Work and Simple

MachinesWhat is work and how do simple machines make work easier?

• In science, the word work has a different meaning than you may be familiar with.

• Scientific Definition of Work: –Using a force to move an object a

distance, when both the force and the motion of the object are in the same direction.

• Work = Force x Distance• A simple machine makes work easier.

What is work?

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Work or Not?• A scientist delivers a speech to an audience

of his peers.

• A body builder lifts 350 pounds above his head.

• A mother carries her baby from room to room.

• A father pushes a baby in a carriage.

• A woman carries a 20 kg grocery bag to her car

• A mouse pushing a piece of cheese with its nose across the floor

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Work or Not?• A scientist delivers a speech to an audience

of his peers.

• A body builder lifts 350 pounds above his head.

• A mother carries her baby from room to room.

• A father pushes a baby in a carriage.

• A woman carries a 20 kg grocery bag to her car• A mouse pushing a piece of cheese with

its nose across the floor

NO

NO

NO

YES

YES

YES

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Formula for work

• The unit of force is Newtons (N)• The unit of distance is meters (m)• The unit of work is Newton-meters (Nm)• One newton-meter is equal to one Joule (J)• So, the unit of work is a Joule (J)

Work(J) = Force(N) x Distance(m)

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Work = Force x Distance

If you push a shopping cart 20 meters with a force of 25 N, how much work have you done? W(J) = F(N) x D(m) 500 Joules

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History of Work

Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.

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Simple Machines

Ancient people invented simple machines that would help them make work easier to do.

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Simple Machines

• A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions: – transferring a force from one place to

another, – changing the direction of a force, – increasing the magnitude of a force, or – increasing the distance or speed of a

force.

The 6 Simple MachinesAll machines are made up of some combination of the simple machines shown.

Lever Pulley

Wheel and Axle

WedgeScrew

Inclined Plane

Let’s review forces

• When a machine is used to do work, two kinds of forces are involved. –Input Force or Effort Force • the force you apply to the machine

–Output Force, Load, or Resistance• the force that the machine applies to the

object to against another force (like friction or gravity)

Forces

Output Force, Resistance, or Load

The size of your push or pull on the machine

Input or Effort Force

The size of the machine’s push or pull on the object

Acts against gravity

150 N

150

N15

0 N

Forces

Output Force, Resistance, or Load The size of the

machine’s push or pull on the object

Acts against gravity

150

N

150 N100 N

Input or Effort Force

The size of your push or pull on the machine

Work = Force x Distance

• Pushing the block of ice up the incline requires only one-fifth the input or effort force that it would take to lift the ice

Work over a distance

• The force exerted on a machine is called the input force, or effort. – This force moves the machine a certain distance,

called the load, resistance or input distance. • The force the machine exerts on an object is called the

output force, load, or resistance.

–This force moves the object a certain distance, called the effort or output distance.

Work over a distance

Output distance

Output distance

Input distance

Input distance

Work = Force x Distance• The man must use five times as much force to

move the block of ice, but the girl must move the block five times farther (up the length of the inclined plane

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Limitations

No machine can increase both the magnitude and the distance of a force at the same time.

When we compare work input to work output (efficiency), we see that no machine is 100 percent efficient, because some output force is lost to friction

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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.

A B C

The amount of work done on an object is the same, with or without a machine

Why do we need simple machines?

• The 6 simple machines make our work easier. • Machines can increase or decrease the force

by changing the distance over which the force is applied. Different amounts of force can do the same amount of work. For example, lifting a heavy box into a truck or sliding it up a ramp into the truck.

• Let’s focus on pulleys and inclined planes…

Inclined Plane•An inclined plane is a flat, slanted surface.•It allows you to use less effort force to move an object. •It makes moving objects easier.

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Inclined Plane

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

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

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Pulley

• A pulley consists of a rope wrapped around a grooved wheel that turns freely in a frame called a block.

• Pulleys reverse the direction of force

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Pulley• Pulleys reverse the

direction of force, making it easier to move an object

• Load and the Input Force move in opposite directions

• It is easier to pull down than to pull up, because when you pull down, you can use your body weight

LOAD

INPUT FORCE

PulleyPulleys are used to raise flags. As the rope is pulled down, the flag goes up.

Pulleys are used for exercise, especially in hospitals, to help patients grow stronger.

PulleysPulleys are used to open and close curtains and blinds.

Pulleys are used to raise and lower sails on sailboats.

