chap10_11

chap10_11 - Energy Work Forms of Energy Conservation of...

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Unformatted text preview: Energy Work Forms of Energy Conservation of Energy Gravitational & Elastic Potential Energy Work - Energy Theorem Conservation of Momentum & Energy Power Simple Machines Mechanical Advantage Work The simplest definition for the amount of work a force does on an object is magnitude of the force times the distance over which it’s applied: W = F x This formula applies when: • the force is constant • the force is in the same direction as the displacement of the object F x Work Example Tofu Almond Crunch 50 N 10 m The work this applied force does is independent of the presence of any other forces, such as friction. It’s also independent of the mass. A 50 N horizontal force is applied to a 15 kg crate of granola bars over a distance of 10 m. The amount of work this force does is W = 50 N · 10 m = 500 N · m The SI unit of work is the Newton · meter. There is a shortcut for this unit called the Joule, J. 1 Joule = 1 Newton · meter, so we can say that the this applied force did 500 J of work on the crate. Negative Work 7 m f k = 20 N A force that acts opposite to the direction of motion of an object does negative work. Suppose the crate of granola bars skids across the floor until friction brings it to a stop. The displacement is to the right, but the force of friction is to the left. Therefore, the amount of work friction does is -140 J. Friction doesn’t always do negative work. When you walk, for example, the friction force is in the same direction as your motion, so it does positive work in this case. v Tofu Almond Crunch When zero work is done 7 m N mg As the crate slides horizontally, the normal force and weight do no work at all, because they are perpendicular to the displacement. If the granola bar were moving vertically, such as in an elevator, then they each force would be doing work. Moving up in an elevator the normal force would do positive work, and the weight would do negative work. Another case when zero work is done is when the displacement is zero. Think about a weight lifter holding a 200 lb barbell over her head. Even though the force applied is 200 lb, and work was done in getting over her head, no work is done just holding it over her head. Tofu Almond Crunch Net Work The net work done on an object is the sum of all the work done on it by the individual forces acting on it. Work is a scalar, so we can simply add work up. The applied force does +200 J of work; friction does -80 J of work; and the normal force and weight do zero work. So, W net = 200 J - 80 J + 0 + 0 = 120 J F A = 50 N 4 m f k = 20 N N mg Note that ( F net ) ( distance ) = (30 N) (4 m) = 120 J. Therefore, W net = F net x Tofu Almond Crunch Tofu Almond Crunch When the force is at an angle x F θ When a force acts in a direction that is not in line with the displacement, only part of the force does work. The component of F that is parallel to the displacement does work, but the perpendicular component of F does zero work. So, a more general formula for work is W = F x cos θ F cos θ F sin θ This formula assumes that F is constant. Work: Incline Example...
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chap10_11 - Energy Work Forms of Energy Conservation of...

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