Work and Simple Machines Chapter 14

I can :

1. Recognize when work is done

2. Calculate how much work is done

3. Explain the relation between work and power.

What is work?

In Science,Work is done when a force causes an object to move in the same direction that the force is applied.

In order for you to do work: Two things must occur:

1.You must apply force on an object.(Force is a push or pull that changes the motion or shape of an object.)

2.The object must move in the same direction as your applied force.

You do work on an object ONLY when the object moves as a result of the force you exerted.

Work involves MOTION, not just effort.

The woman may think she is working by holding the bags. However, if she is not moving, she is NOT doing work because she is not causing something to move.

To do work, how must a force make an Object move?

In the direction of the force.

When the boywalks forward NO WORK is being doneby the upward force that his arms exert.

Work is done only when the FORCE you exert on an object is in the same direction as the object’s motion; Lifting a clothes basket is work, but carrying itwhile walking is not work.

The boys arms DO work when they exert an Upward force on the weights and the weights move Upward.

Whenever you apply force to an object,the object moves in the direction of the force.

True or False:

Applying a force ALWAYS results in work being done.

Force in two directions:Sometimes only part of the force you exert moves an object.

Think about what happens when you push a lawn mower.You push at an angle to the ground.

Only the part of theForce that is in the SAMEdirection as the motion ofthe mower does work.

Calculating Work

Work is done when a force makes an object move.More work is done when the force is increased or the object is moved a greater distance.

In SI units, the unit for work is the joule, named for James Prescott Joule

Work EquationWork ( joules) = force ( newtons ) x distance( meters)

W=Fd

Distance in the work equation is the distance an object moves only WHILE the force is

applied.

Applying Math

Work - A painter lifts a can of paint that weights 40 N a distanceOf 2 m. How much work does she do?

Hint: to lift a can weighing 40 N, the painter must exert aForce of 40 N.

This is what you know: force: F = 40 N distance: d = 2m

This is what you need to find out: work: W = ?J

This is the procedure you need to use:

Substitute the known values F = 40 N and d = 2 mInto the work equation:

W =Fd = (40 N) (2m) = 80 Nm = 80 J

Check your answer by dividing the work you Calculated by the distance given in the problem. The result should be the force given in the problem.

Practice Problems:

1.As you push a lawn mower, the horizontal force is 300 N. If you push the mower a distance of 500 m, how much work do you do?

2.A librarian lifts a box of books that weighs 93 N a distance of 1.5 m. How much work does he do?

Answers:

1.Work = force x distance = 300 N x 500 m = 150,000J

2.Work = force x distance = 93N x 1.5 m = 140 J

What is Power?

Imagine two weightlifters lifting the same amountof weight the same vertical distance. They both dothe same amount of work. However, the amountof power they use depends on how long it took to doThe work.

Power is the rate at which work is done.(How quickly workIs done.)

The weightlifter who lifted the weight in less time is more powerful.

In SI units, the unit of power is WATT, in honor Of James Watt, a nineteenth-century BritishScientist who invented a practical version of the steam engine.

Calculating Power:

Power can be calculated by dividing the amount of WORK DONE by the TIME NEEDED to do the work.

Power Equation power = work (joules) time (seconds)

P = W T

Applying Math

Power - You do 200 J of work in 12 s. How much powerDid you use?

Work : W = 200J ; time: t = 12 sPower: P = ? Watts

P=W =200 J = 17 watts T 12 s

Check your answer by multiplying the power youCalculated by the time given in the problem.The results should be the work given in the problem.

Practice Problems

1.In the course of a short race, a car does 50,000 J ofwork in 7 s . What is the power of the car during the race?

2. A teacher does 140 J of work in 20 s. How much power didUse?

Answers:1.Power = work done/time needed50,000 J/7s = 7,143 watts

2. P =W/t = 140 J/20s = 7 watts

What is Energy? The ability to do work.

Example: I must have energy to run the mile.