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Mechanical Advantage

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

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

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Mechanical Advantage

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

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

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Inclined Plane

• The mechanical advantage of this inclined plane is equal to the length of the slope divided by the height of the inclined plane.

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

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Inclined Plane

• The mechanical advantage of this inclined plane could also be calculated by dividing output (resistance) force by the input (effort) force.

Input Force20 N

Output Force100 N

MA = 100N/20 NMA = 5

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Pulley

• A pulley can be used to simply change the direction of a force or to gain a mechanical advantage, depending on how the pulley is arranged.

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

Work = 7 m X 50 N X 2 = 700 N-m or J

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 back to its original position?

Work is defined as a force applied to an object, moving that object a distance in the direction of the applied force. The bricklayer is doing more work.

Inclined Plane

• Where have we seen inclined planes?• How much force is needed to pull the car

straight up? How far is the car pulled? How much work is done?

• How much force is needed to pull the car up the inclined plane? How far is the car pulled? How much work is done?

Pulleys LabHYPOTHESIS: ?

• As we change the number and arrangement of pulleys, the mechanical advantage will change.

• As the Mechanical Advantage increases, the force required to move the same mass decreases (the same amount of work can be done using less force over a longer distance).

DEPENDENT VARIABLEINDEPENDENT VARIABLENumber and Arrangement of Pulleys

Effort Force/Effort Distance Mechanical Advantage

Mechanical Advantage

• We will first measure the amount of force needed to lift a weight a certain distance without using a pulley, and calculate the amount of work done (Station 1).

• We are interested in how pulleys make work easier, so first we must establish how work is done without a pulley

• Control group allows us to measure the effects of changes made to the independent variable

• Is this step important? • Why?

The Purpose of this Investigation is to illustrate how simple machines (in this case a pulley) make work easier

The purpose of this investigation is to illustrate how simple machines (in this case a pulley) make work easier• We will then measure the Input

Force and Distance and the Output Force and Distance for three pulley systems, and then calculate and compare the amount of work done by each system (Stations 2, 3, and 4).

INPUT FORCE

INPUTDISTANCE

OUTPUT DISTANCE

OUTPUT FORCE

Output = Load = Resistance

Input = Effort

REMEMBER:

The purpose of this investigation is to illustrate how simple machines (in this case a pulley) make work easier

• Using our measurements of Input and Output Force and Input and Output Distance, we will calculate and compare the mechanical advantage for each pulley system.

INPUT FORCE

INPUTDISTANCE

OUTPUT DISTANCE

OUTPUT FORCE

The purpose of this investigation is to illustrate how simple machines (in this case a pulley) make work easier

3. Using our measurements of Input and Output Force and Input and Output Distance, we will calculate and compare the mechanical advantage for each pulley system.

2. We will then measure the Input Force and Distance (effort) and the Output Force and Distance (load) for three pulley systems, and then calculate and compare the amount of work done by each system (Stations 2, 3, and 4).

1. We will first measure the amount of force needed to lift a weight a certain distance without using a pulley, and calculate the amount of work done (Station 1).

Lab on Pulleys - Safety

• When doing this lab:• One person will hold the base of the pulley

stand.• One person will pull the yellow string – straight down.

• One person will measure and record data.

• Jobs will rotate after each station.

Station 1 – Force Needed Without a Pulley• Lift the weight (mass) 30 cm using only the spring scale. • Draw an arrow showing the direction of the force on the

diagram below.• In the box below, record the distance you moved the

spring scale. (Remember we chose this distance) • *Be sure to convert cm to meters!• Read the spring scale and record the force in Newtons in

the box below. • Calculate the amount of work done:• W = d x F

Station 1 – Force Needed Without a Pulley

• Draw an arrow showing the direction of the force on the picture.

• F = • d =

• Calculate the amount of work done:• W (work) = d x F =

10 N30 cm = 0.3 m 10 N (weight,

force of gravity)

10 N (force applied by experimenter)

W (work) = 0.3 m x 10 N = 3 J

3 J3 Nm =

A pulley is said to be a fixed pulley if it does not rise or fall with the load being moved. A fixed pulley changes the direction of a force; however, it does not create a mechanical advantage.

Fixed Pulleys

Station 2 – Single Fixed Pulley

• Use the pulley with the string wrapped over the top once. • Lift the weight (mass) 30 cm by pulling straight down on the spring

scale attached to the yellow string. • *Be sure to hold the base of the stand!• Draw an arrow showing the direction of the force in the box

below. (Your hand creates the force.)• Record the distance you pulled the string in the box below. • *Be sure to convert cm to meters!• Read the spring scale and record the force in Newtons in the box

below. • Calculate the amount of work and mechanical advantage (MA).

