Defining work and power

Work, Power,

and Machines

AKS 15b – analyze the relationship between work, power, and timeAKS 15c – explain how machines make work easier

Work, Power, and MachinesBrainstorming:

What does it mean to “WORK”?

Anticipation Guide:After completing the Before Reading portion, work with your group to write one question that would help you determine whether a statement (or statements) is true or false.

Before reading Statement After

reading Text evidence for or against

(include the page # and paragraph # along with your citation)

Write an “A” if you agree or “D” if you disagree for each statement.

A force acting on an object does work

even if the object doesn’t move.

A mover who pushes a piece of furniture up a ramp and into a truck does the same work as a mover who

simply lifts the piece of furniture directly onto the truck.

Can it be proven mathematically? HOW?

You are using more power if you do the same amount of work in a shorter time.

Machines put out more work than

we put in.

Machines can increase the size of

a force that is applied to a load

The work you do on a machine (work input) is the same as the work the

machine does on a load (work output)

Work, Power, and MachinesQuestions (1st Period):

1. Is force the same as work?2. How can we calculate work?3. What is a load?4. If you push something and it doesn’t move, is it

still work?5. How much work can machines put out?6. What is work?7. Does direction affect the force applied?8. What is the difference between work input and

work output?

Work, Power, and MachinesQuestions (2nd Period):

1. What is the difference between power and work?2. How much force is being applied to an object?3. What is the sci. definition of work?4. Can machines increase the size of force applied to a

load?5. Is the work you do on a machine the same as the work

the machine does on a load?6. Does the quality of the machine affect the amount done

by the machine?7. What is the difference between force and work?8. How can you calculate the work done by simple

machines?

Work, Power, and MachinesQuestions (3rd Period):

1. What is the difference between work input and work output?

2. What effect do machines have on work and power?3. How does angle/direction affect the work done on an

object?4. Do machines apply the same amount of force and

distance as a human?5. What is work?6. Does an object need to move for work to be done?7. What is a machine?8. Can machines increase the magnitude of a force applied

to a load?

Work, Power, and MachinesQuestions (4th Period):

1. What is work?2. How do machines make work easier?3. In a shorter amount of time, if you use more power are

you putting more work?4. Does the amount of time you spend doing work affect

the amount of work you do?5. What are the differences between work input and work

output?6. Can machines increase the magnitude of a force applied

to a load?7. EPIC FAIL!8. Do machines or people do more work?

What is Work?1. Analyze the picture and

formulate your own scientific definition of WORK.

2. Compare and contrast your definition of work to the definition on p. 129 of your Interactive Textbook. How are they similar and different?

3. SHOW AND TELL: Work with a classmate to prepare a short demonstration of WORK and NOT WORK.• Define WORK• 1 example of WORK w/

explanation• 1 non-example of WORK

w/ explanation

Calculating Work

W = F x DA rock weighing 2N was lifted 3m. How much work was done on the rock?

A man used 600N of force to move a car 0m. How much work was done on the car?

2N X 3m = 6N•m or 6 Joules (J)

600N X 0m = 0N•m or 0 Joules (J)

Calculating Work

W = F x DWhich of the following actions do more work on an object?• Lifting an 80 N box 1 m up off the floor• Lifting a 160 N box 1 m up off the floor• Lifting a 90 N box 2 m up off the floor• Lifting a 100 N box 1.5 m up off the floor

Work– In which situation is a person doing work on an

object?a) A school crossing guard raises a stop sign that

weighs 10N.b) A student walks 1m/s while wearing a

backpack weighing 15N.c) A man exerts 500N of force on a rope

attached to a house but the house doesn’t move.

d) A worker holds a box 1m off the floor.

WorkWhich example does NOT involve work being done?

a) A waiter carrying a food tray across a dining room.

b) A person places a heavy box up on a high shelf.

c) A rocket accelerates into space.d) A truck pulling a trailer

Calculating Work

• You lift a chair that weighs 50N to a height of 2m and carry it 10m across the room. How much work do you do on the chair?

W = F x D

Calculating Work

• You apply a 200N force to move a 500N boulder 5m to the edge of a cliff. The boulder falls 100m. How much work did you do to the boulder?

W = F x D

Calculating Work

• You apply a 200N force to move a 500N boulder 5m to the edge of a cliff. The boulder falls 100m. How much work did gravity do to the boulder?

W = F x D

Calculating Work

• A rope is thrown over a beam and one end is tied to a 300N bundle of lumber. You pull the free end of the rope 2m with a force of 400N to lift the lumber off the ground. How much work have you done?

W = F x D

Calculating Power

A cart is pushed 10 m down the hall with a force of 100N in 5s.

• How much work is done?

• How much power is used?

P = W / T

Power DemoRecord the fastest times in the chart below:

Name Time (s) AnalysisCopy and complete the following questions:

1. What was the work done in this demonstration?

2. Who did the most work? How do you know?

3. Which student was the most powerful? How do you know?

4. What factors (variables) may have affected your results?

What is Power?

– What is the advantage of using a power tool over a hand tool to do the same amount of work?

a) It uses less force.b) It uses less energy.c) It requires less time.d) It requires more time.

– REMEMBER: More power means you can work faster!

