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Work and Power Work and Power

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History of WorkHistory of Work

Before engines and motors were invented, Before engines and motors were invented, people had to do things like lifting or pushing people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, heavy loads by hand. Using an animal could help, but what they really needed were some clever but what they really needed were some clever ways to either make work easier or faster. ways to either make work easier or faster.

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What is work?What is work?

In science, the word In science, the word workwork has a has a different meaning than you may be different meaning than you may be familiar with.familiar with.

The scientific definition of work is: The scientific definition of work is: using a force to move an object a using a force to move an object a distance (when both the force and distance (when both the force and the motion of the object are in the the motion of the object are in the same direction.)same direction.)

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Work or Not?Work or Not?

According to the According to the scientific definition, scientific definition, what is work and what is work and what is not?what is not? a teacher lecturing a teacher lecturing

to her classto her class a mouse pushing a a mouse pushing a

piece of cheese piece of cheese with its nose across with its nose across the floorthe floor

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Work or Not?Work or Not?

According to the According to the scientific definition, scientific definition, what is work and what is work and what is not?what is not? a teacher lecturing a teacher lecturing

to her classto her class a mouse pushing a a mouse pushing a

piece of cheese piece of cheese with its nose across with its nose across the floorthe floor

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WhatWhat’’s work?s work? A scientist delivers a speech to an A scientist delivers a speech to an

audience of his peers. audience of his peers. A body builder lifts 350 pounds A body builder lifts 350 pounds

above his head. above his head. A mother carries her baby from room A mother carries her baby from room

to room. to room. A father pushes a baby in a carriage.A father pushes a baby in a carriage. A woman carries a 20 kg grocery bag A woman carries a 20 kg grocery bag

to her car? to her car?

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WhatWhat’’s work?s work? A scientist delivers a speech to an A scientist delivers a speech to an

audience of his peers. audience of his peers. NoNo A body builder lifts 350 pounds A body builder lifts 350 pounds

above his head. above his head. YesYes A mother carries her baby from room A mother carries her baby from room

to room. to room. NoNo A father pushes a baby in a carriage. A father pushes a baby in a carriage. YesYes A woman carries a 20 km grocery A woman carries a 20 km grocery

bag to her car? bag to her car? NoNo

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Formula for workFormula for work

Work = Force x DistanceWork = Force x Distance

The unit of force is newtonsThe unit of force is newtons The unit of distance is metersThe unit of distance is meters The unit of work is newton-metersThe unit of work is newton-meters One newton-meter is equal to one jouleOne newton-meter is equal to one joule So, the unit of work is a So, the unit of work is a joulejoule

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W=FDW=FD

Work = Force x Work = Force x DistanceDistance

Calculate: If a Calculate: If a man pushes a man pushes a concrete block 10 concrete block 10 meters with a meters with a force of 20 N, force of 20 N, how much work how much work has he done?has he done?

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W=FDW=FD

Work = Force x Work = Force x DistanceDistance

Calculate: If a Calculate: If a man pushes a man pushes a concrete block 10 concrete block 10 meters with a meters with a force of 20 N, how force of 20 N, how much work has he much work has he done? done? 200 joules200 joules(W = 20N x 10m)(W = 20N x 10m)

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PowerPower Power is the rate at which work is Power is the rate at which work is

done. done.

Power = WorkPower = Work**/Time/Time **(force x distance)(force x distance)

The unit of power is the watt.The unit of power is the watt.

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Check for UnderstandingCheck for Understanding

1.1.Two physics students, Ben and Bonnie, are Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head kg barbell the same distance over his head 10 times in 10 seconds. 10 times in 10 seconds.

Which student does the most work? Which student does the most work?

Which student delivers the most Which student delivers the most power?power?

Explain your answers. Explain your answers.

LetLet’’s say Gary is lifting s say Gary is lifting weights at the gym. It weights at the gym. It takes him 2 sec. to lift a takes him 2 sec. to lift a 120 N weight over his 120 N weight over his head (0.5 m).head (0.5 m).

How much work did Gary How much work did Gary do?do?

How much power did he How much power did he generate?generate?

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Ben and Bonnie Ben and Bonnie do the same amount do the same amount of work; they apply of work; they apply the same force to lift the same force to lift the same barbell the the same barbell the same distance above same distance above their heads. their heads.

Yet, Ben is the Yet, Ben is the most powerful since most powerful since he does the same he does the same work in less time. work in less time.

Power and time Power and time are inversely are inversely proportional.proportional.

  

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2. How much power will it take to 2. How much power will it take to move a 10 kg mass at an move a 10 kg mass at an acceleration of 2 m/s/s a distance of acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This 10 meters in 5 seconds? This problem requires you to use the problem requires you to use the formulas for force, work, and power formulas for force, work, and power all in the correct order.all in the correct order.

Force=Mass x Acceleration Force=Mass x Acceleration

Work=Force x DistanceWork=Force x Distance

Power = Work/TimePower = Work/Time

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2. How much power will it take to move a 10 kg 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and you to use the formulas for force, work, and power all in the correct order.power all in the correct order.

Force=Mass x Acceleration Force=Mass x Acceleration Force=10 x 2Force=10 x 2Force=20 NForce=20 N

Work=Force x DistanceWork=Force x DistanceWork = 20 x 10Work = 20 x 10

Work = 200 JoulesWork = 200 Joules

Power = Work/TimePower = Work/TimePower = 200/5Power = 200/5

Power = 40 wattsPower = 40 watts