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Work and Machines

- Chapter 5
- Sections 1-3

C5- Work Machines

- Section 1- Work slides 3-13
- Section 2- Using Machines slides 14-30
- Section 3- Simple Machines slides 31-50

Section 1- Work

- What Youll Learn
- What work is
- How work energy are related
- How to calculate work power

Work

- Work is done when a force causes an object to

move in the same direction that the force is

applied. - When a force makes an object move, energy is

transferred from one object to another.

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How do you know if you are doing work?

- Two things must happen
- A force must be applied to the object
- The object must move in the same direction as the

force

What does direction have to do with work?

- When you pick up a stack of books, your arms

apply a force upward. - You start walking with the books.
- The direction of motion has changed your arms

are no longer working, instead your legs supply

the forward direction

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Work Energy

- Energy is the ability to cause change or to do

work. - When work is done energy is transferred from one

object to another.

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Calculating Work

- You multiply force times distance to calculate

work. - Work (joules) force (N) X distance (m)
- WFd
- Calculate the amount of work a painter does when

he lifts a can of paint weighing 40 newtons 2

meters. - WFd(40N) (2m) 80 J

When is work done?

- When a pitcher throws a ball to the catcher, he

applies force to the ball only when it is in his

hand. - The ball moves 10 m after it leaves his hand but

work occurred only while it was in his hand for

1m.

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Power

- Power is the rate at which work is done.
- Something has more power if it can do the same

amount of work in less time.

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How do you calculate power?

- To calculate power, divide the amount of work

done by the time it takes to do it. - Power (in watts) work (in joules)/time(s)
- PW/t The SI unit for power is the watt.
- Find the power of the machine that can do 5,000

joules of work in 20 seconds. - PW/t5000J/20 s250 watts. The power of the

machine is 250 watts.

How is power calculated when energy is

transferred?

- You can also calculate power by dividing the

amount of energy transferred by the time it took

to transfer it. - Power (in watts)energy transferred (j)/ time (s)

or PE/t.

How is energy transferred when no work is done?

- Suppose a light bulb changes electrical energy

into light heat at a rate of 100 j /2 s. How

many watts of power will the light bulb have? - PE/t 100j/2s 50 watts

Section 2- Using Machines

- What Youll Learn
- How machines make work easier
- How to calculate mechanical advantage
- How to calculate the efficiency of a machine

What is a machine?

- A machine is a device that makes work easier.
- Knives, scissors, and doorknobs are simple

machines. - Engines are more complex machines

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Making Work Easier

- There are 3 ways machines make work easier
- Increasing force on an object
- Increasing the distance over which a force is

applied or - Changing the direction of an applied force.

How can force be increased using a machine?

- If Work force X distance, then either force or

distance increases. - If distance increases, then force decreases.
- Machines such as a car jack, increase the

distance a force is applied thus reducing the

amount of force needed to do the same amount of

work.

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How does increasing distance decrease force?

- A rake makes the work easier by increasing the

distance over which you apply force. - Your hands move a small distance at the top of

the handle while the handle moves across a wider

distance.

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Why do you want a machine that will change

direction?

- Sometimes its easier to apply force in a

different direction - To raise a flag up a pole, its easier to pull

down on a rope than to climb to the top. - When you use an axe to split wood, a downward

force on the handle changes into a sideways force

by the blade.

The Work Done by Machines

- Pushing down on the handle raising the lid.
- You are doing work on the crowbar that opposes

the friction of the nails in the lid and crate.

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What are input forces output forces?

- A machine changes the way you do the work making

it easier. - The force you apply to the machine is called the

input force or Fin. - The force applied by the machine is called the

output force or Fout.

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What are input work output work?

- When you use a machine there are 2 kinds of work
- Input work or Win , done by you
- Output work or Wout, done by machine

How do machines use conservation of energy?

- When you do work on a machine, you transfer your

energy to the machine which then transfers energy

to the object. - So, Wout is never greater than Win.
- However, a machine does not transfer all its

energy to the object. Due to friction, some of

the energy changes to heat. - This means that Wout is always smaller than Win.

What is an ideal machine?

- If you could build perfect machine without any

friction, the input work would equal the output

work. - WinWout or FinX dinFoutX dout
- If the machine could increase the input force,

then work output would be greater than the work

input.

Mechanical Advantage

- Some machines make work easier by making the

output force greater than the input force. The

number of times the applied force is increased by

a machine is called the mechanical advantage (MA)

of the machine. Mechanical advantage is the

ratio of the output force to the input force. MA

Fout/Fin

Mechanical Advantage

- Using a pulley system you only need 300 N to lift

a piano that weighs 1500 N. What is the MA? - MA Fout/Fin 1500 N/300 N 5
- Notice this ratio cancels out units of newtons.

What is ideal mechanical advantage?

- The MA of a machine without friction is called

the ideal MA or IMA. - You can calculate this by dividing the input

distance by the output distance.

