Work & Machines

Open Ended

Name two things in your life that require work. Name two things you would use to do work.

Review

• force

New

•work

•machine

•simple machine

•compound machine

•efficiency

•mechanical advantage

Work and Machines
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Vocabulary

Match

Vocab Pre-Check

• Work

• Machine

• Simple Machine

• Mechanical Advantage

• Efficiency

Force applied through a distance

Change force or increase motion for work

Machine that works with one movement

ratio of input F to output F

how much input W becomes useful output W

• Force applied through a distance

• Change force or increase motion for work

• Machine that works with one movement

• ratio of input F to output F

• how much input W becomes useful output W

What is work?

Work is force applied through a distance.

• Remember that a force is a push or a pull. Work requires both force and motion.

• If you push against the desk and nothing moves, then you haven't done any

work.

• Suppose you give a book a push and it slides along a table for a distance of 1 m

before it comes to a stop.

• Even though the book moved 1 m, you do work on the book only while your

hand is in contact with it.

Work and Machines
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Doing Work

There are two conditions that have to be satisfied for work to be done on an object.

1. the applied force must make the object move.

2. the movement must be in the same (or opposite) direction as the applied

force.

Work and Machines
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If the applied force is opposite to the displacement, the work is negative.
If the applied force is PERPENDICULAR to the displacement, NO WORK IS DONE.

Force and Direction of Motion

When you lift a stack of books, your arms apply a force upward and the books move
upward.

• Because the force and distance are in the same direction, you have done work

on the books.

When you carry books while walking, you might think that your arms are doing work.
However, in this case, the force exerted by your arms does no work on the books.

• The force exerted is upward, but they are moving horizontally

• The force you exert is at right angles to the direction they are moving.

Work and Machines
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Force and Direction of Motion

Work and Machines
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Categorize

Options (3)

a box is lifted off the ground

a box is held in place above the ground

a box is lowered to the ground.

For each of the following three actions, determine if positive, negative, or no work is done on the box:

Positive

Negative

No Work

Calculating Work

The amount of work done depends on the amount of force exerted and the distance
over which the force is applied.

When a force is exerted and an object moves in the direction of the force, the
amount of work done can be calculated as follows:

• One joule is about the amount of work required to lift a baseball a vertical

distance of 0.7 m.

Work and Machines
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Math Response

You push a refrigerator with a horizontal force of 100 N. If you move the refrigerator a distance of 5 m while you are pushing, how much work do you do?

Type answer here

Correct Answer: 500

Deg^°

Rad

Work and Machines
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SOLVE FOR WORK

EVALUATE THE ANSWER

Check to see whether the units match on
both sides of the equation.
units of W = (units of F ) × (units of d)
= N × m = J

Use with Example Problem 1.
Problem
You push a refrigerator with a horizontal
force of 100 N. If you move the
refrigerator a distance of 5 m while you
are pushing, how much work do you do?

Response
ANALYZE THE PROBLEM

KNOWN

applied force: F = 100 N

distance: d = 5 m

UNKNOWN

work: W

SOLVE FOR THE UNKNOWN

•Equation

W = Fd

•Substitute and Solve

W = (100 N)(5 m) = 500 J

Multiple Choice

The SI Unit for Work

• Newton (N)

• Joule (J)

• Second (s)

• Watt (W)

Machines

A machine is a device that changes the force or increases the motion from work.

• Some machines, such as knives, scissors, and doorknobs make doing work

easier.

• Some machines, such as bicycles, increase speed.

• Some machines, such as an axe, change the direction of force.

• Some machines, such as a car jack, increase force.

Work and Machines
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Types of Simple Machines

A simple machine is a machine that does work with only one movement of the
machine.

There are 6 simple machines:

• lever

• pulley

• wheel and axle

• inclined plane

• screw

• wedge

Work and Machines
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Multiple Choice

A machine that does work with only one movement.

• mechanical advantage
• efficiency
• simple machine
• compound machine
• inclined plane

Simple Machines

Multiple Choice

What type of simple machine is this?

• wheel and axle
• gear
• lever
• inclined plane

Compound Machines

A compound machine is a combination of two or more simple machines.

• A pair of scissors is a compound machine that combines two wedges and two

levers.

• A bicycle is also a compound machine made up of two wheel and axle systems.

Work and Machines
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Calculating Efficiency

Efficiency is a measure of how much of the work put into a machine is changed into
useful output work by the machine.

Every machine is less than 100% efficient.

Work and Machines
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Math Response

You do 20 J of work in pushing a crate up a ramp. If the output work from the inclined plane is 11 J, then what is the efficiency of the inclined plane?

Type answer here

Correct Answer: 55

Deg^°

Rad

Work and Machines
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SOLVE FOR EFFICIENCY

EVALUATE THE ANSWER

The work out is about half of the work in. Therefore, an answer close to 50
percent is reasonable.

