11

Work and Simple Work and Simple MachinesMachines

22

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.)

33

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

44

55

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. 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?

66

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

77

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

88

W=FDW=FDWork = Force x Work = Force x DistanceDistance

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

99

W=FDW=FD

Work = Force x Work = Force x DistanceDistance

Calculate: If a man Calculate: If a man pushes a concrete pushes a concrete block 10 meters with a block 10 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 (W = 20N x 10m)10m)

1010

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

work is done. work is done.

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

distance)distance)

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

1111

Check for UnderstandingCheck for Understanding1.1.Two physics students, Ben and Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg in one minute; Ben lifts the 50 kg barbell the same distance over his barbell the same distance over his head 10 times in 10 seconds. head 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.

1212

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

  

1313

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

Force=Mass x Force=Mass x Acceleration Acceleration

Work=Force x DistanceWork=Force x Distance Power = Work/TimePower = Work/Time

1414

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

Work=Force x DistanceWork=Force x DistanceWork = 20 x 10 Work = 200 JoulesWork = 20 x 10 Work = 200 Joules

Power = Work/TimePower = Work/TimePower = 200/5 Power = 40 wattsPower = 200/5 Power = 40 watts

1515

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 transferring a force from one place

to another, to another, changing the direction of a force, changing the direction of a force, increasing the magnitude of a increasing the magnitude of a

force, or force, or increasing the distance or speed of increasing the distance or speed of

a force. a force.

1616

Mechanical AdvantageMechanical Advantage

It is useful to think about a machine It is useful to think about a machine in terms of the in terms of the input forceinput force (the (the force you apply) and the force you apply) and the outputoutput forceforce (force which is applied to the (force which is applied to the task).task).

When a machine takes a small input When a machine takes a small input force and increases the magnitude of force and increases the magnitude of the output force, a the output force, a mechanical mechanical advantageadvantage has been produced. has been produced.

1717

Mechanical AdvantageMechanical Advantage Mechanical advantage is the ratio of output force Mechanical advantage is the ratio of output force

divided by input force. If the output force is divided by input force. If the output force is bigger than the input force, a machine has a bigger than the input force, a machine has a mechanical advantage greater than one.mechanical advantage greater than one.

If a machine increases an input force of 10 If a machine increases an input force of 10 pounds to an output force of 100 pounds, the pounds to an output force of 100 pounds, the machine has a mechanical advantage (MA) of 10. machine has a mechanical advantage (MA) of 10.

In machines that increase distance instead of In machines that increase distance instead of force, the MA is the ratio of the output distance force, the MA is the ratio of the output distance and input distance.and input distance.

MA = output/input MA = output/input

1818

Inclined PlaneInclined Plane The mechanical The mechanical

advantage of an inclined advantage of an inclined plane is equal to the plane is equal to the length of the slope length of the slope divided by the height of divided by the height of the inclined plane. the inclined plane.

While the inclined plane While the inclined plane produces a mechanical produces a mechanical advantage, it does so by advantage, it does so by increasing the distance increasing the distance through which the force through which the force must move. must move.

1919

Although it takes less force for car A to get to the top of the ramp, Although it takes less force for car A to get to the top of the ramp, all the cars do the same amount of work.all the cars do the same amount of work.

A B C

2020

Inclined PlaneInclined Plane A wagon trail on a A wagon trail on a

steep hill will often steep hill will often traverse back and forth traverse back and forth to reduce the slope to reduce the slope experienced by a team experienced by a team pulling a heavily loaded pulling a heavily loaded wagon.wagon.

This same technique is This same technique is used today in modern used today in modern freeways which travel freeways which travel winding paths through winding paths through steep mountain passes. steep mountain passes.

2121

EfficiencyEfficiency We said that the input force times We said that the input force times

the distance equals the output the distance equals the output force times distance, or:force times distance, or:

Input Force x Distance = Output Input Force x Distance = Output Force x DistanceForce x DistanceHowever, some output force is lost However, some output force is lost

due to friction.due to friction. The comparison of work input to The comparison of work input to

work output is called efficiency. work output is called efficiency. No machine has 100 percent No machine has 100 percent

efficiency due to friction.efficiency due to friction.

2222

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

2. How much work is done in pushing an 2. How much work is done in pushing an object 7.0 m across a floor with a force object 7.0 m across a floor with a force of 50 N and then pushing it back to its of 50 N and then pushing it back to its original position? How much power is original position? How much power is used if this work is done in 20 sec?used if this work is done in 20 sec?3. Using a single fixed pulley, how heavy 3. Using a single fixed pulley, how heavy a load could you lifta load could you lift??

2323

Practice QuestionsPractice Questions44. . Give an example of a machine Give an example of a machine in which friction is both an in which friction is both an advantage and a disadvantage.advantage and a disadvantage.

5. Why is it not possible to have 5. Why is it not possible to have a machine with 100% a machine with 100% efficiency?efficiency?

6. What is effort force? What is 6. What is effort force? What is work input? Explain the work input? Explain the relationship between effort relationship between effort force, effort distance, and work force, effort distance, and work input.input.

2424

Practice QuestionsPractice Questions1. Explain who is doing more work and why: a 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall bricklayer carrying bricks and placing them on the wall of a building being constructed, or a project supervisor of a building being constructed, or a project supervisor observing and recording the progress of the workers observing and recording the progress of the workers from an observation booth. from an observation booth. The bricklayer is doing more work. 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 across a floor with a force of 50 N and then pushing it back to its original position? How much power is used back to its original position? How much power is used if this work is done in 20 sec? 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 3. Using a single fixed pulley, how heavy a load could 3. Using a single fixed pulley, how heavy a load could you lift?you lift?Since a fixed pulley has a mechanical advantage of one, it will only change the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction.

2525

Practice QuestionsPractice Questions4. 4. Give an example of a machine in which friction is Give an example of a machine in which friction is

both an advantage and a disadvantage. both an advantage and a disadvantage. One answer might be the use of a car jack. Advantage of friction: It allows a car to be raised to a desired height without slipping. Disadvantage of friction: It reduces efficiency.

5. Why is it not possible to have a machine with 100% 5. Why is it not possible to have a machine with 100% efficiency? efficiency? Friction lowers the efficiency of a machine. Work output is always < work input, so an actual machine cannot be 100% efficient.

6. What is effort force? What is work input? Explain 6. What is effort force? What is work input? Explain the relationship between effort force, effort distance, the relationship between effort force, effort distance, and work input. and work input. The EF is the force applied to a machine. Work input is the work done on a machine. The work input of a machine is = to the EF times the distance over which the effort force is exerted.