CHAPTER 14 : WORK, POWER, AND MACHINES

14.1 Work and Power

What is Work?

Define Work:

The product of force and distance.

Work Requires Motion

What condition must exist in order for a force to do work on an object?

Some of the force must act in the same direction as the object moves.

If there is no movement, no work is done.

Work Depends on Direction

The amount of work done on an object depends on what two things? Direction of the force Direction of the movement

Does all the force have to act in the direction of movement to do work? No

Think of an original idea where this is true.

Calculating Work

How do you calculate work?

Multiply the force acting in the direction of motion by the distance the object moves

Work = Force x Distance

Tomorrow when I am gone, you will do math practice with this formula

Units of Work

What are the two ways you can label work? Newton * Meter

force (N) x distance (m)

Joule (J)

1 N*m = 1 J

Using the Work Formula

How much work does a 25 N force do to lift a potted plant from the floor to a shelf 1.5 m high? Work = Force x Distance

Work = 25 N x 1.5 m

Work = 37.5 N*m

Work = 37.5 J

What is Power?

Define Power:

The rate of doing work.

What are two ways to increase power?

Increase the amount of work done in a given time.

Do a given amount of work in less time.

How does doing work at a faster rate affect the power required? More power is required.

Think of an original example of two things that do the same task, where one requires more power, but less time and the other requires less power, but more time. Plowing a field with a horse & chisel Plowing a field with a tractor

Calculating Power

How do you calculate power? Divide the amount of work done by the time needed to

do the work.

Power = Work/Time

What are two ways you can label power? Joules/seconds

work (J) / time (sec)

Watt (W)

You lift a large bag of flour from the floor to a 1 m high countertop, doing 100 J of work in 2 sec. How much power do you use to lift the bag of flour? Power = Work/Time

Power = 100 J / 2 sec

Power = 50 J/sec

Power = 50 W

James Watt and Horsepower

What is another unit for power? Horsepower (hp)

Compare this unit with watts.1 horsepower = 746 watts

Section 14.1 Assessment

1. How much work is done when a vertical force acts on an object moving horizontally?

Section 14.1 Assessment

2. A desk exerts an upward force to support a computer resting on it. Does this force do work? Explain.

Section 14.1 Assessment

3. Two cars have the same weight, but one of the cars has an engine that is twice as powerful as the other. Which car can make it to the top of a mountain pass first?

Which car does more work to reach the pass?

Section 14.1 Assessment

4. You carry two heavy bags of groceries upstairs to your kitchen. Will you do more work on the bags if you carry them one at a time? Explain.

Section 14.1 Assessment

5. You lift a book to a bookshelf 1 m above the floor. How much power is used if the upward force is 15 N and you do the work in 2 seconds?

CHAPTER 14 : WORK, POWER, AND MACHINES

14.2 Work & Machines

Machines Do Work

Define machine: A device that changes a force.

What is the main purpose of machines? They make work easier to do.

In which ways can machines do this? Change the size of the force needed. Change the direction of the force. Change the distance over which a force acts.

Increasing Force

How can a machine increase a force? Increase the distance over which a small force is

exerted.

A small force exerted over a large distance becomes a large force exerted over a short distance.

Think of an original example of a machine that increases force: Bolt Cutters vs. Scissors

Increasing Distance

How can a machine increase distance? Exert a greater force over a smaller distance.

A machine that decreases the distance through which you exert a force, increases the amount of force required.

Think of an original example of a machine that increases distance: Gears on a bike Broom? Golf club? Bat?

Changing Direction

How can a machine change the direction of the applied force?

Think of an original example of a machine that changes direction: Levers Pry bars

Work Input & Work Output

How does the work done ON a machine compare to the work done BY a machine? Work done ON a machine is bigger than work done BY

it More work goes in than comes out.

What does friction have to do with this? Friction causes work/force to be lost. It can be lost as:

Heat Light Sound

Work Input to a Machine

Define input force: The force you exert on a machine.

Define input distance: The distance over which you exert a force.

Define work input: Work done by you.

Write an equation that uses the last three terms: Work Input = Input Force x Input Distance

Work Output of a Machine

Define output force: The force exerted by a machine

Define output distance: The distance over which a machine exerts a force

Define work output : The work done by a machine

Look at Figure 7 (pg. 419). What is the input distance? The length of the path over which the oar handle

moves (the arc)

How does the input distance compare to the output distance? It is less

Section 14.2 Assessment

1. A machine produces a larger force than you exert to operate the machine. How does the input distance of the machine compare to its output distance?

Section 14.2 Assessment

2. You do 200 J of work pulling the oars of a rowboat. What can you say about the amount of work the oars do to move the boat? Explain.

Section 14.2 Assessment

3. How can you increase the work output of a machine?

Section 14.2 Assessment

4. When you swing a baseball bat, how does the output distance (the end of the bat) compare with the input distance (the distance your hands move)? Why might it be useful to know this?

Section 14.2 Assessment

5. An ad claims that a new wrench reduces the force needed to tighten a bolt. If this ad is true, what do you know about the input distance?

CHAPTER 14 : WORK, POWER, AND MACHINES

14.3 Mechanical Advantage & Efficiency

Mechanical Advantage

Define Mechanical Advantage The number of times the machine increases an input

force

Actual Mechanical Advantage

Define AMA: The ratio of the output force to the input force

Write the equation: AMA = O. Force/ I. Force

Ideal Mechanical Advantage

Define IMA: The MA of a machine in the absence of friction

Write the equation: IMA = I. Distance/O. Distance

Why is the AMA always less than the IMA? Because there is always friction

What can be done to make the AMA closer to the IMA? Use low-friction materials Ball bearings Oil/grease Stream-lining

Efficiency

Define Efficiency: The percentage of the work input that becomes work

output

Write out the equation: Efficiency = Work O. / Work I. X 100%

Why is efficiency of any machine always less than 100%? Because there’s always friction.

