3.1

## UNSW Sydney

Skip to 0 minutes and 9 secondsDo you see how this folding washing line works? When it's raised, a link engages in each end of the frame and props it up. How strong does the pin have to be for each connecting link? That will be your design task. First, you need to work out the loads. There's the weight of the frame itself, the weight of the wet washing, and maybe the weight of a child swinging on the frame. It might happen. Once you have decided on the loads, you will use equilibrium to find the forces in the pins. It's made up of rigid bodies, and you'll need new skills to analyse it. You'll need a new equation.

Skip to 0 minutes and 58 secondsLet's see why you can't do it with what you've learned already. Here's a sheet of cardboard with a vertical centre line marked on it and a hole on the centre line to hang it from. Its weight is 0.2 Newtons. Its centre of gravity, where the weight acts, is in the middle of the sheet, because of symmetry. First, we'll suspend it from the centre hole. Here's a question for you. What's the tension in the string? Have you got it? The tension in the string is the weight of the cardboard. That's 0.2 Newtons, because of equilibrium. Now, we'll hang it from two lengths of string. The strings are vertical, and each is 70 millimeters from the centre line.

Skip to 1 minute and 53 secondsWhat's the tension in the string on your left? Did you get it? The answer is half the weight. Half of 0.2 Newtons. That's 0.1 Newton, of course. Each string takes half the load by symmetry. Now, we'll hang a 0.6 Newton weight on the centre line. What's the tension in each string now? The extra weight is shared equally between the two strings as before, because of symmetry. So the total tension in each string is 0.1 Newtons plus half of 0.6 Newtons. Giving 0.4 Newtons per string. Now, we'll move the weight off centre by 80 millimetres. And the card is now hanging at an angle. What to can engineering intuition tell us about the tensions?

Skip to 2 minutes and 55 secondsIt might tell us that the string on your left takes a bigger share of the load. It might tell us that the total of the two tensions equals the total weight, because of equilibrium. But it can't tell us what each tension is. We'll need to know about the twisting effect of a force to work that out. Once you can handle twisting effects, you'll be ready to apply equilibrium, Newton's first law, to a rigid object. You'll learn how to draw complicated free-body diagrams, which is a real highlight of this course.

Skip to 3 minutes and 37 secondsAll of this will be in two dimensions. Luckily, a lot of object in engineering can be treated as 2D.

Skip to 3 minutes and 48 secondsYou'll use all this to design the pins for the wall bracket. As well as applying equilibrium, you'll learn something about shear stress in bolts.

Skip to 4 minutes and 2 secondsNow, you're ready for the experiments.

# It's so simple, yet we can't analyse it!

Suppose forces on an object aren’t acting at a point, and you can’t use sliding vectors or other tricks to convert them.

How can you apply equilibrium to find the forces you need for design?

We’re into the realm of the ‘rigid body’.

Sometimes you can find what you need from symmetry. But for most cases you’ll need something more. You’ll need to consider the twisting effects of forces.

This video introduces these concepts, and shows you why you need them for design.