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Work and Simple Work and Simple MachinesMachines

What is work?What is work?In science, the word In science, the word workwork has a has a

different meaning than you may different meaning than you may be familiar with.be familiar with.

The scientific definition of work The scientific definition of work is: is: using a force to move an using a force to move an object a distanceobject a distance (when both (when both the force and the motion of the the force and the motion of the object are in the object are in the samesame direction.)direction.)

Work or Not?Work or Not? According to the

scientific definition, what is work and what is not? a teacher

lecturing to her class

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

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What’s work?What’s work? A scientist delivers a speech to an A scientist delivers a speech to an

audience of his peers. audience of his peers. A body builder lifts 350 pounds A body builder lifts 350 pounds

above his head. above his head. A mother carries her baby from A mother carries her baby from

room to room.room to room. A father pushes a baby in a A father pushes a baby in a

carriage. carriage. A woman carries a 20 kg grocery A woman carries a 20 kg grocery

bag to her car?bag to her car?

What’s Work?A scientist delivers a speech to an

audience of his peers. NoA body builder lifts 350 pounds

above his head. YesA mother carries her baby from

room to room. NoA father pushes a baby in a

carriage. YesA woman carries a 20 kg grocery

bag to her car? No

Formula for work

Work = Force x Distance Work = (newton x meter) One newton-meter is equal to

one joule (1 J)The unit of work is a joule

(J)

W = F x dWork = Force x

Distance

CalculateCalculate: If a : If a man pushes a man pushes a concrete block 10 concrete block 10 meters with a meters with a force of 20 N, force of 20 N, how much work how much work has he done?has he done?

W= F x dW= F x dWork = Force x Work = Force x

DistanceDistanceCalculateCalculate: If a man : If a man

pushes a concrete pushes a concrete block 10 meters with block 10 meters with a force of 20 N, how a force of 20 N, how much work has he much work has he done? done? W= f x dW= f x d

W = 20N x 10mW = 20N x 10m

W= 200 W= 200 JoulesJoules

PowerPower is the rate at which work

is done. Power = Work (Joules) Time (second)The unit of power is the watt.

Named by James Watt. He was trying to find a way to compare how much power his

steam engine produced, compared to a known unit. At the time people used horses.

Check for Understanding

1.A man applies a force of 500 N to push a truck 100 m down a street. How much work does he do?

And the answer is…..

Set up your formula:Set up your formula:

Work = force x distance

= 500N x  100 m

= 50,000 J

In which situation do you do In which situation do you do more work?more work?

a. You lift a 75 N bowling ball 2 m off the floor.

b. You lift two 50N bowling balls 1 m off the floor.

A. Work = force x distance = 75 N x 2m = 150 J

B. Work = force x distance = 50N x 1m = 50 J per ball (= 100 J for both)

So you do more work lifting the 75 N bowling ball 2m off the floor- A is

correct!

Review for PowerReview for Power

You and a friend together apply a force of 1000N to a 3000N car to make it roll 10m in 1 minute 40 seconds.

1.What was your work?

2.What was your power??

Answer to Power questionW = f x dW = f x d = 1000N x 10m= 1000N x 10m = 10,000 J= 10,000 J P = work /tP = work /t = = 10,000 J10,000 J 100 sec100 sec = 100 watts= 100 watts

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 MachinesAncient people invented

simple machines that would help them overcome resistive forces and allow them to do the desired work against those forces.

Simple MachinesAll machines come from six simple

machines. They are:

Lever Wheel and Axle Pulley Inclined Plane Wedge Screw

Simple MachinesA machine is a device that helps A machine is a device that helps

make work easier to perform by make work easier to perform by accomplishing one or more of accomplishing one or more of the following functions: the following functions: transferring a force from one transferring a force from one

place to another, place to another, changing the direction of a changing the direction of a

force, force, increasing the magnitude of a increasing the magnitude of a

force, or force, or increasing the distance or increasing the distance or

speed of a force.speed of a force.

Mechanical AdvantageIt 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.

Mechanical AdvantageMA = output force input force

Ex.If you apply an input force of 5 pounds to wall with your hammer but the output force is 25 pounds onto the nail, the machine has a mechanical advantage of 25 or 5.

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The LeverA lever is a rigid

bar that rotates around a fixed point called the fulcrum.