What are some ways to increase efficiency? Ball bearings Grease/Oil Smoother surfaces…

Can you think up a machine that would have 100% efficiency?

Section 14.3 Assessment

1. You test a machine and find that it exerts a force of 5 N for every 1 N of force you exert operating the machine. What is the AMA of the machine?

Section 14.3 Assessment

2. How can 2 machines appear identical and yet not have the same AMA?

Section 14.3 Assessment

3. What information would you use to calculate the efficiency of a machine?

Section 14.3 Assessment

4. When is the IMA of a machine greater than 1?

Section 14.3 Assessment

5. Suppose you are an inventor in 1900. You are constructing a bicycle of your own design. What could you do to ensure your bicycle efficiently changes the work input into forward motion?

Section 14.3 Assessment

6. You have just designed a machine that uses 1000 J of work from a motor for every 800 J of useful work the machine supplies. What is the efficiency of your machine?

Section 14.3 Assessment

7. If a machine has an efficiency of 40% and you do 1000 J of work on the machine, what will be the work output of the machine?

Section 14.3 Assessment

8. A construction worker moves a crowbar through a distance of 0.5 m to lift a load 0.05 m off the ground. What is the IMA of the crowbar?

Section 14.3 Assessment

9. The IMA of a simple machine is 2.5. If the output distance of the machine is 1 m, what is the input distance?

CHAPTER 14 : WORK, POWER, AND MACHINES

14.4 Simple Machines

What are the 6 types of simple machines?

Lever

Wheel & Axle

Inclined Plane

Wedge

Screw

Pulley

Levers

Define lever:

A rigid bar that is free to move around a fixed point

Define fulcrum:

The fixed point the bar rotates around

Levers are classified into 3 categories based on the locations of what?

The locations of the input force, output force, and fulcrum

Define input arm:

The distance between the input force and fulcrum

Define output arm:

The distance between the output force and the fulcrum

How do you calculate the IMA of a lever? Divide the input arm by the output arm

First-Class Levers

Where is the fulcrum located on a first-class lever? Between the input and output force

Draw and label a sketch of a first-class lever

What is the MA of a first-class lever? Greater than, equal to, or less than 1

Name some examples of simple machines that use first-class levers:

Teeter-totter Scissors

Second-Class Levers

Where is the output force located on a second-class lever? Between the input force and fulcrum

Draw and label a sketch of a second-class lever:

What is the MA of a second-class lever? Greater than 1 Requires less force, but more distance

Name some examples of simple machines that use second-class levers:

Wheelbarrow Nut cracker

Third-Class Levers

Where is the input force located on a third-class lever? Between the fulcrum and the output force

Draw and label a sketch of a third-class lever:

What is the MA of a third-class lever? Less than 1 The output distance is greater then the input distance

Name some examples of simple machines that use third-class levers: Broom Hockey stick Baseball bat Golf club

Wheel & Axle

Define a wheel and axle: A simple machine that consists of 2 disks or cylinders,

each w/ a different radius

Draw and label a sketch of a wheel and axle:

How do you calculate the IMA of a wheel and axle?

Divide the radius (diameter) of the input force by that of the output force

What is the MA of a wheel and axle?

Greater or less than one

Depends which end is the input

Name some examples of simple machines that use a wheel and axle: Steering wheel and column Screwdriver

Inclined Planes

Define an inclined plane: Slanted surface along which a force moves an object

to a different elevation

Draw and label a sketch of an inclined plane:

How do you calculate the IMA of an inclined plane? The distance along the plane divided by the change in

height

What is the MA of an inclined plane? Greater than 1

Name some examples of simple machines that use inclined planes: Moving ramp Wheelchair ramp Switchbacks on a mountain

Wedges

Define a wedge: A V shaped object whose sides are 2 inclined planes

sloped towards each other

Draw and label a sketch of an wedge:

What has a greater IMA:

A thin wedge 8 cm long?

A thick wedge 8 cm long?

Name some examples of simple machines that use wedges:

Knife blade

Zipper

Axe blade

Screws

Define a screw: An inclined plane wrapped around a cylinder

Draw and label a sketch of a screw:

What has a greater IMA:

A screw with close-together threads?

A screw with far-apart threads?

Name some examples of simple machines that use screws: Screws

Nuts

Bolts

Corkscrew

Pulleys

Define a pulley: A simple machine that consists of a rope that fits into

a groove in a wheel

How do you find the IMA of a pulley or a pulley system? It is equal to the number of rope sections supporting

the load being lifted

Fixed Pulleys

Describe a fixed pulley: Wheel attached at a fixed location Rotates in place Direction of force is changed, but the size of the force

is not

Draw and label a sketch of a fixed pulley:

What is the MA of a fixed pulley? Always 1 IF = OF (disregarding friction)

Name some examples of fixed pulleys: Flag pole Mini-blinds

Moveable Pulleys

Describe a moveable pulley: The pulley is attached to the object being moved

Draw and label a sketch of a moveable pulley:

What is the MA of a moveable pulley? Greater than 1

Name some examples of moveable pulleys: Sails on a ship Skyscraper window washers’ platforms

Pulley Systems

Describe a pulley system: Fixed and moveable pulleys together

Draw and label a sketch of a pulley system:

What is the MA of a pulley system? Greater than 1 Even larger than a moveable pulley

Name some examples of pulley system: Train car lifters

Compound Machines

Define a compound machine: A combination of 2 or more simple machines that

work together

Name some examples of compound machines: Car engine Washing machine Watch Scissors …