Work and Energy Calculating Power Chapter 5 Section

Learning Targets Use the equations for work and power to calculate multiple variables Describe

Reviewing Work When calculating work, it is only important to know how much force

The rate at which work is done varies ◦ It would take a

Power The quantity that measures the rate at which work is done is known

Calculating Power: Method Two The expression for power is work/time. ◦ Since the expression

Since the expression for velocity is displacement/time, the expression for power can be

Units of Power The standard metric unit of power is the Watt. ◦ Since

Power Ratings Most machines are designed and built to do work on objects. ◦

Machines with different power ratings do the same work in different time intervals The

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Work and Energy: Calculating Power Chapter 5: Section 3

Learning Targets Use the equations for work and power to calculate multiple variables Describe the unit of power Explain power ratings on machines

Reviewing Work When calculating work, it is only important to know how much force is applied to an object in order to cause a certain displacement W=F*d Work does not take into account, the amount of time that the force acts upon the object

The rate at which work is done varies ◦ It would take a rock climber more time to elevate her body up a few meters along the side of a cliff then it would if she walked along the path cut on the side of the cliff Although both scenarios illustrate the same amount of force and displacement (work) hiking will take considerably less time than climbing

Power The quantity that measures the rate at which work is done is known as power. More generally, power is the rate of energy transfer

Calculating Power: Method Two The expression for power is work/time. ◦ Since the expression for work is force*displacement, the expression for power can be rewritten as:

Since the expression for velocity is displacement/time, the expression for power can be rewritten once more as : This new equation for power reveals that a powerful machine is both strong (big force) and fast (big velocity)

Units of Power The standard metric unit of power is the Watt. ◦ Since work is measured in joules and time is measured in seconds, one watt is equivalent to one Joule/second (J/s) For historical reasons, horsepower is occasionally used to describe the power delivered by a machine. ◦ One horsepower is equivalent to approximately 750 Watts.

Power Ratings Most machines are designed and built to do work on objects. ◦ These machines are typically described by a power rating. The power rating indicates the rate at which that machine can do work upon other objects. A car engine is an example of a machine that is given a power rating.

Machines with different power ratings do the same work in different time intervals The greater the power rating, the more efficient the machine