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This content is taken from the UNSW Sydney's online course, Through Engineers' Eyes - Introducing the Vision: Engineering Mechanics by Experiment, Analysis and Design. Join the course to learn more.
4.1

## UNSW Sydney

Skip to 0 minutes and 9 seconds This is the most important week in the course– how to apply equilibrium in two dimensions, a powerful capability. So far, we’ve analysed forces that act at a point. Now we’ll analyse forces that act anywhere on a two-dimensional rigid body. That’s one you can represent on a sheet of paper. In this week’s design task, we’ll choose the bolts that connect the parts of a folding washing line like this one. Here’s an FBD we’ll use. It’s very different from the loudspeaker task in week 2. To see why, we’ll look at a simpler rigid body– this sheet of cardboard. We’ll hang a weight pan on the centreline. Last week, we saw how to find forces that act at a point, like this.

Skip to 0 minutes and 57 seconds But what if the strings don’t meet in a point? We’ll try hanging the cardboard from two parallel lengths of string. Parallel lines never meet. Each string is 70 millimetres from the centreline. Because it’s symmetrical, the combined weight acts on the centreline of the sheet. What’s the tension in the string on your left? Did you get it? From equilibrium, the two strings share the total load. And because it’s symmetrical, the tension in each string is the same. So each string takes half the load. Recognising symmetry can make problems much simpler. But our washing line FBD isn’t symmetrical. To represent this, we’ll move the load on our cardboard off-centre so it’s not symmetrical anymore.

Skip to 2 minutes and 34 seconds Luckily, many objects in engineering can be treated as 2D even though they might seem like 3D. You’ll see this in the design task. It’s time for the most important week in the course.

# 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.