SC PHYS 201 - Power (4 pages)

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Lecture number:
Lecture Note
University Of South Carolina-Columbia
Phys 201 - General Physics I

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Phys 201 1nd Edition Lecture 15 Outline of Last Lecture I Non conservative work and energy with a spring Outline of Current Lecture II External vs Internal Work III Power A Power in terms of Speed IV Air Resistance V Linear Momentum A Impulse Current Lecture External vs Internal Work Remember that work can be positive or negative If work is positive then energy is going into a system If work is negative then energy is going out of a system Work done by an internal conservative force does not leave the system Wext E External work can change the total energy of a system For example if a ball is thrown straight up in the air and the forces working on it are gravity and air resistance then compared to the time it takes for the ball to go up it takes more time for the ball to come down This is because due to air friction the ball is continuously losing kinetic energy Power Power describes the speed at which work is done Essentially power is calculated as a change in energy over a change in time The equation for this is Power W t Because power is energy over time the unit for power is J s which are also called Watts If you think about a machine or appliance with a certain wattage this same equation can be used to find out how much energy the appliance uses in a certain amount of time just convert the equation to solve for work These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Example An electric motor lifts a 50kg mass a distance of 6m The motor can put out 1000 Watts of power How long will it take the motor to move the object P W T P F X t t FX P t mgX P 50 9 81 6 1000 2 94s Power in Terms of Speed If you consider the ball example from above the loss of kinetic energy as the ball is falling causes the speed of the ball to decrease Because work is a measure of kinetic energy this means that the speed of an object can also affect the power of that object If you think

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