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Experiments on a rigid object

Although the significance of twist mightn't be an obvious part of these experiments, when you come to analyse them twist is vital.
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We’ve seen why we can’t analyse this simple suspended system using what we’ve learned already, so how can we do it? These experiments will help us find the answer. From week two, we’ll need our force transducers, the load pans and washers, and the ruler. We’ve hung the transducers from the support beam, resting it on chairs as before. For our rigid object, we’ll use this cardboard rectangle. You saw it in the introduction. We chose thick cardboard, corrugated is good. We drew a centreline on it and made various holes on the top, connecting it to the force transducers, and holes in the bottom for hanging a weight pan from. We made it as symmetrical as we could. And we made hooks with paperclips.
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Download the details if you want to do this yourself. Here’s how we set it up. We suspended the cardboard shape from the force transducers, positioning them so they are vertical, and then we adjusted the string as necessary until the top of the cardboard was horizontal. Each hole is 70 millimetres from the centreline, that looks about right. Now we are ready for the measurements. First, we’ll weigh the cardboard shape by finding the force in each force transducer in the usual way and recording the result. That’s the first experiment done. For the second experiment, we’ll hang a loaded pan from the centre bottom hole and repeat the measurements.
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For the final experiment with this set up, we’ll move the loaded pan and weights to the offset hole. This hole is 40 millimetres from the centreline. The cardboard rotated, did you expect that? Anyway, we’ll adjust it to be square again, then measure and record the forces. That’s the third and final experiment in this set. Next we’ll look at generating a pure torque.
Trust us. Although the significance of twist mightn’t be an obvious part of these experiments, when you come to analyse them twist is vital.
If you have already made the force transducers you’ve got most of the kit you need, and the experiments won’t take long. If you haven’t made the rubber band transducers, it’s not too late. Why not make them now.
You’ll get a physical appreciation of the importance of considering twist. And you’ll be ready for the next experiments.
You can download instructions to the experiment in the Downloads section below.

Talking points

  • What would happen if the cardboard wasn’t square when the measurements were made?

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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