19

Power Lab

20

Record the appropriate information in the spaces below:

Student Name

Force

Distance

Time

Work

Power

Analysis1. What was the work done in this

experiment?2. Who did the most work?3. Which student was the most

powerful? How do you know?4. What factors (variables) may

have affected your results?

2.2 kg = _____ N1.1 kg = _____ N Power =

Work (J)

time (s)

Work and PowerMr. Cox and Mrs. Steele each lift an identical stack of books the same distance onto a table, but Mr. Cox does the job twice as fast. Therefore his actions involve twice as much

a) Work inputb) Work outputc) Powerd) Efficiency

Work and Power

Work = force and motion in the same direction.Units = Joules (J)

Power = the rate at which work is done.Units =Watts (W)

Work and Power

Work = ____ and ______ in the same ________.Units = ________

Power = the _____ at which ______ is done.Units = ________

Machines– Mechanical Advantage and Efficiency• What does it mean to have an ADVANTAGE?• What does EFFICIENT mean?

Machines– Mechanical Advantage• Some machines can make work easier by

increasing force applied to an object.• Mechanical advantage is the number of times a

machine multiplies the force.

Mechanical Advantage (MA) = Output force (N)Input force (N)

Machines

– Mechanical Advantage• A mechanic uses a pulley to lift an engine out of a

car. She applies a force of 200N to one end of the rope and the rope applies a force of 600N to the engine. What is the mechanical advantage of the pulley?

Mechanical Advantage (MA) = Output force (N)Input force (N)

Machines

– Mechanical Advantage• Suppose that you exert 60N on a machine and the

machine exerts 240N on another object. What is the machine’s mechanical advantage?

Mechanical Advantage (MA) = Output force (N)Input force (N)

Machines

– Mechanical Advantage• A worker uses a lever to lift a load. She applies a

force of 100N and the lever applies a force of 25N. What is the mechanical advantage of the lever?

Mechanical Advantage (MA) = Output force (N)Input force (N)

Machines– Mechanical Efficiency• Some of the work done by a machine is always

used to overcome the friction created by the machine.

• Mechanical efficiency tells you what percentage of the work input gets converted into work output.

Mechanical Efficiency (ME) = Work output (J)

X 100Work input (J)

Machines

– Mechanical Efficiency• What is the mechanical efficiency of a machine

whose work input is 1200J and work output is 900J?

Mechanical Efficiency (ME) = Work output (J)

X 100Work input (J)

Machines

– Mechanical Efficiency• You do 500J of work to a rope attached to a pulley.

The pulley does 400J of work to lift a load. What it is ME of the pulley?

Mechanical Efficiency (ME) = Work output (J)

X 100Work input (J)

Machines

–MA and ME• You are measuring force and distance for a lab

activity. Your measurements indicate that you apply 100N to move a lever 2m. The lever applies 300N to move a load 1m. 1. What is the MA of the lever?2. What is the ME of the lever?3. How much work was used to overcome the

friction of the machine?

Machines– Define machine, input force, and output force (p. 216-217)

Copy and complete the following chart with a picture, a description (definition), and a real-world example (p. 222):Lever Pulley

Wheel and Axle Inclined Plane

Wedge Screw

Machines– Levers

• Levers

Machines– Pulleys

Machines– Wheel and Axle

Machines– Inclined Plane

Machines• Wedges

Machines– Screw

Machines1. Mr. Cox wants to remove the lid from a paint

can. What simple machine should he use as a tool?

2. What type of simple machine is used to split things apart?

3. Both the wedge and the lever are used in the operation of which tool?

a) Shovelb) steering wheelc) Wheelbarrowd) block and tackle (p. 224)

Investigating LeversLoad

Fulcrum

Effort sideResistance side

Experiment1. Place the fulcrum in the middle (50cm) and the load (100g) at one

end. Observe how much force is needed to lift the load.2. Move the fulcrum closer to the load and observe how much force

is needed to lift the load.3. Move the fulcrum farther from the load and observe how much

force is needed to lift the load.4. Try to balance different masses at each end by adjusting the

position of the fulcrum.

Investigating Levers

Analysis1. Where should you move the fulcrum to make the load easier to

lift?2. The ____________ (closer/farther) the fulcrum is to the load,

the ____________ (more/less) force needed to lift the load.3. The greater the length of the effort side, the __________

(greater/less) force needed to lift the load. 4. The greater the length of the resistance side, the __________

(greater/less) force needed to lift the load.

Load

Fulcrum

Effort sideResistance side

Investigating Levers

Sample Test QuestionA lever has a mechanical advantage of 2. The input force is 100N. What is the output force?

Load

Fulcrum

Effort sideResistance side

Machines

Two children are playing on a seesaw in the park. Sarah, who weighs less than Jeff, is suspended in the air. Where could you move the fulcrum in order to raise Jeff into the air?

Machines

– Which of the following items is most similar to the lever in the diagram? (p. 223)

a) Seesawb) Wheelbarrowc) Bottle openerd) Arm lifting a weight

Machines– What simple machines are found on a pair of scissors?– What do you call a machine made up of 2 or more

simple machines?

LeverWedge

Fulcrum

Machines– What simple machines are used to make a

wheelbarrow?

Machines– What simple machines are found on this

compound machine?