Work and Energy

If you push a chair and make it move, you do workon the chair and change its energy. Recall that when something is moving it has energy of motion, or kinetic energy. By making the chair move you increase its kinetic energy.

You also change the energy of an object when you do work and lift it higher. An object has potential energy that increases when it is higher above Earth’s surface. By lifting an object, you do work and increase its potential energy.

Power and Energy:

When you do work on an object you INCREASE the KINETIC energy of the object. Because energy can never be created or destroyed, if the object gains energy then you must lose energy.

When you do work on an object you transfer energy to the object, and your energy decreases. The amount of work done is the amount of ENERGY TRANSFERRED in a certain amount of time.

Sometimes energy is transferred even when no work is done, such as when heat flows from a warm to a cold object. In fact, there are many ways energy can be transferred even if no work is done. Power is always THE RATE at which energy is transferred- the amount of energy transferred divided by the time needed.

Is carrying a book while walking work?Why or why not?

Today we have learned:

1. Work is done when a force causes an objectto move in the same direction that the force is applied.

2. Power is the rate at which work is done.

3. Something is more powerful if it can do a given amount of work in less time.

Review Questions:

1. When a force causes motion to occur in the Same direction in which the force has been applied, we say that _____________has been done.

2. Suppose you are waiting for a train. WhileYou are standing on the platform, your arms are becoming more and more tired from holding heavySuitcases. Are you doing work?

In SI, the unit for work is the ______________.

A.AmpereB.JouleC.NewtonD.Watt

I can:

Explain how a machine makes work easier

Calculate the mechanical advantages and efficiency of a machine.

Explain how friction reduces efficiency.

What is a machine?A machine is simply a device that

makes doing work easier.

What is a Simple Machine? A simple machine

has few or no moving parts.

Simple machines make WORK EASIER.

Even a sloping surface can be a machine.

Mechanical AdvantageMachines do not

decrease the amount of work you need to do.

A machine changes the WAY in which you do work.

You exert a force over some distance.

Input Force The force you apply

on a machine is the INPUT FORCE.

The work you do on the machine is equal to the input force times the distance over which your force moves the machine.

The work that you do on the machine is the input work.

Output Force The machine also

does work by exerting a force to move an object over some distance.

The resistance force.

When using a machine, the output work can never be GREATER than the input work.

When you use a machine, the output work can Never be greater than the input work.

Changing Force Some machines make doing work easier by

reducing the force you have to apply to do work.

This type of machine increases the input force, so that the output force is greater than the input force.

Changing ForceThe number of times a machine

increases the input force is the mechanical advantage of the machine.

Mechanical AdvantageWhat is the advantage of using a

machine?

A machine makes work easier by changing the amount of force you need to exert, the distance over the force exerted, or the direction in which you exert your force.

Mechanical AdvantageNumber of times

the input force is multiplied by a machine-Equation:

MA=F out/F in

EfficiencyThe ability of a machine to convert input

work to output work; calculated as :

efficiency=output work/input work x 100%

eff = W out x 100

W in

Friction To help understand friction, imagine pushing

a heavy box up a ramp.

As the box begins to move, the bottom surface of the box slides across the top of the ramp.

Neither surface is perfectly smooth-each has high spots and low spots.

Friction Reduces efficiency

by converting some work into HEAT.

The efficiency of a real machine is always LESS THAN 100 percent because of friction.

Friction and Efficiency One way to reduce friction

between two surfaces is to add oil.

Oil fills the gaps between surfaces and keeps many of the high spots from making contact.

More input work is converted to output work by machine.

Infer how the output work done by a machine changes when friction is reduced.

Discussion Question:What is the

efficiency of a machine and how can it be improved?

We have learned there are three main advantages to a machine.

1.By changing the amount of force you need to exert.

2 .Changing the distance over which the force is exerted

3. Changing the direction in which you exert the force.

Exit slip:

What is the force that you apply on a machine?

