Author: admin
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The Yo-Yo
A yo-yo is a physics lab that you can fit in your pocket. If a yo-yo rolls down its string for a distance h, it loses potential energy in amount mgh but gains kinetic energy in both translational and rotational forms. As it climbs back up, it loses kinetic energy and regains potential energy. In a modern yo-yo, the string…
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The Forces of Rolling
Friction and Rolling If a wheel rolls at constant speed, as in Fig. 11-3, it has no tendency to slide at the point of contact P, and thus no frictional force acts there. However, if a net force acts on the rolling wheel to speed it up or to slow it, then that net force causes acceleration of…
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The Kinetic Energy of Rolling
Let us now calculate the kinetic energy of the rolling wheel as measured by the stationary observer. If we view the rolling as pure rotation about an axis through P in Fig. 11-6, then from Eq. 10-34 we have in which ω is the angular speed of the wheel and IP is the rotational inertia of the wheel about the axis through P. From the…
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Rolling as Translation and Rotation Combined
Here we consider only objects that roll smoothly along a surface; that is, the objects roll without slipping or bouncing on the surface. Figure 11-2 shows how complicated smooth rolling motion can be: Although the center of the object moves in a straight line parallel to the surface, a point on the rim certainly does not. However, we can…
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What Is Physics?
As we discussed in Chapter 10, physics includes the study of rotation. Arguably, the most important application of that physics is in the rolling motion of wheels and wheel-like objects. This applied physics has long been used. For example, when the prehistoric people of Easter Island moved their gigantic stone statues from the quarry and across…
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Work and Rotational Kinetic Energy
As we discussed in Chapter 7, when a force F causes a rigid body of mass m to accelerate along a coordinate axis, the force does work W on the body. Thus, the body’s kinetic energy can change. Suppose it is the only energy of the body that changes. Then we relate the change ΔK in kinetic energy to the work W with the work–kinetic energy…
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Newton’s Second Law for Rotation
A torque can cause rotation of a rigid body, as when you use a torque to rotate a door. Here we want to relate the net torque τnet on a rigid body to the angular acceleration α that torque causes about a rotation axis. We do so by analogy with Newton’s second law (Fnet = ma) for the acceleration a of a body…
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Torque
A doorknob is located as far as possible from the door’s hinge line for a good reason. If you want to open a heavy door, you must certainly apply a force; that alone, however, is not enough. Where you apply that force and in what direction you push are also important. If you apply your…
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Calculating the Rotational Inertia
If a rigid body consists of a few particles, we can calculate its rotational inertia about a given rotation axis with Eq. 10-33 that is, we can find the product mr2 for each particle and then sum the products. (Recall that r is the perpendicular distance a particle is from the given rotation axis.) If a rigid body consists of a…
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Kinetic Energy of Rotation
The rapidly rotating blade of a table saw certainly has kinetic energy due to that rotation. How can we express the energy? We cannot apply the familiar formula to the saw as a whole because that would give us the kinetic energy only of the saw’s center of mass, which is zero. Instead, we shall treat…