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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.
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## UNSW Sydney

Skip to 0 minutes and 10 seconds Now that we can draw free-body diagrams of rigid objects - not just particles - things have become more realistic. To make this happen, we needed to introduce twisting effects. There are obvious twisting effects, like using a spanner to turn a nut or the twisting effect of a motor, which is known as torque. But less obviously, we can consider twisting effects anywhere on a rigid object by using our free-body diagram. If the object is in equilibrium, the total twisting effect everywhere on a rigid object must be zero. That gives us an extra equation. This way, we get equations for calculating forces. It’s a magic moment. In time, you will absorb it into your engineer’s world view.

# Through Engineers' Eyes

The concept wheel shows how the last four weeks fit together. The final short video reviews the whole four weeks. The chances are that you have developed engineers’ eyes.

Take your mind back to week two. We used equilibrium (and FBDs of course) to find unknown forces where forces met at a point. We had two equations. That’s useful, but we’ve seen that where forces don’t meet a point two equations aren’t enough, rigid bodies for example.

In this final week, twist gave us an extra equation - taking the sum of moments about a point (more precisely, an axis). This gave us what we needed for finding forces in planar rigid bodies (planar means flat - two-dimensional). And with Newton’s 3rd law we could analyse systems of rigid bodies, such as the washing line in the design task.

It’s ‘two dimensional’, but that needn’t be too much of a limit. With a bit of inventiveness you can often solve 3D problems with 2D analysis; the washing line was 3D, but that wasn’t a problem. You’ve just got to find the way to look at it. This powerful knowledge and skill you can gain from this course is at the heart of the engineering world view.