Work, Power, and Energy
Mrs Sedlock
Principles of Chemistry and Physics
Review
• Newton’s Laws were used to predict and
describe an object’s motion
• In this unit we will discuss motion in terms of
energy and work
Work
• Work - when force acts on an object and
causes displacement of the object
– Force
– Displacement
– Cause
• In order for work to be done there must be a force that
causes a displacement
Examples of Work
Examples of work
A teacher applies a force to a wall and
becomes exhausted.
A book falls off a table and free falls
to the ground.
A waiter carries a tray full of meals
above his head by one arm straight
across the room at constant speed.
(Careful! This is a very difficult
question that will be discussed in
more detail later.)
A rocket accelerates through space.
Work
• Any part of a force that does not act in the
direction of motion does NO WORK in an
object
Negative Work
• Sometimes force acts in the opposite direction
of the displacement to prevent motion
– Ex:
• car skidding to a stop
• Baseball player sliding into home plate
Calculating Work
• Work = force x displacement
• Unit of work = J (Joules)
• 1 J = 1 N*m
• Ws work problems
Power
Power
• Power – is the rate of doing work
– Doing work at a faster rate requires more power
– to increase power, increase the amount of work
done in a given amount of time
– Or do the same amount of work in less time
Power
• The snow blower can do more work in less
time – so it has more power than the person
shoveling
Calculating Power
Power = work
time
Units
Work is in Joules (J)
Time is in seconds (s)
Power is in watts (W)
which is 1 Joule /second
Ex: a 40-watt lightbulb
requires 40 Joules each
second that it is lit
Calculating Power
• You exert a vertical force of 72 Newtons to lift
a box to a height of 1.0 meter in a time of 2.0
seconds. How much power is used to lift the
box?
• (hint: remember that work = force x
displacement )
• 36 Watts
Horsepower
• One horsepower (hp) = 746 Watts
• slapshot physics
Work and Machines
drones
Machine
• Machines make work easier to do
– Change the size of the force
– Or the direction of the force
– Or distance over which a force acts
• Increase force
• Each rotation applies a small force over a large
distance, but each rotation lifts the car a short
distance
• If a machine increases the distance over which
you exert a force, then it decreases the
amount of force you need to exert
• Increasing distance
• The oars act as a machine to push the boat
through the water
• Pulling the oar short distance near the boat
translates to a large distance in the water –
but you increase the force needed
• A machine that decreases the distance
through which you exert a force increases the
amount of force
• Change of direction
• Some machines change the direction of the
applied force
• Pulling back on the handle of the oar causes
its other end to move the opposite direction
• Machines can change the direction of the
force
Work Output
• The force that is exerted by a machine is called
the output force
• The distance the output force is exerted
through is the output distance
• work output = output force x output distance
•
Work Input and Work Output
• Because of friction, the work done by a
machine is always less than the work done on
the machine
Work input and work output
• The force you exert on a machine is called the
input force
• the distance the input force acts through is
called the input distance
• The work done in this process is called the
work input
• Work input = input force x input distance
Work Input
• For the oar– the input force is the force exerted on the handle
– The input distance is the distance the handle
moves
– The work input is the work you do to move the
handle
• You can increase the work input by increasing
the input distance, the input force, or both
• The force the oar on the water causes an
equal and opposite reaction force to be
exerted by the water on the oar – this reaction
force propels the boat through the water
• The only way to increase the work work
output is to increase the amount of work you
put into the machine
Mechanical Advantage and
Efficiency
Mechanical Advantage
• Mechanical advantage of a machine is the
number of times that the machine increases
an input force
Actual Mechanical Advantage
• A loading ramp is a machine used to move
heavy items into a truck
– The longer the ramp, the less force is needed to
lift a refrigerator into the truck
Actual Mechanical Advantage (AMA)
• AMA = output force
•
input force
Actual Mechanical Advantage
– If the ramp has a rough surface it will have less
mechanical advantage than a ramp with a
smooth surface
• It takes a greater force to overcome the friction
Ideal Mechanical Advantage(IMA)
• Ideal mechanical advantage of a machine is
the mechanical advantage in the absence of
friction
– Because friction is always present, the actual
mechanical advantage of a machine is always less
than the ideal mechanical advantage
Ideal Mechanical Advantage(IMA)
IMA = input distance
output distance
Ideal Mechanical Advantage(IMA)
• A woman drives her car up onto wheel ramps
to perform some repairs. If she drives a
distance of 1.8 meters along the ramp to raise
the car 0.3 meter, what is the ideal mechanical
advantage of the wheel ramps?
• IMA = input distance = 1.8 m = 6
output distance 0.3 m
Efficiency
• Efficiency – the percentage of work input that
becomes work output- usually expressed as a
percentage
• The efficiency of ANY machine is always less
than 100%
Efficiency
Efficiency = work output x 100%
work input
Quiz Review
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What is the unit for force?
What is the unit for power?
What is the unit for distance/displacement?
What is the unit for time?
What is the unit for work?
Quiz review
• You must exert a force of 4.5 newtons on a
book to slide it across a table. If you do 2.7
Joules of work in the process, how far have
you moved the book?
Quiz Review
• A catcher picks up a baseball from the ground.
If the unbalanced force on the ball is
7.25 x 10 -2 Joules of work is done to lift the
ball, how far does the catcher lift the ball?
Quiz Review
• A machine has a work output of 8 joules and
requires 10 joules of work input to operate.
What is the machine’s efficiency?
