Work, Energy, and Work, Energy, and PowerPower

Identify each of the situations Identify each of the situations below that represent work being below that represent work being

done.done. A book in free fallA book in free fall Pushing on a wallPushing on a wall Lifting a boxLifting a box Carrying a box across the roomCarrying a box across the room Pushing a box across the roomPushing a box across the room Holding a weight over your headHolding a weight over your head Reading a bookReading a book Writing notesWriting notes

WorkWork

When a When a forceforce acts upon an object to cause acts upon an object to cause a a displacementdisplacement of the object, it is said of the object, it is said that that workwork was done upon the object. was done upon the object.

There are three key There are three key componentscomponents to work - to work - forceforce, , displacementdisplacement, and , and causecause. .

In order for a force to qualify as having In order for a force to qualify as having done done workwork on an object, there must be a on an object, there must be a displacement displacement and the force must and the force must causecause the displacement. (the force must be in the the displacement. (the force must be in the same same directiondirection as the displacement) as the displacement)

Identify each of the situations Identify each of the situations below that represent work being below that represent work being

done.done. A book in free fallA book in free fall Pushing on a wallPushing on a wall Lifting a boxLifting a box Carrying a box across the roomCarrying a box across the room Pushing a box across the roomPushing a box across the room Holding a weight over your headHolding a weight over your head Reading a bookReading a book Writing notesWriting notes

Vector or Scalar?Vector or Scalar?

Work is a Work is a vectorvector quantity- it has a quantity- it has a magnitudemagnitude and a and a directiondirection..

Since work is a vector quantity, it can Since work is a vector quantity, it can have a have a negativenegative value. value.

Negative workNegative work occurs when the occurs when the force doesn't force doesn't causecause the displacement the displacement but rather but rather hindershinders it or when the it or when the force moves the object in the force moves the object in the negativenegative direction. direction.

Calculating WorkCalculating Work

The SI unit for work is the The SI unit for work is the joulejoule, J. , J. A joule is equal to a force of 1 A joule is equal to a force of 1 newtonnewton

applied over a distance of 1 applied over a distance of 1 metermeter, or , or

1 1 newton-meternewton-meter.. Formula: work = Formula: work = forceforce x x distance distance W = F x dW = F x d

Sample Problem #1Sample Problem #1

How much work is done by a person How much work is done by a person who uses a force of 27.5N to move a who uses a force of 27.5N to move a grocery buggy 12.3m?grocery buggy 12.3m?

W = 338 JW = 338 J

Sample Problem #2Sample Problem #2

55, 000J of work is done to move a 55, 000J of work is done to move a rock 25m. How much force was rock 25m. How much force was applied?applied?

F = 2200 JF = 2200 J

Sample Problem #3Sample Problem #3

You and 3 friends apply a combined You and 3 friends apply a combined force of 489.5N to push a piano. The force of 489.5N to push a piano. The amount of work done is 1762.2J. amount of work done is 1762.2J. What distance did the piano move?What distance did the piano move?

d = 3.6 md = 3.6 m

Sample Problem #4Sample Problem #4

John pulls a 4.5 kg sled across level John pulls a 4.5 kg sled across level snow with a force of 225 N on a rope snow with a force of 225 N on a rope that is 35o abovethat is 35o above the horizontal. If the horizontal. If the sled moves a distance of 65.3 m, the sled moves a distance of 65.3 m, how much work does John do?how much work does John do?

Sample Problem #5Sample Problem #5

Sharon is pushing a lawn mower with Sharon is pushing a lawn mower with a force of 88.0 N along a handle that a force of 88.0 N along a handle that makes an angle of 41.0makes an angle of 41.0oo with the with the horizontal. How much work is done horizontal. How much work is done in moving the lawn mower 1.2 km to in moving the lawn mower 1.2 km to mow the lawn?mow the lawn?

Sample problem #6Sample problem #6

A 4200 N piano is to be slid up a 3.5 A 4200 N piano is to be slid up a 3.5 m frictionless ramp at a constant m frictionless ramp at a constant speed. The plank makes an angle of speed. The plank makes an angle of 30.030.0oo with the horizontal. Calculate with the horizontal. Calculate the work done by the person sliding the work done by the person sliding the piano up the ramp.the piano up the ramp.

Sample Problem #7Sample Problem #7

Marcus slides a 60.0 kg crate up a ramp that Marcus slides a 60.0 kg crate up a ramp that is 2.0 m long and attached to a platform 1.0 is 2.0 m long and attached to a platform 1.0 m above the floor level. A 400.0 N force, m above the floor level. A 400.0 N force, parallel to the ramp, is needed to slide the parallel to the ramp, is needed to slide the crate up the ramp at a constant speed.crate up the ramp at a constant speed.

a) How much work does Marcus do in sliding a) How much work does Marcus do in sliding the crate up the ramp? the crate up the ramp? 

b) How much work would be done if Marcus b) How much work would be done if Marcus simply lifted the crate straight up from the simply lifted the crate straight up from the floor to the platform?floor to the platform?

