'll explore balance. First, we'll put the same number of washers on each hook. The card hangs square.

Skip to 2 minutes and 33 secondsThat's our introduction to the twisting effect of a force, the tendency to rotate. One weight tries to rotate the card clockwise, the other weight tries to rotate the card counterclockwise, and the two effects cancel. We've got rotational equilibrium.

Skip to 2 minutes and 57 secondsNow we'll put a different number of washers on each side and measure the horizontal distance from each hole to the plumb line. Record the distances and the number of washers. There's a relationship between these distances and the number of washers. You'll find out all about it in the first analysis video. You could watch that video now, or you could do the next experiment. The next experiments are on a rigid object.

Experiment: Balancing out twisting effects

This really simple experiment shows how twisting effects are generated and how they balance out.

It’s another example of equilibrium - simple, elegant and surprisingly accurate.

If you haven’t given the other experiments a go, why not try these?

Talking points

The balancing experiment has some hidden features.

• The cardboard is pinned in its centre. What influence does its weight have on the balance condition?
• How does the vertical distance between the centre hole and the holes for the hangers influence the sensitivity of the system?
• What do you think of the precision that can be obtained from this simple balance?