# rot2 - Physics 53 Rotational Motion 2 Right handers go over...

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Unformatted text preview: Physics 53 Rotational Motion 2 Right handers, go over there, left handers over here. The rest of you, come with me. — Yogi Berra Torque Motion of a rigid body, like motion of any system of particles, is changed by the effects of external forces. A rigid body's motion consists of motion of the CM plus rotation about the CM; external forces can change one or both of these parts of the motion. Shown is a rigid body, with an external force F applied to it at a point speciFed by the position vector r , relative to the CM as origin. At Frst we take the force vector F to lie in the plane of the drawing. This vector can be broken up into a component along r ( F r ) and a component perpendicular to r ( F ⊥ ). Acting alone, the radial component F r would make the CM accelerate to the right along the line of r , but it would not produce any rotation about the CM. Now suppose there is a Fxed axle passing through the body, perpendicular to the page and passing through the CM. Then normal forces exerted on the body by the axle would cancel the effect of F r and the body would not move at all. We will ignore this component in what follows. On the other hand, F ⊥ would make the body rotate about the axle. It is this rotational effect that we are interested in here. As the body turns counter-clockwise through angle d θ during time dt , the point at which F ⊥ is applied moves through distance ds = r d θ , so the work done by it is rF ⊥ d θ ....
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rot2 - Physics 53 Rotational Motion 2 Right handers go over...

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