Chapter 5 5-4 Power

Objectives Define power Calculate power Explain the effect on machines the effect of machines on work and power Objectives

Power is the rate of work done in a unit of time Power is the rate of work done in a unit of time. It can be misunderstood by most of the students. They think that more power full machine does more work. However, power just shows us the time that the work requires. For example, same work is done by two different people with different time. Say one of them does the work in 5 seconds and the other does in 8 seconds. Thus, the man doing same work in 5 seconds is more power full. The shorter the time the more power full the man. What is power

Power Mathematically, it is computed using the following equation. The standard metric unit of power is the Watt. As is implied by the equation for power, a unit of power is equivalent to a unit of work divided by a unit of time. Thus, a Watt is equivalent to a Joule/second. For historical reasons, the horsepower is occasionally used to describe the power delivered by a machine. One horsepower is equivalent to approximately 750 Watts.

Example#1 Example Find the power of the man who pushes the box 8m with a force of 15N in a 6seconds.

A tired squirrel (mass of approximately 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm in order to do a mere 0.50 Joule of work. If the tired squirrel does all this work in 2 seconds, then determine its power. Solution: The tired squirrel does 0.50 Joule of work in 2.0 seconds. The power rating of this squirrel is found by P = W / t = (0.50 J) / (2.0 s) = 0.25 Watts Example#2

When doing a chin-up, a physics student lifts her 42 When doing a chin-up, a physics student lifts her 42.0-kg body a distance of 0.25 meters in 2 seconds. What is the power delivered by the student's biceps? solutioh To raise her body upward at a constant speed, the student must apply a force which is equal to her weight (m•g). The work done to lift her body is W = F * d = (411.6 N) * (0.250 m) W = 102.9 J The power is the work/time ratio which is (102.9 J) / (2 seconds) = 51.5 Watts (rounded) Example#3

Student guided practice Do worksheet Student guided practice

The expression for power is work/time The expression for power is work/time. And since the expression for work is force*displacement, the expression for power can be rewritten as (force*displacement)/time. Since the expression for velocity is displacement/time, the expression for power can be rewritten once more as force*velocity. This is shown below. Power

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Closure Today we learned about power Next we are going to learn about momentum and collisions Closure