Work and Energy

What is work?

In your study of motion, you have learned that forces can cause motion. But in some cases, a force that is applied is balanced by another opposite force, and there is no net motion as a result. Look at the following illustrations, and identify the forces and motion in each one. (See illustrations on following slide.)1. In one drawing, no motion is likely to occur. Which drawing is it?2. Describe the forces that are acting in this diagram. If the person exerts slightly more force, what happens to the opposite force? Does it increase to match the new force of the person, stay the same, or decrease?

Chapter 12

Work• Work: when a force causes change in the

position or direction of an object– The object will move in the direction of the force– Work = force x distance– W = F x d– Measured in Nm also known as Joules– 1 Nm = 1 J = 1 kg m2/s2

– You may apply a lot of force to try and move the object, but if the object does not move, then you have not done any work –in the physics sense. (Although it may feel like you have done work, unless it moves, you haven’t done work)

The object must move or change direction to have had work done to it. Is holding a book above your head doing

work?

Ex 1: A crane uses an average force of 5200 N to lift a girder 25m. How much work does the crane do on the girder?

Ex 2: While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar 0.800 m. How much work does he do in 30 strokes?

Power• Running up a flight of stairs does not

require more work than walking up slowly does, but it is more exhausting.

• The amount of time it takes to do work is an important factor when considering work and machines.

• The quantity that measures work in relation to time is POWER

• Power = work P = Wtime t

Power = work P = W time t

How are power and work related, if time is constant?

How are power and time related, if work is constant?

Power is measured in Watts (W) (or hp- horsepower)

A watt is the amount of power required to do 1 J of work in 1 second.

Problems: W = F x d P = W t

1. It takes 100 kJ of work to lift an elevator 18

m. If this is done in 20 s, what is the average power of the elevator during the process?

2. Anna walks up the stairs on her way to class. She weighs 565 N and the stairs go up 3.25 m vertically. Calculate the power output if she climbs the stairs in 12.6 seconds.

3. What if Anna climbs the stairs in 10.5 seconds, what would be her power output?

MACHINE

Any device that makes work easier

You may not think of a door as a simple machine, but it is one. It functions like a lever. Like other levers, when you exert a force on it (an input force), a force is exerted along the entire door (the output force).

Chapter 12

1. For all levers, one point along the lever stays still while the rest of the lever moves. This point is called the fulcrum. Where is the fulcrum of a door?2. You can push at any point along the width of a door and it will open. Which position requires the least force: pushing the door near the hinges, in the middle, or near the side farthest from the hinges? (Hint: Which of these feels easiest to do?)3. If you are trying to prop the door open, but your only doorstop is not very heavy, is it likely to work best near the hinges, in the middle, or near the side farthest from the hinges?

Machines and Mechanical Advantage

Which is easier, lifting a car yourself or using a jack?

Machines can be used to take advantage of the fact that force and distance are inversely proportional. So increasing one will decrease anotherThe longer the distance, the less force

needed to do the same work

Machines do not increase the quantity of work that one can do

Force and Work

Chapter 12

Why is it easier to push a box up a ramp to a truck, rather than lift it up to a truck?Because you are increasing the distance,

thus lowering the force needed to do the same work

Machines

Machines help us to do work by redistributing the work that we put into them.Machines can change the direction of an

input forceMachines can increase or decrease the

force by changing the distance

Some machines amplify force and some amplify distance and thus speed.

A baseball bat is a machine that increases speed by increasing the distance

In other words

Make work easier by redistributing the work

Change direction Increase or decrease force by changing the

distance over which it is applied

Work in = Work out

Fin din = Foutdout

If force decreases, distance increases so the work remains equal

Mechanical Advantage

M. A. is a ratio that measures how much a machine multiplies force or distance

M.A. = Input distance = Output force Output distance Input force

All of the calculations that you will learn for the MA of different machines are variations of this equation!

EXAMPLE OF WHAT A MACHINE DOES FOR US

Using a screwdriver to open a can of paint…

Changes direction of forceChanges size of force

Simple Machines

The most basic machines are called simple machines

There are six simple machines

Other machines are just combination of the six simple machines

Two families of simple machinesThe lever family and the inclined plane

family

A simple machine does the work with only one movement

Advantages?

Changes the force you exert: In size

In direction In both size and direction

6 TYPES of SIMPLE MACHINES

(in 2 categories)

LEVER WHEEL AND AXLE PULLEY

INCLINED PLANE SCREW WEDGE

The lever family• Simple lever, pulley, wheel and axle are

the three types of simple machines in the lever family

• Simple lever: Like a hammer pulling out a nail– All levers have a rigid arm that turns around a

point called a fulcrum (the pivot point)– Force is transferred from one part of the arm

to another

LEVER

A lever is a simple machine.  It is a board or bar that rests on a turning point.  This turning point is called the fulcrum.  An object that a lever moves is called the load.  The closer the object is to the fulcrum, the easier it is to move.  

