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

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Presentation transcript:

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:  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

Review problem (continued)

 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?

Power  The rate at which work is performed  In other words…the rate at which energy is transferred

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

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

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?