Station 2 – Single Fixed Pulley• Draw an arrow showing the

direction of the force on the picture.

• F = • d =

• Calculate the amount of work done:• W (work) = d x F =

10 N

30 cm = 0.3 m

W (work) = 0.3 m x 10 N = 3 J

3 J3 Nm =

10 N (force applied by experimenter)10 N (weight,

force of gravity)

¿30𝑐𝑚30𝑐𝑚 = 1

A moveable pulley rises and falls with the load that is being moved. A single moveable pulley creates a mechanical advantage; however, it does not change the direction of a force.

The mechanical advantage of a moveable pulley is equal to the number of ropes that support the moveable pulley.

Moveable Pulleys

Two or More Pulleys

• When two or more pulleys are connected together, they permit a heavy load to be lifted with less force.

• The trade-off is that the end of the rope must move a greater distance than the load.

Station 3 – Single Fixed Pulley and Moveable Pulley

• Use the pulley with the string wrapped around the top (fixed) and bottom (movable) pulley and then over the top again.

• Lift the weight (mass) 30 cm by pulling the spring scale attached to the yellow string straight down.

• *Be sure to hold the base of the stand!• Draw an arrow showing the direction of the force in the box below.

(Your hand creates the force.)• In the box below, record the distance you pulled the string.• *Be sure to convert cm to meters!• Read the spring scale and record the force in Newtons in the box

below. • Calculate the amount of work and mechanical advantage (MA).

Station 3 – Single Fixed Pulley and Moveable Pulley

• Draw an arrow showing the direction of the force on the picture.

• F = • d =

• Calculate the amount of work done:• W (work) = d x F =

5 N

60 cm = 0.6 m

W (work) = 0.6 m x 5 N = 3 J

3 J3 Nm =

5 N (force applied by experimenter)10 N (weight,

force of gravity)

¿60𝑐𝑚30𝑐𝑚 = 2

Station 4 – Double Fixed Pulley and Movable Pulley

• Use the pulley with the string wrapped around the top (fixed) and bottom (movable) pulley and then over the top twice.

• Lift the weight (mass) 30 cm by pulling the spring scale attached to the yellow string straight down.

• *Be sure to hold the base of the stand!• Draw an arrow showing the direction of the force in the box below.

(Your hand creates the force.)• In the box below, record the distance you pulled the string.• *Be sure to convert cm to meters!• Read the spring scale and record the force in Newtons in the box

below. • Calculate the amount of work and mechanical advantage (MA).

Station 4 – Double Fixed Pulley and Moveable Pulley

• Draw an arrow showing the direction of the force on the picture.

• F = • d =

• Calculate the amount of work done:• W (work) = d x F =

2.5 N

120 cm = 1.2 m

W (work) = 1.2 m x 2.5 N = 3 J

3 J3 Nm =

2.5 N (force applied by experimenter)10 N (weight,

force of gravity)

¿120𝑐𝑚30𝑐𝑚= 4

Which weight requires the least force to move?

INPUT FORCE: 98 N INPUT FO

RCE: 49 N49 N

WEIGHT: 98 N

LOAD: 98 N

WEIGHT: 98 NLOAD: 98 N

If the pulley is fixed, then the force required is equal to the weight. If the pulley is moveable then the force is equal to half of the weight.

Which weight requires the least force to move?

. A – Weight A requires a force equal to 5 Kg whereas weight B requires a force equal to 10 Kg. Remember to divide the weight by the number of sections of rope supporting it to get the force needed to lift the weight.

How much force is required to move the weight?

 

A) 100 kg C) 50 kgB) 150 kg D) 60 kg

C – The weight is 300 Kg and there are 6 sections of rope supporting it. Divide 300 by 6 to get 50 Kg. In all cases, just divide the weight by the number of sections of rope supporting it to get the force needed to lift the weight.

Warm-Up

Fill in the blanks to complete this statement about inclined planes:

As the _________ of the inclined plane increases, the ________ needed to move the object up the inclined plane decreases.Write three sentences explaining how pulleys can change the amount of force needed to move an object.

Forces

Welcome to a world full of machines!

There are machines all around us!

Turn to your neighbor and discuss what kinds of machines you saw on your way to school today.

Interactive Website on Pulleys

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https://fuse.education.vic.gov.au/content/8aeb7e58-da45-4a5d-b0b9-23ff51e6887b/p/index.html