Three parts of a lever

FULCRUM…the pivot point

OUTPUT …resistance, the load, transmitted out of the machine

INPUT …effort, the force you put into the machine

Types of LeversLook for which of the three parts of the lever is in between the other two.

1st Class Fulcrum is in between2nd Class Output is in between3rd Class Input is in between

Mechanical Advantage = input length/output length

MA=Lin/Lout (measure from fulcrum to input and from fulcrum to output)

First Class Lever1st Class lever: the fulcrum is center, input at

one end and output at the other. They either multiply force or increase distance (hammer). The fulcrum is between the effort and the loadFulcrum is in between input and output

See-saw, rowboat oar

MA can be greater

or less than 1

Second Class Lever2nd Class Lever: The fulcrum is at one end and input force is at the other so as to multiply force. The output force is in the middle (wheelbarrow). The load is located between the fulcrum and the effort. The fulcrum is at one end and the load is in the middle..the effort is at the opposite end.Output in between fulcrum and input

Wheelbarrow, bottle opener, nutcracker

MA is always

greater than 1

Third Class Lever3rd Class Lever Input is in between output and fulcrum

Hockey stick, tweezersThe fulcrum is at one end and the input force is

in the middle. The output force is at one end. They always increase distance. (the human arm) The effort is between the fulcrum and the load. The fulcrum is at one end and the load is at the other….the effort is in the middle

MA is always

less than 1

Used to increase distance

Or increase speed

Pulleys: are modified leversThe point in the middle of the pulley is like the

fulcrum….it is the pivot pointPulleys are like 1st class levers because the

“pivot” point is in the center….between the input and the out…between the effort and the load/strength.

Pulleys can be added together to amplify the advantage

The Lever Family

PULLEYThis simple machine is made up of a wheel and a rope.  The rope fits on the groove of the wheel.  One part of the rope is attached to the load.  When you pull on one side of the pulley, the wheel turns and the load will move.  Pulleys let you move loads up, down, or sideways.  Pulleys are good for moving objects to hard to reach places.  It also makes the work of moving heavy loads a lot easier. MA= # pulleys if pulling downMA= #pulleys + 1 if pulling up 

EXAMPLES OF PULLEYS

Flag Poles

Clothes Lines

Sailboat

Blinds

Crane

The Lever Family

Wheel and Axle: lever or pulley attached to a shaftLike a steering wheel

When the wheel is turned, the axel also turns. When a small force is applied to turn the wheel,

the force is multiplied to become a large output force applied to the steering column, which turns the front wheels of the car.

Screwdriver and cranks

WHEEL AND AXLE

The wheel and axle is another simple machine.  The axle is a rod that goes through the wheel.  This lets the wheel turn.  It is easy to move things from place to place with wheels and axles. Gears are a variation in which wheels move together because they are connected by teeth or a chain.

MA of wheels=dw/da=rw/ra

MA of gears=de/dr (the effort is often the larger gear)

EXAMPLES OF WHEEL AND AXLE

Cars

Roller Skates

Wagons

Door Knobs

Gears in Watches, Clocks, and Bicycles

The Inclined Plane Family• An inclined plane, a ramp

• A wedge: modified inclined plane

• A screw: an inclined plane wrapped around a cylinder

Inclined Plane Family

INCLINED PLANEA simple machine that is a flat surface that is higher on one end.  You can use this machine to move an object to a lower or higher place.  Inclined planes make the work of moving things easier.  You would need less energy and force to move objects with an inclined plane. 

MA=Lslope/hslope

EXAMPLES OF INCLINED PLANES

Ramp

Slanted Road

Path up a Hill

Slide

Materials move along an inclined plane

Ramp

• An inclined plane : ramp– Changes magnitude and direction of force– Pushing a box up a ramp requires less

force than lifting it directly.– The work is spread over a greater distance

Wedge

Modified inclined plane– Turns downwards force into two forces

directed out to the sides, like a nail

WEDGEA wedge is a simple machine used to push two objects apart.  A wedge is made up of two inclined planes.  These planes meet and form a sharp edge.  This edge can split things apart. A wedge moves through material to transfer a downward force sideways.

MA: the thinner the wedge, the greater the MA  

EXAMPLES OF WEDGES

Knives

Axes

Forks

Nails

A Screw

• An inclined plane wrapped around a cylinder– Jar lids and spiral stair cases are examples– Gentle slopes of the threads of a screw

make it easier because it requires less force.

SCREW

  A screw is a simple machine that is made from another simple machine.  It is actually an inclined plane that winds around itself.  A screw has ridges and is not smooth like a nail.  Some screws are used to lower and raise things.  They are also used to hold objects together. MA: the smaller the pitch,

the greater the MAPitch is the distance between threads

EXAMPLES OF SCREWS

Jar Lids

Light Bulbs

Stools

Clamps

Jacks

Wrenches

Key Rings

Spiral Staircase

Types of Simple Machines Review

What type of simple machines are these?

