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Unformatted text preview: 1 Chapter 7 Work and Energy 2 71 Work Done by a Constant Force Work is the product of the magnitude of the displacement times the component of force parallel to the displacement. In the SI system, work is measured in joules. Work is a scalar quantity. [W] = (N)(m) = joules 3 Work Done by a Constant Force W = F d W = Fd cos 4 A B = AB cos Therefore we can write the work done by a constant force as the dot product of the force and displacement: W = F d = Fd cos Scalar Product of Two Vectors 5 Important Must specify (as with force) if work is done on the object or by the object. Must specify if work is done by one particular force or by the net force. If net force, you get net (or total) work. No net work done by the pulling force! Example A car is taking a turn. How much work does friction on the car? F friction d Friction is directed radially The displacement is tangent to the path There is no force parallel to the displacement! W = F friction d = 0 Example What is the work that the Earth does on the Moon? Force acting on the Moon: Gravitational Force W = 0 v F G perpendicular to displacement 6 Example 71 Work done on a create. A 50kg create is pulled 40 m along a horizontal floor by a constant force exerted by a person, F P = 100 N, which acts at a 37 o angle as shown in the figure. The floor is rough and exerts a friction force F fr = 50 N. Determine the work done by each force acting on the create, and the net work done on the create. 7 Work on a carried backpack. ( a ) Determine the work a hiker must do on a 15.0kg backpack to carry it up a hill of height h = 10.0 m, as shown in the figure. ( b ) Determine the work done by gravity on the backpack. ( c ) Determine the net work done on the backpack. For simplicity, assume the motion is smooth and at a constant velocity (i.e., a =0). 8...
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This note was uploaded on 04/23/2011 for the course JAPAN 7b taught by Professor Wallace during the Spring '11 term at University of California, Berkeley.
 Spring '11
 wallace

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