Transcript lever - wbm-physical

Work and Energy
Physical Science
Chapter 13
Work
Examples?
Scientific definition: Work is the transfer of
energy through motion.
In order for work to take place, a force
must be exerted through a distance.
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Work
In order for work to be done, there has to
be motion, and the motion has to be in the
direction of the applied force.
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Work Equation
Work  force  distance
W  F d
Work, like energy, is measured in joules.
1 J = 1 N ∙ m.
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Example
A student’s backpack weighs 10 N. She
lifts it from the floor to a shelf 1.5 m high.
How much work is done on the backpack?
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You try
A dancer lifts a 400-N ballerina overhead a
distance of 1.4 m and holds her there for
several seconds. How much work is done
on the ballerina?
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You try
A carpenter lifts a 45-kg beam 1.2 m high.
How much work is done on the beam?
Remember that weight equals mass times
acceleration due to gravity.
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Power
Power is the rate at which work is done.
work
power 
time
W
P
t
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Watts
Power is measured in watts, named after
James Watt, who invented the steam
engine.
1 W = 1 J/s
Very small unit, so we often use kW.
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Discuss
1. Define work and power. How are work
and power related?
2. Determine if work is being done in the
following situations:
a. Lifting a spoonful of soup to your mouth
b. Holding a large stack of books motionless
over your head
c. Letting a pencil fall to the ground
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Power
Power is the rate at which work is done.
work
power 
time
W
P
t
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You try
While rowing across the lake during a
race, John does 3960 J of work on the
oars in 60/0 s. What is his power output in
watts?
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You try
Anna walks up the stairs on her way to
class. She weights 565 N, and the stairs
go up 3.25 m vertically.
If Anna climbs the stairs in 2.6 s, what is her
power output?
What is her power output if she climbs the
stair in 10.5 s?
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Machine
A device that makes work easier
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Work and machines
Work is done when a force is exerted
through a distance
Machines make work easier by changing
the size or direction of the force, or both.
Opening a paint can with a screwdriver
Changes size – you can use less force
Changes direction
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Mechanical advantage
The number of times a machine multiplies
the effort force
output force input distance
mechanical advantage=
=
input force output distance
MA 
Foutput
Finput

dinput
d output
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Examples
1. Find the mechanical advantage of a ramp
that is 6.0 m long and 1.5 m tall.
2. Alex pulls on the handle of a claw
hammer with a force of 15 N. If the
hammer has a mechanical advantage of
5.2, how much force is exerted on the nail
in the claw?
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Conservation of energy
You can never get more work out of a
machine than you put in
Win  Wout
Finput  dinput  Foutput  d output
If force increases, distance must
decrease.
Machines often allow you to use less
force, but require you to exert that force
over a larger distance.
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Discuss
Describe how a ramp can make lifting a
box easy without changing the amount of
work that can be done.
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Simple machine
A device that does work with only one
movement
There are six types.
They can be divided into two families
The lever family
o Simple lever
o Pulley
o Wheel and axle
The inclined plane
family
o Simple inclined plane
o Wedge
o Screw
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Levers
Examples
Crowbars
Seesaws
Baseball bat
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Definitions
A lever is a bar that is free to pivot, or
turn, about a fixed point.
A fulcrum is the fixed point of a lever.
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First class levers
The fulcrum is in the middle
Seesaw
crowbar
Effort
force
Output
force
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Second class levers
The output is in the middle, with the
fulcrum and one end and the input at the
other
wheelbarrow
output
force
fulcrum
input
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Third class levers
The input is in the middle, with the fulcrum
at one end and the output at the other
Baseball bat
broom
output
force
Input
force Physical Science chapter 13
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Pulley
grooved wheel with a rope or chain
running along the groove
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Pulley
Acts like a first class lever
The center acts like the fulcrum
See the top of page 440
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Fixed pulley
Attached to something that doesn’t move
Change only the direction of a force
MA of 1
F
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F
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Moveable pulley
Attached to the object being moved
MA greater than 1
Finput
Foutput
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Block and tackle
System of fixed and moveable pulleys
Has MA greater than one
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Wheel and axle
Consists of a wheel and a shaft (or axle)
that rotate together
The input force is applied to the wheel
The shaft exerts the output force
Examples: doorknob, water faucet, gears,
meat grinder
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Inclined plane
A ramp
Lifting something along an inclined plane
means you cover more distance than
lifting it straight up, but you get to use a
smaller force
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screw
An inclined plane wrapped in a spiral
around a cylinder.
As you drive in a screw, the inclined plane
slides through the wood.
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Wedge
Two inclined planes placed back-to-back
Examples
Chisels
Knives
Axe blades
The material stays in place while the
wedge moves through it.
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Compound machine
A combination of two or more simple
machines.