The bar may be either straight or curved.

Levers are used to apply a force to a load.

The 3 Classes of LeversThe class of a

lever is determined by the location of the fulcrum, load and input force.

First Class LeverIn a first-class lever the fulcrum is

located between the input force and the load.Examples of first-class levers include crowbars, scissors, pliers, and seesaws.

A first-class lever always changes the direction of force (ex. a downward input force on the lever results in an upward movement of the output force).

Fulcrum is between input and the load. Fulcrum is between input and the load. Multiplies output force and Multiplies output force and changes its changes its

directiondirection..

Second Class LeverWith a second-class lever, the load is located between the fulcrum and the input force.

Examples: nutcrackers, wheelbarrows, doors, and bottle openers.

Load is between fulcrum and input Load is between fulcrum and input force. Multiplies input force, but force. Multiplies input force, but does not change its directiondoes not change its direction

Third Class LeverThird Class LeverWith a third-class lever, the input force is applied between the fulcrum and the load.Examples of third-class levers include tweezers, hammers, and shovels.

A third-class lever does not change the direction of force and it does not increase the input force.

Input force is between fulcrum and load.

Does not multiply the input force Multiplies the distance through

which the output force is exerted.

Wheel and AxleThe wheel and axle

is a simple machine consisting of two circular objects of different sizes. The axle is the smaller of the two objects.

When either the wheel or axle turns, the other part also turns.

PulleyPulley A pulley consists of a grooved

wheel that holds a rope or cable. A load is attached to one end and the input force is applied to the other..

Two kinds: Fixed or movable. They can combine to make a block and tackle.

Fixed- CHANGES direction of force, does NOT increase it.

Movable- does NOT change direction of force but DOES increase force

Inclined Plane

An inclined plane is an straight sloping surface. The inclined plane makes it easier to move a weight from a lower to higher elevation.

WedgeWedgeA wedge is a

double inclined plane that moves. When you move a wedge through a distance you apply force on an object. Greater distance= greater force.

Examples-doorstop, chisels, axe head, knife

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ScrewScrew

The screw is an inclined plane wrapped in a spiral.

While this may be somewhat difficult to visualize, it may help to think of the threads of the screw as a type of circular ramp (or inclined plane).

Here is how…

Remember:MACHINES DO NOT “SAVE”

WORK THEY ONLY MAKE IT A WHOLE LOT EASIER TO

DO!!

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Practice QuestionsPractice Questions1. Explain who is doing more work and why: a bricklayer carrying 1. Explain who is doing more work and why: a bricklayer carrying bricks and placing them on the wall of a building being bricks and placing them on the wall of a building being constructed, or a project supervisor observing and recording the constructed, or a project supervisor observing and recording the progress of the workers from an observation booth. progress of the workers from an observation booth. 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.

2. How much work is done in pushing an object 7.0 m across a 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 floor with a force of 50 N and then pushing it back to its original position? How much power is used if this work is done in 20 sec? position? How much power is used if this work is done in 20 sec? Work = 7 m X 50 N X 2 = 700 N-m or J; Power = 700 N-m/20 sec = 35 W

3. Using a single fixed pulley, how heavy a load could you lift?3. Using a single fixed pulley, how heavy a load could you lift?Since a fixed pulley has a mechanical advantage of one, it will only change the direction of the force applied to it. You would be able to lift a load equal to your own weight, minus the negative effects of friction.

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Practice QuestionsPractice Questions4. 4. Give an example of a machine in which friction is both an Give an example of a machine in which friction is both an

advantage and a disadvantage. advantage and a disadvantage. One answer might be the use of a car jack. Advantage of friction: It allows a car to be raised to a desired height without slipping. Disadvantage of friction: It reduces efficiency.

5. Why is it not possible to have a machine with 100% efficiency? 5. Why is it not possible to have a machine with 100% efficiency? Friction lowers the efficiency of a machine. Work output is always less than work input, so an actual machine cannot be 100% efficient.

6. What is effort force? What is work input? Explain the relationship 6. What is effort force? What is work input? Explain the relationship between effort force, effort distance, and work input. between effort force, effort distance, and work input. The effort force is the force applied to a machine. Work input is the work done on a machine. The work input of a machine is equal to the effort force times the distance over which the effort force is exerted.