• Scissors:• Hammer:• Boat Oar:• Can opener:• Flag Pole: • Bottle opener: • Door knob:• Axe:• Jar:• Tweezers:

Compound Machines

Many of the devices that we use are a combination of more than one simple machine

Car Jack: uses a lever and a screw

Bicycle: uses a variety of simple machines

A compound machine

Any device that uses more than one simple machine

Example: Bicycle

Wheels and axles Levers Screws

A compound machine

Any device that uses more than one simple machine

Example: Bicycle

Wheels and axles Levers Screws

In the chapter on matter, you learned that energy is conserved. Instead of being created or destroyed, it is just changed from one form to another. The energy of the sunlight that reaches Earth is the ultimate source of most of the energy around us. Look at the illustration below, and identify the types of energy involved.

1. How did energy from sunlight provide the energy the girl needed to swing the bat? (Hint: What do you need to have energy?)2. When the girl hits the ball, she exerts a force on it. Does she do work on the ball in the scientific sense of the term? Explain why.3. After the girl hits the ball, the ball moves very fast and has energy. When the ball hits the fielder’s glove, it stops moving. Given that energy can never be destroyed but merely changes form, what happened to the energy the ball once had? (Hint: If you are the fielder, what do you hear and feel as you catch the ball?)

You give yourself and your sled gravitational potential energy as you pull your sled to the top of a snowy hill. You get on board your sled and slide to the bottom of the hill, speeding up as you go.

1. When does the sled have the most potential energy? When does it have the least potential energy?

2. Where does the sled have the most kinetic energy? the least kinetic energy?

3. What happens to the relative amounts of potential and kinetic energy as the sled slides down the hill? What happens to the total energy?

4. After the sled reaches the bottom of the hill, it coasts across level ground and eventually stops. What happened to the energy the sled had?

Energy and WorkWhen you stretch a sling shot, you do work and you transfer energy to the elastic band. The elastic band then does work on the rock by transferring energy.

Energy can not be created or destroyed

Energy can be transferred

Energy can be defined as the ability to do work, so both use Joules as the unit

Energy can be present but undetected. It may only get noticed when it is actually transferred.

Potential energy (PE): energy of position or energy that is stored and unusedElastic PE: energy stored in stretch or

compressed elastic materialGravitational PE: any two objects separated

by a distance (like an apple falling from a tree- the greater the height, the greater the PE)

Kinetic energy (KE): energy of motion or energy that is usedKE depends on the mass and speed of the

objectThe faster an object is going the more KE it

hasKE = ½ mv2

The units are joules (J)KE depends on speed more than mass which

is why it is squared. This is why a car crash at high speeds is so much

more dangerous than at lower speeds despite the mass being the same.

Increasing temperature will increase movement and thus increase Kinetic Energy

Also, the more Kinetic Energy you have the higher the temperature. KE ↑ b/c Temp ↑

Problems

1. Calculate the Kinetic energy of a 1500 kg car moving at 29 m/s.

2. Calculate the kinetic energy of 2000 kg car moving at 13 km/hr.

3. A 35 kg child has 190 J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill?

Other Types of EnergyMechanical: the amount of work an object could do based off of the object’s potential and kinetic energy.

Chemical Energy: the energy from chemical reaction.

Electrical Energy: results from a flow of charged particles through conductive materialsMoving electrons can cause light or magnetic

fields

Flow of Energy• People get energy from living things

– We eat sugars and fats and carbs to get energy

• Living things get energy from the Sun– This energy travels through electromagnetic radiation

known as ultraviolet and visible light– Photosynthesis: when plants use energy from the sunlight

and convert it to chemical energy, which is stored as sugars

• The Sun gets energy from nuclear reactions.– Nuclear reactions are a form of potential energy– Fusion: when two nuclei are combined or fused to form a

heavier nucleus– Fission: when a heavy nucleus is split into two lighter

nuclei

Energy can be transferred• But not created or destroyed!• If total energy cannot be changed then when KE goes up, PE

goes down. • KE and PE are inversely related• TE = KE + PE• This is how a tennis ball can bounce

– If you drop a tennis ball it will bounce up to the height it was dropped (in a perfect world)

– If you throw the ball downward, the KE will be transferred into elastic PE as it compresses and then back to KE as it bounces back up to you.

– Mechanical energy can turn into sound energy or heat energy, thus a bouncing ball will not return to its original height.

– Friction and air resistance can also be a source of transferred energy

Transfer of energy can result in loss of work:

Because of friction and other factors, only some of the work done by a machine is applied to the task at hand. Some may be “lost” or transferred to some other form of energy.

There is a difference between the total work and “useful” work

Can something be in perpetual motion? Why?