Physics Day 90LO: Use concept of Conservation of Energy

AGENDADo NowNotesWorksheet

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HW28 due Thu Jan 8

Worksheet Solutions

Power

6.7 Power

• The idea of power incorporates both the concepts of work and time.

6.7 Power

• The idea of power incorporates both the concepts of work and time.

• Power is work done per unit time.

Another Question

• If a small machine does 2,500 joules of work on an object to move it a distance of 100 meters in 10 seconds, what is the force needed to do the work? What is the power of the machine doing the work?

6.7 Power

• Average power, P is the average rate at which work W is done, and it is obtained by dividing W by the time t required to perform the work:

d

6.7 PowerThe idea of power incorporates both the concepts of work and time.

Power is work done per unit time.

Average power, P is the average rate at which work W is done, and it is obtained by dividing W by the time t required to perform the work:

UnitsTABLE 6.3     Units of Measurement for Power

System Work ÷ Time = Power

SI joule (J) second (s) watt (W)

CGS erg second (s) erg per second (erg/s)

BEfoot · pound (ft · lb)

second (s) foot ·pound/sec (ft · lb/s)

                                                                                                                                                                                                                  

Metabolic Rates for a young 70-kg male

Activity Metabolic Rate (W)

Running (15 km/h) 1340

Skiing 1050

Biking 530

Walking (5 km/h) 280

Sleeping 77

Power and Force

• P = W/t = Fd/t (for linear motion)

Therefore: P = Fv

• What force must the 230,000 W engine of a 2000 kg car exert on the wheels to maintain a speed of 20 m/s?

Ans: 11,500 N

Work and Angle