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Efficiency

- Some of the energy put into a real machine is

changed into heat by friction. - So, the output work of a machine is always less

than the work put into it. - Efficiency is the comparison of the amount of

work put into a machine to the amount of work the

machine puts out. - High-efficiency means less heat from friction!

How do you calculate efficiency?

- Divide the output work by the input work to get a

percentage. - Efficiency Wout/in X 100
- To calculate the efficiency of a machine with Win

of 50 joules a Wout of 40 joules 40/500.8, or

80. - The efficiency of a real machine is always less

than 100 due to friction.

How can machines be made more efficient?

- Reducing friction by adding oil or grease to the

surfaces that rub together fills the gaps between

them so the surfaces slide across each other more

easily.

Section 3- Simple Machines

- What Youll Learn
- Six types of simple machines
- How simple machines make work easier
- How to calculate the ideal mechanical advantage

of simple machines

Types of Simple Machines

- A simple machine is a machine that does work with

only one movement of the machine. - There are six types lever, pulley, wheel axle,

inclined plane, screw, and wedge. - The screw and wedge are different forms of the

inclined plane.

Levers

- A wheelbarrow, a rake a baseball bat are all

examples of levers. - A lever is a bar that pivots, or turns around, a

fixed point called the fulcrum. - The input arm is the distance from the fulcrum to

the point where the input force is applied the

output arm is the distance from the fulcrum to

the point where the lever exerts the output force.

What are the 3 classes of levers?

- The class of a lever is based on the location of

the fulcrum, the input force and the output force.

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First-Class Lever

- The top figure shows a first-class lever with the

fulcrum located between the input and output

forces. - The first-class lever always changes the

direction of the force. - Examples include a crowbar, scissors, and a

seesaw.

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Second-Class Lever

- The middle figure represents a second-class lever

with the output force between the input force and

the fulcrum. - Both input and output forces move in the same

direction. - The wheelbarrow is a classic example.

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Third-Class Lever

- The bottom figure, a third-class lever, shows the

output force is farther away from the fulcrum

than the input force. - The output force is always less than the input

force in a third-class lever such as a baseball

bat, but the advantage is that it increases the

distance over which the output force is applied.

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How is ideal mechanical advantage of a lever

calculated?

- To calculate IMA of any machine, divide the input

distance by the output distance. - For a lever, the input distance is the length of

the input arm output distance is the length of

the output arm. - IMA Lin/Lout

Pulleys

- To raise a sail upward, a sailor pulls down on a

rope wrapped around a pulley. - A pulley is a grooved wheel with a rope, chain or

cable wrapped around it. - Pulleys may be fixed or movable or in systems.

What is a fixed pulley?

- This modified first-class lever changes the

direction of the input force like on a sail or a

flagpole. - An elevator also uses a fixed pulley with a cable.

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What is a movable pulley?

- A movable pulley has one end of the rope fixed

the wheel is free to move. - The movable pulley doesnt change the direction

of the force, but it does decrease the amount of

input force needed to lift the object. MA2

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What is a block and tackle?

- A block and tackle is a system of fixed movable

pulleys used together. - The more sections of the rope a system uses to

pull up an object, the greater the output force

is. IMA of sections

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Wheel and Axle

- Simple machine with an axle attached to the

center of a larger wheel and both turn together. - Doorknobs and ferris wheels are examples.

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What is the IMA of the wheel axle?

- A wheel and axle is a modified lever with the

center of the axle as the fulcrum. - To calculate the IMA of a wheel and axle, use

this equation - IMA radius of wheel (m)/radius of axle (m) or

IMA rw/ra - To increase IMA, simply increase the radius of

the wheel.

How do gears work?

- A gear is a wheel and axle with teeth around the

rim of the wheel. - One gear makes the other turn with the smaller

gear turning more times than the larger one.

Output force direction can be changed with a

gear.

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Inclined Planes

- An inclined plane is a sloping surface that

reduces the amount of force it takes to do work. - Examples include ramps and stairways.

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How does an inclined plane make work easier?

- You do the same work by lifting a box straight up

or pushing it up a ramp. - As the inclined plane becomes longer, the force

needed to move the object becomes less. - The input force is applied over a longer

distance, so it takes less input force. - IMAlength of slope(m)/ height of slope(m)

The Screw

- A screw is an inclined plane wrapped in a spiral

around a post. - The inclined plane forms the threads on the

screw. - Apply force by turning the screw friction holds

it in place. - Examples jar lid, corkscrew, drill bit, light

bulb.

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The Wedge

- A wedge is an inclined plane with one or two

sloping sides. - Like the screw, the inclined plane moves through

the object. Knives are wedges. - The IMA increases as it gets longer thinner.

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Compound Machines

- Some machines, like this can opener, are made of

several simple machines. - Two or more working together are called a

compound machine. - The handles are levers, a wedge pierces the can,

a wheel axle turns to open the can.

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