Transcript Work and Energy

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=Fxd
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=W
time
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=Fxd
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
Chapter 12
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).
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 another
 The longer the distance, the less force
needed to do the same work

Machines do not increase the quantity of
work that one can do
Chapter 12
Force and Work
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 force
 Machines 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 force
Changes 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 machines

The 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
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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 Levers
Look for which of the three parts of the
lever is in between the other two.
1st Class Fulcrum is in between
2nd Class Output is in between
3rd 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 Lever
1st 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
load
 Fulcrum is in between input and output
See-saw, rowboat oar

MA can be greater
or less than 1
Second Class Lever
2nd 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 Lever
3rd Class Lever

Input is in between output and fulcrum
Hockey stick, tweezers
 The 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
The Lever Family
Pulleys: are modified levers
The point in the middle of the pulley is like the
fulcrum….it is the pivot point
 Pulleys 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

PULLEY
This 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 down
MA= #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 shaft

Like 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 PLANE
A 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
–
–
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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
WEDGE
A 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 MA
Pitch 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?
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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
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
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Wheels and axles
Levers
Screws
A compound machine
Any device that
uses more than one
simple machine
Example:

Bicycle
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

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 Work
When 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 unused
Elastic PE: energy stored in stretch or
compressed elastic material
 Gravitational PE: any two objects separated
by a distance (like an apple falling from a treethe greater the height, the greater the PE)

Kinetic energy (KE): energy of motion or
energy that is used
KE depends on the mass and speed of the
object
 The faster an object is going the more KE it
has
 KE = 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 Energy
Mechanical: 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
materials

Moving 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?