Use with Example Problem 2.
Problem
You do 20 J of work in pushing a crate
up a ramp. If the output work from the
inclined plane is 11 J, then what is the
efficiency of the inclined plane?

Response
ANALYZE THE PROBLEM

KNOWN

work in: Win = 20 J

work out: Wout = 11 J

UNKNOWN

efficiency: e

SOLVE FOR THE UNKNOWN

•Set Up the Problem

e = Wout

Win × 100

•Solve the Problem

e = 11 J

20 J × 100

e = 55 percent

Mechanical Advantage

The ratio of the output force to the input force is the mechanical advantage of a
machine.

• Two forces are involved when a machine is used to do work. The force that is

applied to the machine is called the input force. Fin = effort force.

• The force applied by the machine is called output force = Fout.

Work and Machines
Copyright © McGraw-Hill Education

Match

Vocabulary Check!

• Work

• Machine

• Simple Machine

• Mechanical Advantage

• Efficiency

Force applied through a distance

Change force or increase motion for work

Machine that works with one movement

ratio of input F to output F

how much input W becomes useful output W

• Force applied through a distance

• Change force or increase motion for work

• Machine that works with one movement

• ratio of input F to output F

• how much input W becomes useful output W

Mechanical Advantage

• Window blinds are a machine

that changes the direction of
an input force.

• A downward pull on the cord

is changed to an upward
force on the blinds.

• The input and output forces

are equal, so the MA is 1.

Work and Machines
Copyright © McGraw-Hill Education

Math Response

A crate weighs 950 N. If you can use a pulley system to lift that crate with a force of only 250 N, then what is the mechanical advantage of the pulley system?

Type answer here

Correct Answer: 3.8

Deg^°

Rad

Describing Energy
Copyright © McGraw-Hill Education

SOLVE FOR MECHANICAL ADVANTAGE

EVALUATE THE ANSWER

The weight of the crate is very close to
four times the force needed to lift the
crate. Therefore, the mechanical
advantage of the crate should be close
to 4. Our answer is close to 4, so it is
reasonable.

Use with Example Problem 3.
Problem
A crate weighs 950 N. If you can use a
pulley system to lift that crate with a force
of only 250 N, then what is the
mechanical advantage of the pulley
system?

Response
ANALYZE THE PROBLEM

KNOWN

output force: Fout = 950 N

input force: Fin = 250 N

UNKNOWN

mechanical advantage: MA

SOLVE FOR THE UNKNOWN

•Set Up the Problem

MA = Fout

Fin

•Solve the Problem

MA = 950 N

250 N

MA = 3.8

Multiple Choice

Stairs

• wedge
• pulley
• inclined plane
• wheel and axle

Multiple Choice

A person pushes a 20 kg box along a flat surface with a constant force of 50 N for 4 meters. What is the work done by the person on the box?

• 50J

• 100J

• 150J

• 200J

Multiple Choice

Identify this simple machine

• Lever
• Wheel & Axle
• Inclined Plane
• Screw

Multiple Choice

If 150 Joules of work is needed to move a box 10 meters, what force was used? 

• 1500 N

• 15 N

• 1.5 N

• 150 N

Multiple Choice

Lightbulb

• wedge
• pulley
• incline plane
• screw

Multiple Choice

Two people are lifting the same weight as shown in the animation. If they both do 10 reps, who will do the most work?

• Bro A

• Bro B

• They both will do the same work

• Can't tell without knowing the weight they are lifting

Multiple Choice

A constant force did 75 J of work on a shopping cart. If the force applied was 25 N and acted parallel to the cart's motion, what was the distance the cart moved?

• 3m

• 50m

• 1875m

• 0.33m

Multiple Choice

If you push against an object and it doesn't move, are you doing work?

• No

• Yes

Multiple Choice

In order for work to be done, what two things need to happen?

• Object needs to move in any direction

• Object needs to have acceleration and speed

• Object must move and must move in the direction of the force applied

• Object needs to feel force, it doesn't matter if it moves

Math Response

A crate is pulled by a physics student with an applied force of 50 N over a distance of 4 m. A 10 N force of kinetic friction opposes the motion of the crate. What is the net work done on the crate?

Type answer here

Correct Answer: 160

Deg^°

Rad

Multiple Choice

When the force is perpendicular to the motion, the work being done is 0.

• True

• False

Multiple Choice

If force and motion are parallel, the work done is...?

• force times the distance

• force

• distance and speed

• force in N

Multiple Choice

A __________ is a combination of two or more simple machines

• Simple Machine

• Compound Machine

• Efficient Machine

• Useful Machine

Multiple Choice

What is the equation for efficiency (%)

• output work divided by input work x 100

• input work divided by output work x 100

• output work divided by net force x 100

• Input work divided by net force x 100

Multiple Choice

Machines change the way work is done in what way?

• Increase speed

• change the direction of a force

• Increase force

• All of the above