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# Week 3 Introduction: the suprising usefulness of twist

Adding forces that act at a point can only take you so far.
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SPEAKER: This week is all about twist and rotation. We’ll concentrate on the statics, but we’re going to start with some dynamics. Here’s a home video. The spin stabilises the flying ring gyroscopically. It’s all done with a flick of the wrist. Here is another way to generate spin. Folded pieces of paper link a doubled length of string, which wraps around the outside of a cardboard ring. One strand of the doubled string on each side of the ring. The other end of the string is tied onto a strand of rubber. When it’s loaded, you hold the ring and string with thumb and forefinger, and stretch the rubber. I find it helps to have a stick to help extend the rubber.
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When you let it go, it translates and it spins. The launch goes too fast for my camera to catch it, so here’s an animation. If you apply the force through the centre of the cardboard ring, there would be no spin. If the force is offset, it does spin. The force generates a twisting effect. That is what we are going to explore. We have many words related to twist– twist, rotation, torsion, torque, moment, couple. Here are some typical uses of these words. Twist-top bottle, torsion bar suspension, output torque, equivalent force plus couple, moment due to a force, axis of rotation. We will discover two categories of twist. Twisting effect of a single force about an axis.
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For example, a spanner on a nut. Twisting effect with no net force. For example, a screw driver on a screw or the output of a motor. Here, you’ll start with an experiment. It’s really simple. Why not give it a go?

Adding forces that act at a point can only take you so far.

To complete the story, you need to know about twisting effects. We will investigate how twist is generated, how to calculate it, and some of its applications.