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Power and Efficiency

Power and Efficiency. And a review of the work-energy theorem. Warm-up. A bungee jumper jumps from a bridge. Explain the energy changes that occur during the jump, what work is being done, and what force(s) is/are doing the work as the jumper completes his jump. Review problem #1:.

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Power and Efficiency

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  1. Power and Efficiency And a review of the work-energy theorem

  2. Warm-up • A bungee jumper jumps from a bridge. Explain the energy changes that occur during the jump, what work is being done, and what force(s) is/are doing the work as the jumper completes his jump

  3. Review problem #1: • A block of wood of mass 1.50 kg is suspended from a fixed point by two light strings so that it is able to swing through the arc of a circle. A bullet of mass 25.0 g, travelling horizontally at 305 m·s-1 hits and embeds into the block. As a result of the impact, the block swings and rises to a maximum vertical height of h meters. • Draw a diagram! • Calculate the kinetic energy of the bullet before impact with the block

  4. Review problem (continued)

  5. A 1.5 kg block slides along a rough table and is brought to rest after travelling a distance of 2.4 m. The frictional force is assumed constant at 3.2 N. • What work is done by friction? • What was the initial velocity of the block?

  6. Power • The rate at which work is performed • In other words…the rate at which energy is transferred

  7. Power • Power can also be defined in terms of how quickly an object moves as a result of a force applied:

  8. Efficiency • The power supplied to a motor is not always the same as the power that is actually used by the motor. • The ratio of Power available to the Power used is a measure of the efficiency • The larger the percentage of available power that is used, the greater the efficiency

  9. Sample: • The motor of an elevator develops power at a rate of 2500 W. • At what constant speed can a 120 kg load be raised? • It was measured that the load was actually lifted at a speed of 1.5 m/s. What is the efficiency of the motor for this elevator?

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