An axe – lever and wedge
Bike – series of wheels and axles
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Discuss
1. Identify the kind of simple machine
represented by each of the following
examples.
a. A drill bit
b. A skateboard ramp
c. A boat oar
2. What class of lever is this?
Input force
3. It is easier to open a door by pushing
near the knob than to open a door by
pushing near the hinges. What class of
lever is a door?
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Energy
Scientific definition: Energy is the ability to
do work or to cause change.
Any sample of matter has energy if it can
produce a change in itself or in its
surroundings.
Energy comes in many forms, including
Radiant, electrical, chemical, thermal, and
nuclear
Energy is measured in joules (J).
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Work and Energy
Work is the transfer of energy through
motion.
When 1 J of work is done on an object, 1 J
of energy has been transferred to the
object.
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Potential energy
Stored energy
Depends on its position or condition
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Gravitational Potential Energy
Potential energy of an object due to height
above the earth’s surface.
The higher the object is, the more potential
energy it has.
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Calculating gravitational potential
energy
PE  mgh
h is relative
oOften measured from the ground, but
it doesn’t have to be
oWe can set h=0 anywhere that is
convenient
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Examples
 Calculate the gravitational potential
energy of the following. Assume h = 0 at
the ground:
1. A 1200 kg car at the top of a hill that is 42 m
high
2. A 65 kg climber on top of Mount Everest
(8800 m high)
3. A 0.52 kg bird flying at an altitude of 550 m
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Kinetic energy
Energy in the form of motion
Amount depends on the mass and velocity
of the object.
Greater mass at the same velocity will
have greater kinetic energy.
Greater velocity for the same mass will
have greater kinetic energy.
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Discuss
1. Explain the relationship between energy
and work.
2. Explain the difference between potential
energy and kinetic energy.
3. What is the potential energy of a 2.5 kg
book held 2.0 m above the ground?
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Calculating kinetic energy
1 2
KE  mv
2
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Examples
1. Calculate the kinetic energy in joules of a
1500 kg car that is moving at a speed of
12 m/s.
2. A 35 kg child has 190 J of kinetic energy
after he sleds down a hill. What is the
child’s speed?
3. A bowling ball traveling 2.0 m/s has 16 J
of kinetic energy. What is the mass of
the bowling ball?
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Chemical potential energy
The energy stored in foods, fuels, and
batteries.
There must be a chemical reaction to get
the energy out.
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Mechanical energy
The sum of kinetic energy and potential
energy
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The sun
Gives energy to living things (starting with
photosynthesis
Gives off energy as heat and light
Gets its energy from nuclear fusion
When small atomic nuclei combine into a larger
nucleus
A type of potential energy
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Discuss
1. What is the kinetic energy of a 0.02 kg
bullet that is traveling 300 m/s? Express
your answer in joules.
2. What is the kinetic energy of a 0.015 kg
snowball that is moving through the air at
3.5 m/s?
3. What is the kinetic energy of an 8500 kg
airplane that is flying at 220 km/h? (Make
sure you convert to m/s first)
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Conservation of Energy
The sum of potential and kinetic energy in
a system is called mechanical energy.
Mechanical energy is conserved. It can
change from one form to another, but it
cannot be created or destroyed.
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Changing from gravitational potential
to kinetic
When something moves from a higher
position to a lower position
Roller coaster
Free-fall
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Changing from kinetic to gravitational
potential
When something moves from a lower
position to a higher position
Roller coaster
Object thrown straight up
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Losses of mechanical energy
Keeps bouncing or swinging objects from
returning from original height.
Friction
Air resistance
Heat
Sound
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Law of Conservation of energy
Energy cannot be created or destroyed.
The total amount of energy in the universe
never changes
It just changes forms
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Open system vs. closed system vs.
isolated system
Open system
Energy and matter are exchanged with
surroundings.
Closed system
Energy, but not matter, is exchanged with
surroundings
Isolated system
Neither energy nor matter are exchanged with
surroundings
rare
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Discuss
1. State the law of conservation of energy in
your own words. Give an example of a
situation that you have either
encountered or know about in which the
law of conservation of energy is
demonstrated.
2. Describe the rise and fall of a thrown
basketball by using the concepts of
kinetic energy and potential energy.
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Energy “lost” in machines
Only some of the input work on a machine
is actually converted to output work.
Friction
Weight of rope (in pulley) or lever itself
Sound
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Efficiency
No machine is 100% efficient
efficiency 
Woutput
Winput
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Example
Alice and Jim calculate that it takes 1800 J
of work to push a piano up a ramp.
However, because they must also
overcome friction, they actually must do
2400 J of work. What is the efficiency of
the ramp?
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You try
It takes 1200 J of work to lift a car high
enough to change a tire. How much work
must be done by the person operating the
jack if the jack is 25% efficient?
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