PowerPower

Power is the Power is the raterate of doing work. of doing work. The SI unit for power is the The SI unit for power is the wattwatt, W., W. A watt is equal to 1 A watt is equal to 1 Joule/secondJoule/second.. Formula: power = Formula: power = workwork / / timetime P = W / tP = W / t

REVIEWREVIEW

In physics, work is defined In physics, work is defined as forceas force

0%

0%

0%

0% 1.1. times timetimes time

2.2. divided by timedivided by time

3.3. times distancetimes distance

4.4. divided by distancedivided by distance

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

The unit of work is theThe unit of work is the

0%

0%

0%

0% 101.1. NewtonNewton

2.2. WattWatt

3.3. metermeter

4.4. JouleJoule

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

___ is the rate at which work ___ is the rate at which work is done.is done.

0%

0%

0%

0% 101.1. PowerPower

2.2. EnergyEnergy

3.3. ForceForce

4.4. FrictionFriction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

The unit of power is theThe unit of power is the

0%

0%

0%

0% 101.1. NewtonNewton

2.2. WattWatt

3.3. metermeter

4.4. JouleJoule

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

When we carry an object across a When we carry an object across a room, without lifting it or setting it room, without lifting it or setting it

down, we do no physical work on it.down, we do no physical work on it.

0%

0% 101.1. TrueTrue

2.2. FalseFalse

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How much work is done on a 10 N How much work is done on a 10 N block that is lifted 5 meters off the block that is lifted 5 meters off the

ground?ground?

0%

0%

0%

0% 101.1. 2 J2 J

2.2. 5 J5 J

3.3. 50 J50 J

4.4. 500 J500 J

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30

You did 170 J of work lifting a 140 N You did 170 J of work lifting a 140 N backpack. How high did you lift it?backpack. How high did you lift it?

0%

0%

0%

0% 101.1. 30 m30 m

2.2. 0.82 m0.82 m

3.3. 1.2 m1.2 m

4.4. 310 m310 m

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You lift a 45 N bag of mulch 1.2 m and You lift a 45 N bag of mulch 1.2 m and carry it a distance of 10 m to the carry it a distance of 10 m to the

garden. How much work was done?garden. How much work was done?

0%

0%

0%

0% 101.1. 54 J54 J

2.2. 504 J504 J

3.3. 450 J450 J

4.4. 540 J540 J

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21 22 23 24 25 26 27 28 29 30

A mouse carries a 1.0 g ant on his A mouse carries a 1.0 g ant on his back across the floor for a distance of back across the floor for a distance of

10 m. How much work was done?10 m. How much work was done?

0%

0%

0%

0% 101.1. 10 J10 J

2.2. 98 J98 J

3.3. 0.098 J0.098 J

4.4. None of the aboveNone of the above

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What power is expended if What power is expended if you lift a 10 kg boulder 1 m you lift a 10 kg boulder 1 m

in 2 s?in 2 s?

0%

0%

0%

0% 101.1. 5 W5 W

2.2. 20 W20 W

3.3. 50 W50 W

4.4. 500 W500 W

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Is work a scalar or vector Is work a scalar or vector quantity?quantity?

0%

0%

0%101.1. ScalarScalar

2.2. VectorVector

3.3. NeitherNeither

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It is not possible for a It is not possible for a person to do negative work.person to do negative work.

0%

0% 101.1. TrueTrue

2.2. FalseFalse

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EnergyEnergy

Energy is the ability to produce Energy is the ability to produce changechange in in itselfitself or the or the environmentenvironment..

The energy can take many forms The energy can take many forms including including thermalthermal, , chemicalchemical, , energyenergy of of motionmotion..

When the position of a moving object When the position of a moving object changes over time, the change in changes over time, the change in position indicates that the object has position indicates that the object has energy. The energy resulting from a energy. The energy resulting from a change in motion is called change in motion is called kinetickinetic energyenergy..

Kinetic EnergyKinetic Energy

The equation for kinetic energy is:The equation for kinetic energy is:

KE = ½ mvKE = ½ mv22

m is the mass of the object and v m is the mass of the object and v is is the velocity of the object.the velocity of the object.

What is the relationship between What is the relationship between kinetic energy and velocity? Between kinetic energy and velocity? Between kinetic energy and mass?kinetic energy and mass?

WorkWork

Work is the process of changing the Work is the process of changing the energyenergy of the system.of the system.

When work is done on an object, a change When work is done on an object, a change in in kinetickinetic energyenergy results. results.

Two conditions must be met for work to Two conditions must be met for work to occur.occur.

a. The object must move through a a. The object must move through a distancedistance

b. A b. A forceforce must act upon the object in the must act upon the object in the directiondirection the object the object movesmoves..