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Skip to 0 minutes and 9 secondsThis system of links will apply a pure twist to the larger cardboard shape. Pure twist means twist without a resultant force. Download the drawings of the links if you want to do the experiments yourself. We cut these shapes from corrugated cardboard, with the corrugations running along those shapes. As shown in the drawings, we made holes in the links, positioning them as precisely as possible. The next part was fiddly. We laid the links out on the table and connected them with string, like this. Then, we suspended the complete linkage assembly from the support beam like this, using the force transducers on the larger shape.

Skip to 0 minutes and 58 secondsWe bent three hooks out of identical paper clips and installed one in each of the two holes in the main shape under each transducer, and one in the middle of the lowest link, like this. Now for the experiment.

Skip to 1 minute and 15 secondsWe'll load each of the hooks on the main shape with the same number of washers. We'll use 15 on each hook. At this point, we'll check that the force gauges are vertical and the larger shape is horizontal. Now, we'll adjust the links of the top mechanism to make them roughly horizontal. It shouldn't be too critical, but do the best you can. This is our datum condition. We'll measure the height from the top of the main shape to the bottom of the support beam and record it. Next, we'll load the bottom link of the twist mechanism. We'll use 10 washers. This applies a pure twist to the main shape. You can see it rotate. It's what we expected.

Skip to 2 minutes and 7 secondsWe'll measure the height again from the top of the main link, halfway between the transducers to the bottom of the support beam. It's not changed. The next part is neat. Take five washers off the right-hand hook on the main shape and put them on the left-hand hook of the main shape. Look at that. We're back to our datum condition. The shape is horizontal again, and the height is as before.

Experiment: Pure twist

If you use a spanner to undo a nut, the twist you generate comes from the force you apply.

The twist and the force come as a package.

If you use a screwdriver to undo a screw, the twisting effect comes without a net force (apart from the force along the axis to keep the blade in place, but that’s another story).

Two parallel, equal, opposite forces generate twist without unbalanced force. This is a ‘couple’ or ‘couple moment’. It has many applications in engineering mechanics.

In this experiment you’ll build a system out of cardboard and thread that generates a pure couple. You can use the force transducers to check it out.

(I think the mechanism is elegant, see what you think. RF)

If you attempt the experiment, take a photo and upload it to our Through Engineers’ Eyes Padlet wall. You can include a link to your photo in the comments for this step (click on your post on the Padlet wall and then copy the web address).

If you don’t have access to a camera, we encourage you to still do the activity and write a description of your experiment on the Padlet wall.

For a guide to using Padlet, please see FutureLearn’s social media help page.

You can download instructions to the experiment in the Downloads section below.

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This video is from the free online course:

Through Engineers' Eyes: Engineering Mechanics by Experiment, Analysis and Design

UNSW Sydney