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Energy Chapter 4

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Work Work is done on an object when a force moves an object through a distance Work = force x distance Work is measured in Joules (Newton-meter) Examples: Pushing a swing Lifting a box Carrying a box – no work is done! If 90° angle, between force and motion

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problems You push a refrigerator with a horizontal force of 100 N. If you move the refrigerator a distance of 5 m with pushing, how much work do you do? Work = force x distance W = fd W = (100 N)(5 m) = 500 J

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problems A couch is pushed with a horizontal force of 80 N and moves a distance of 5 m across the floor. How much work is done on the couch? Work = force x distance W = fd W = (80 N)(5 m) = 400 J

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problems How much work do you do when you lift a 100 N child 0.5 m? Work = force x distance W = fd W = (100 N)(0.5 m) = 50 J

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problems The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 40 m while the brakes are being applied, how large is the average force that the brakes exert on the car? W = fd F = W/d F = (240,000 J)/(40 m) = 6000 J

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**Energy Is defined as the ability to do work or cause a change**

Work is the transfer of energy Measured in Joules System: anything around which you can imagine a boundary

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**Types of energy Kinetic energy Energy of motion**

Depends upon mass and velocity Kinetic energy (in J) = ½ mass (in kg) x [velocity (in m/s)]2 KE = ½ mass x velocity2 = mass x velocity2 2 KE ↑ as mass ↑ KE ↑ as velocity ↑

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**Potential Stored energy It has the potential to do work**

Elastic PE – Energy that is stored by compressing or stretching an object Chemical PE – Energy due to chemical bonds Gravitational PE – depends on height Mass (kg) x gravity (N/kg) x height (m) GPE = mgh where g = 9.8 N/kg

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Problems A jogger moving forward with a mass of 60.0 kg is moving forward at a speed of 3.0 m/s. What is the jogger’s kinetic energy from this forward motion? KE = ½ (mass x velocity2 ) KE = ½ (60.0 kg) (3.0 m/s)2 KE = ½ (60.0 kg)(9.0 m2/s2) = 270 J

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Problems A baseball with a mass of 0.15 kg is moving at a speed of 40 m/s. What is the baseball’s kinetic energy from this motion? KE = ½ (mass x velocity2 ) KE = ½ (0.15 kg) (40.0 m/s)2 KE = ½ (0.15 kg)(1600 m2/s2) = 120 J

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problems A 4.00 kg ceiling fan placed 0.25 m above floor. What is the gravitational potential energy of the Earth-ceiling fan system relative to the floor? GPE = mass x gravity x height GPE = mgh GPE = (4.00 kg)(9.8 N/kg)(0.25 m) = 9.8 N•m = 9.8 J

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problems An 8.0 kg history textbook placed on 1.25 m high desk. How large is the gravitational potential energy of the textbook-Earth system relative to the floor? GPE = mass x gravity x height GPE = mgh GPE = (8.0 kg)(9.8 N/kg)(1..25 m) = 98 N•m = 98 J

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**Conservation of energy**

Energy cannot be created or destroyed. Energy can only be converted from one form to another or transferred from one place to another. Mechanical energy is the sum of the kinetic and potential energy of the objects in a system Example: running water, swing Swing

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