The force that the machine applies is the_________.

Simple Machines

I can statements:

1.Distinguish among the different simple machines2.Describe how to find the mechanical advantage of each machine.

Simple machine does work with only ONE movement; a machine made of a COMBINATIOM of simple machinesis a COMPOUND machine.

The six simple machines are :1. Incline plane2. lever3. Wheel and axle4. Screw5. Wedge6. Pulley

Examples of a compound machine: can opener and bicycle

Inclined Plane

To move limestone blocks weighing more than 1,000 kg each, archaeologists

hypothesize that the Egyptians built

enormous ramps.

A simple machine known as an incline

plane.

Inclined Planes An inclined plane is

a flat surface that is higher on one end (sloped surface)

Inclined planes make the work of moving things easier

Less FORCE is needed to move an object from one height to another using an inclined plane than is needed to lift the object.

As the inclined plane gets longer, the force needed to move the object gets SMALLER.

The mechanical advantage of an inclined plane is the length of the inclined plane divided by its height.

Wedges Two inclined planes

joined back to back. Wedges are used to

split things. Inclined plane that

moves-wedge changes the direction of the applied force.

Example: Your Front teeth

Wedges in Your Body

The teeth of meat eaters, or carnivores,are more wedge shaped than the teeth of plant eaters.

Carnivores use their teeth to cut and rip meat.

Screws

A screw is an inclined plane wrapped around a shaft, cylinder or post.

The inclined plane allows the screw to move itself when rotated.

The mechanical advantage of the screw is the length of the inclined plane wrapped around the screw divided by the length of the screw.( drill bit, screws)

LEVER- any rigid rod or plankThat pivots about a point.

The point about which the lever pivots is FULCRUM

Mechanical advantage-divide the distance from the fulcrum to the INPUT force by the distance from the fulcrum to the OUTPUT force.

Levers can be divided into classes depending on the position of the FULCRUM.

Levers-First Class In a first class lever

the fulcrum is in the middle and the load and effort is on either side

Think of a see-saw

Levers-Second Class In a second class

lever the fulcrum is at the end, with the load in the middle

Think of a wheelbarrow

Levers-Third Class In a third class lever

the fulcrum is again at the end, but the effort is in the middle

Think of a pair of tweezers

Lever Notes:

“F R E”1st Class- Fulcrum is between effort and resistance “ E F R”

2nd Class-Resistance is between effort and fulcrum “ E R F”

3rd Class-Effort is between resistance and fulcrum “R E F “

Wheel and axle- Two circular objects of different sizes that rotate together.

Examples: Steering wheel, doorknob, screw driver

The advantage of a wheel and axle is found by dividing the radius of the wheel by the radius of the axle

In some cases, the input force turns the wheel, and the axle exerts the output force, resulting in a mechanical advantage greater than one’ examples are doorknob, a steering wheel and a screwdriver.

In other cases, the input force turns the axle, and the wheel exerts the output force resulting in a mechanical advantage LESS than one; example Fan and a Ferris wheel

Wheels and Axles The wheel and axle

are a simple machine

The axle is a rod that goes through the wheel which allows the wheel to turn

Gears are a form of wheels and axles

Pulleys Pulley are wheels

and axles with a groove around the outside

A pulley needs a rope, chain or belt around the groove to make it do work

Two types of Pulleys:

1.Fixed 2. Moveable

Fixed Pulleys, such as on window blinds or flagpoles, are attached to an overhead structure and change the direction of the force you exert; they have mechanical advantage of One.

Moveable pulleys are attached to the object being lifted and allow you to exert a smaller force; they have a mechanical advantage of two

Pulley system: a combination of fixed and moveable pulleys

Simple Machines Simple Machines

can be put together in different ways to make complex machinery

What is the difference between a simple machine and a compound machine?

A simple machine makes only ONEMovement; a compound machine is Made up of multiple simple machines And makes more than one movement.