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Work and PowerWork and Power

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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.

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The scientific definition of The scientific definition of work is:work is:

using a force to move an object a using a force to move an object a distance distance

Both the force and the motion of the Both the force and the motion of the object must be in the same direction.object must be in the 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

WHY?!WHY?!

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

audience of his peers. audience of his peers.

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

audience of his peers. audience of his peers.

NO!!!NO!!!

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What’s work?What’s work? A body builder lifts 350 pounds A body builder lifts 350 pounds

above his head. above his head.

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What’s work?What’s work? A body builder lifts 350 pounds A body builder lifts 350 pounds

above his head. above his head.

YES!!!YES!!!

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What’s work?What’s work? A mother carries her baby from room A mother carries her baby from room

to room. to room.

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What’s work?What’s work? A mother carries her baby from room A mother carries her baby from room

to room.to room.

NO!!! NO!!!

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What’s work?What’s work? A father pushes a baby in a carriage.A father pushes a baby in a carriage.

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What’s work?What’s work? A father pushes a baby in a carriage.A father pushes a baby in a carriage.

YES!!!

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What’s work?What’s work? A woman carries a 20 kg grocery bag to A woman carries a 20 kg grocery bag to

her car? her car?

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What’s work?What’s work? A woman carries a 20 kg grocery bag to A woman carries a 20 kg grocery bag to

her car? her car?

NO!!!

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What’s work?What’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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How Do We How Do We Calculate Work?Calculate Work?

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The FormulaThe Formula

Work = Force x DistanceWork = Force x Distance

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W = F x dW = F x d The unit of Force is NewtonsThe unit of Force is Newtons The unit of distance is metersThe unit of distance is meters Thus, the unit of work is Newton-Thus, the unit of work is Newton-

metersmeters One N-m is equal to one jouleOne N-m 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 = F x dW = F x d

Calculate: If a Calculate: If a man pushes a man pushes a concrete block concrete block 10 meters with a 10 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 = F x dW = F x d

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

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What is Power?What is Power? Power is the rate at which work is Power is the rate at which work is

done. done.

The amount of energy consumed per The amount of energy consumed per given unit timegiven unit time

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The FormulaThe Formula

Power = Work/TimePower = Work/Time

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P = W / tP = W / t The unit of power is N-m/s.The unit of power is N-m/s.

We know this better as the We know this better as the WattWatt

The amount of energy consumed per The amount of energy consumed per given unit timegiven unit time

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P = W / tP = W / tRemember that we know Work from Remember that we know Work from

our previous formula:our previous formula:

W = F x dW = F x d

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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.

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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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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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Simple MachinesSimple Machines

Ancient people invented simple Ancient people invented simple machines that would help them overcome machines that would help them overcome resistive forces and allow them to do the resistive forces and allow them to do the desired work against those forces. desired work against those forces.

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Simple MachinesSimple Machines

We will learn more about simple We will learn more about simple machines later in this course!machines later in this course!

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Simple MachinesSimple Machines

The six simple machines are:The six simple machines are: Lever Lever Wheel and Axle Wheel and Axle Pulley Pulley Inclined Plane Inclined Plane Wedge Wedge Screw Screw

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Simple MachinesSimple Machines

A machine is a device that helps A machine is a device that helps make work easier to perform by make work easier to perform by accomplishing one or more of the accomplishing one or more of the following functions: following functions: transferring a force from one place to transferring a force from one place to

another, another, changing the direction of a force, changing the direction of a force, increasing the magnitude of a force, or increasing the magnitude of a force, or increasing the distance or speed of a increasing the distance or speed of a

force. force.

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Practice QuestionsPractice Questions1. Explain who is doing more work and why: a bricklayer 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building carrying bricks and placing them on the wall of a building being constructed, or a project supervisor observing and being constructed, or a project supervisor observing and recording the progress of the workers from an observation recording the progress of the workers from an observation booth. booth.

2. How much work is done in pushing an object 7.0 m 2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back across a floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work to its original position? How much power is used if this work is done in 20 sec?is done in 20 sec?

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Practice QuestionsPractice Questions1. Explain who is doing more work and why: a bricklayer carrying 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the constructed, or a project supervisor observing and recording the progress of the workers from an observation booth. progress of the workers from an observation booth. Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work.

2. How much work is done in pushing an object 7.0 m across a 2. How much work is done in pushing an object 7.0 m across a floor with a force of 50 N and then pushing it back to its original floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? position? How much power is used if this work is done in 20 sec? Work = 7 m X 50 N X 2 = 700 N-m or J; Power = 700 N-m/20 sec = 35 W