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Lecture series from Conceptual Physics 8 th Ed.

This lecture series covers topics such as work, power, potential energy, kinetic energy, the work-energy theorem, conservation of energy, machines, efficiency, and the comparison of kinetic energy and momentum in Conceptual Physics.

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Lecture series from Conceptual Physics 8 th Ed.

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  1. Lecture series from Conceptual Physics 8th Ed.

  2. Work p101 If he lifts 100kg 2.0 meters off the floor, then Work = force x distance W = F d W = Fd =1000N x 2.0 m W = (N)x(m) = N-m W = 2000 N-m A N-m is a Joule. The unit of work is the Joule, J. W = F d W = F x 0 W = 0 J How many Joules of work is the convict doing? How many J of work is the person doing if she slides the crate 3.0 m? W = Fd = (75N)(3.0m) W = 225J

  3. Power p101 work done Power = time interval If a force of 50 N pushes an object 15 m in 5 seconds, how much work is done? W Fd 50 N x 15 m 150 W P = = = = t t 5 s Yes! Watt is power? A J/s is a Watt. There are also…kW and MW. And, 100 kW = 134 HP WATT??

  4. Mechanical Energy p 102 Potential energy Gravitational PE All of these are expressed in Joules of energy. Chemical PE GPE is in Joules. How much GPE does the 10 kg ball have when its 3 m above the ground? GPE = F x d = mg x h = mgh = (10)(9.8)(3) = 300J GPE and work are both energy, J. Elastic PE

  5. MOTION. The energy of Kinetic Energy p 105 KE = ½ mv2 What’s the energy of a 1000kg car moving at 20 m/s? KE = ½ mv2 = ½ (1000)(20)2 = 200,000J = 200kJ

  6. Work-Energy Theorem p105 When you throw a ball, you give it energy of motion. Throw is work is Fd. Energy of motion is KE is ½ mv2. SO Fd = ½ m(Δv)2 Work = change in KE How much work does it take to accelerate a 500kg car from 25 m/s to 50 m/s? W = Δ KE = ½ m (Δ v)2 = ½(500)(50-25)2 = 156kJ

  7. Conservation of Energy p106 Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.

  8. Machines p108 A lever is a simple machine. The work done on one side equals the work done on the other. A lever is a FORCE MULTIPLIER!

  9. Five ropes around the pulleys. One rope is pulled. Rope is pulled 50cm Mass is lifted 10 cm 100 N of force on this side. 500 N of force on this side. (F x d)left = (F x d)right (100 N) (50 cm) = (500 N) (10 cm) Pulleys change the direction of force. Pulley systems multiply force.

  10. Efficiency p 109 40,000,000J/L W W = F / d = 80 km/L d = = F 500 N J N-m 1 m L x = = L N N L

  11. Efficiency (cont.) Useful energy output Efficiency = Total energy input What if our buddy pulled with a force of 110 N for 1.0 m and the weight of 500N was lifted 0.20 m. (Fx d)in = (F x d)out output input (F x d)out (500)(0.2) Efficiency = = = 0.91 (F x d)in (110)(1.0)

  12. Comparison of Kinetic Energy and Momentum p110 Metal bullet delivers its momentum as it penetrates wood. A little wobble as momentum is transferred to block Rubber bullet delivers its momentum. Then, it does it again as it bounces. Twice as much wobble because of bounce. If the speed of the metal bullet is doubled, then it would cause the same wobble as the rubber bullet above. But, doubling the speed would increase the KE by four times. With four times the damage. Momentum and Kinetic Energy are not the same thing.

  13. This time dart bounces Note needle in nose so that it doesn’t bounce Wood is not damaged by impact. KE drives needle into wood causing damage. The End

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