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p111_lecture11

# p111_lecture11 - Chapter 6 Work and Energy 6.1 Work Done by...

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Chapter 6 Work and Energy

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6.1 Work Done by a Constant Force Fs W = ( ) J joule 1 m N 1 = ! Work involves force and displacement .
6.1 Work Done by a Constant Force

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6.1 Work Done by a Constant Force ( ) s F W ! cos = 1 180 cos 0 90 cos 1 0 cos ! = = = o o o More general definition of the work done on an object, W , by a constant force, F , through a displacement, s , with an angle, θ , between F and s :
6.1 Work Done by a Constant Force Example 1 Pulling a Suitcase-on-Wheels Find the work done if the force is 45.0 N, the angle is 50.0 degrees, and the displacement is 75.0 m. ( ) ( ) [ ] ( ) J 2170 m 0 . 75 0 . 50 cos N 0 . 45 cos = = = o s F W !

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6.1 Work Done by a Constant Force ( ) Fs s F W = = 0 cos ( ) Fs s F W ! = = 180 cos A weightlifter doing positive and negative work on the barbell: Lifting phase --> positive work. Lowering phase --> negative work.
Example . Work done skateboarding. A skateboarder is coasting down a ramp and there are three forces acting on her: her weight W (675 N magnitude) , a frictional force f (125 N magnitude) that opposes her motion, and a normal force F N (612 N magnitude). Determine the net work done by the three forces when she coasts for a distance of 9.2 m. W = ( F cos θ ) s , work done by each force W = (675 cos 65 o )(9.2) = 2620 J W = (125 cos 180 o )(9.2) = - 1150 J W = (612 cos 90 o )(9.2) = 0 J The net work done by the three forces is: 2620 J - 1150 J +0 J = 1470 J

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6.1 Work Done by a Constant Force Example 3 Accelerating a Crate
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