Quiz review
• What is the output distance of a machine with
an input distance of 3.0 cm and an ideal
mechanical advantage of 12?
Simple Machines
6 Types of Simple Machines
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Lever
Wheel and axle
Inclined plane
Wedge
Screw
Pulley
Simple machines
• Many mechanical devices are combinations of
the six types of simple machines
Lever
• Lever- a rigid bar that is
free to move around a
fixed point
- the fixed point is known as
the fulcrum
Lever
• There are 3 classes of levers based on the
locations of
– input force,
– output force,
– and the fulcrum
Lever
• First Class lever
– Fulcrum of a first class lever is always between the
input force and the output force
– Mechanical advantage depends on location of the
fulcrum
Lever
• Second class lever- output force is between
the input force and the fulcrum
• Input distance is larger than output distance ,
which means you need less force
• The mechanical advantage of a second class
lever is always greater than 1
Lever
• Third class lever – input force is between the
fulcrum and the output force
• Input distance is smaller than output distance
• Mechanical advantage is less than 1
Levers
• Levers
•
Paul Rabil
Lever
Wheel and Axle
• Wheel and axle is a simple machine that
consists of two disks or cylinders, each with a
different radius
– The outer disk is the wheel and the inner disk is
the axle
– The input force can be applied to the wheel or the
axle
Wheel and Axle
• To calculate the mechanical advantage of the
wheel and axle
• Can have a mechanical advantage less than or
greater than 1
Mechanical advantage = radius of input
radius of output
Wheel and Axle
Inclined Plane
• If the input distance is greater than the output
distance, the input FORCE is DECREASED
Inclined plane
• Inclined plane- a slanted surface along which a
force moves an object to a different elevation
• The ideal mechanical advantage of an inclined
plane is the distance along the inclined plane
divided by its change in height
• IMA = distance of inclined plane
change in height
Teacher demo
Wedges and Screws
• Wedges- v-shaped object that has inclined
planes on the sides sloped toward each other
• The sloping sides push the wood a small
distance apart
• Mechanical advantage
is greater than 1
Wedges and Screws
• Screw- an inclined plane wrapped around a
cylinder
– Screw that have threads closer together have a
greater ideal mechanical advantage
Pulleys
• Pulley is a simple machine that consists of a
rope that fits into a groove in a wheel
– Pulleys produce an output force that is different in
size, direction, or both from the input force
Pulleys
• The ideal mechanical advantage (IMA) of a
pulley system is equal to the number of rope
sections supporting the load being lifted
– Three types of pulleys
• Fixed Pulley
• Movable pulley
• Pulley system
Pulleys
• Fixed Pulley
– Wheel attached in a fixed location
– Changes direction of the exerted force
– IMA is 1 because the rope will lift the load the
exact distance you pull the rope
Pulleys
• Movable Pulley
– Is attached to the object being moved
– Reduce the input force
Pulleys
• Pulley system
– Mechanical advantage depends on how the
pulleys are arranged
– Each segment of the rope exerts a force equal to
the force you exert on the rope
– Pulleys
Compound Machines
• Combination of two or more simple machines
that operate together
– The output force of one of the simple machines
becomes the input force
for another
– Ex:
• Clocks
• Bicycles
Compound Machines
Simple machines
• Bill Nye Simple MAchines
Energy
Energy
• Energy is the ability to do work
– Energy is transferred by a force moving an object
over a specific distance
– Sooooo
– Work is a transfer of energy
– Both are measured in Joules
Types of Energy
• Kinetic energy
–Energy of motion
• Potential energy
–Energy of position, stored energy
Kinetic Energy
• The kinetic energy (KE) of an object depends
on its mass (in kg) and speed (velocity v in
meters per second)
KE=½ mv2
if you double the mass, the KE is doubled
if you double the speed, the KE is
quadroupled!
Practice problem
• A 70 kg man is walking at a speed of 2.0 m/s.
What is his kinetic energy?
• KE = ½ mv2
m= 70 kg
• KE = ½(70 kg) (2.0 m/s)2
• KE = 140 J
v = 2.0 m/s
Potential Energy
• Potential energy is the energy with the
potential to do work
– Two common forms
• Gravitational
• Elastic
Gravitational Potential Energy
• Potential energy that depends on an objects
height is called gravitational potential energy
Gravitational Potential Energy
• An object’s gravitational potential energy
depends on its mass (m), height (h), and
acceleration due to gravity (g)
• Potential Energy (PE) = mgh
Elastic Potential Energy
• Potential Energy of an object that is stretched
or compressed is known as elastic potential
energy
– Something is considered to be elastic if it springs
back to its original shape
– Rubber band- energy you add is stored as
potential energy
Forms of Energy
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Mechanical energy
Thermal energy
Chemical energy
Electrical energy
Electromagnetic energy
Nuclear energy
Mechanical Energy
• Energy associated with motion
Thermal Energy
• All particles of matter are in constant motion
so they have kinetic energy
• The total of the potential and kinetic energy of
all microscopic particles make up its thermal
energy
Thermal Energy
Chemical Energy
• Energy stored in chemical bonds
– When bonds are broken, energy is released that
can do work
Chemical Energy
Electrical Energy
• The energy associated with electrical charges
– Electric charges can exert a forces that do work
Electrical energy
Electromagnetic Energy
• Form of energy that travels as a wave
Nuclear Energy
• Energy stored in atomic nuclei is nuclear
energy
Nuclear Energy
How a Nuclear Power Plant
Produces Electricity
Energy
• Bill Nye Energy