3.17

## UNSW Sydney

Skip to 0 minutes and 10 secondsNow 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

This short video reflects on what you have encountered and what is to come. The concept wheel below shows how it all fits in.

Week 3 concept wheel (Click to expand)

In week 3 you saw how to analyse rigid bodies that are ‘planar’ - that is, two-dimensional, flat. You can often solve three dimensional problems with two dimensional models.

But first you needed to understand twisting effects. We used various terms - moments, couples and torque. Did you find this a tricky concept? Often, when learners have studied some of this work before, it begins to feel new at about this stage.

FBDs are even more important with rigid bodies, and the interactions you must represent are more complicated. Luckily there are standard methods for this - ‘conventional interactions’.

And so, you used equilibrium to find forces in planar rigid bodies. This is the basic capability that part 1 offers. But we went further. By calling upon Newton’s 3rd law you extended this capacity to ‘two dimensional statics of systems of rigid bodies’ - in your case a folding washing line.

That’s it. Part 1. You can now find unknown forces in systems of two-dimensional rigid bodies.

It’s time to celebrate.

### Talking points

These are to get you started. Share any other thoughts you’ve had.

• What concepts and techniques do you need if you want to find unknown forces on a rigid body?