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Statics For Dummies by James H. Allen, III, PE, PhD

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Chapter 12. Special Occasions in the Life of a Force Vector: Moments and Couples

In earlier chapters, I discuss the concept of forces and explain how a force (such as a bat) pushing on an object (such as a baseball) causes that object to move in the direction of the applied force (known as translating). However, not all actions cause an object to move or translate; some cause rotation.

In this chapter, I illustrate the rotational behaviors, including moments, couples, and concentrated moments, of objects and present the equations that let you calculate the behaviors that cause these rotational effects.

I Need a Moment: Exploring Rotation and Moments of Force

Think about a pinwheel. By blowing on the pinwheel, you're actually applying a force to it. Unlike the examples in previous chapters, the pinwheel doesn't move in the direction of the force because it's tied to a stick. But it does spin or rotate. The pinwheel stays in place, but it spins. The harder you blow (the more force you apply) to a pinwheel, the faster it spins.

Rotational behaviors can also occur in objects that are translating. Rolling is a combination of both linear motion (or translation) and rotation. The old tumblin' tumbleweeds that you see in classic Westerns are a great example. These dried plants move along the dusty countryside (translation), ...

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