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Experiment: Pure twist (Repeat)

If you use a spanner to undo a nut, the twist you generate comes from the force you apply.
SPEAKER 1: This 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 the 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.
We bent three hooks out of identical paperclips 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.
We will 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 will check that the force gauges are vertical and the larger shape is horizontal. Now we’ll adjust the links of the taut 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.
We’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 are back to our datum condition. The shape is horizontal again and the height is as before.

This experiment appeared in Week 3. We have repeated it here because we will use our new analytical skills to get a deeper understanding of the concept of a couple – that is, pure twist.

Remember, 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.

Talking points

  • Was it difficult to understand this the first time around?
  • Do you still think the mechanism is elegant, see what you think? (Robin Ford)

Share your experiment

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).

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Through Engineers' Eyes - Introducing the Vision: Engineering Mechanics by Experiment, Analysis and Design

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