Download WORK and ENERGY

WORK and ENERGY
Work
Occurs when a force causes a change in
position or motion of an object
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W = force x distance
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W = f x d
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Unit of measurement is the Joule (J)
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1 Joule = 1 Newton meter or 1 kg m2/s2
Work
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The applied force must make the
object move, or else work is zero
The movement of the object must be
parallel to the applied force
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If the force and direction of the object’s
motion are in the same direction, the work
is positive
If the force and direction of the object’s
motion are in opposite directions, the work
is negative
Example of Work
How much work does a student do when he
exerts a force of 190N to lift a box 2.0 m?
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Energy
Energy is the ability to do work
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Work = energy!! If an object has energy, it’s
because work is done on it
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Whenever work is done, energy is
transformed or transferred from one
system to another
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Energy can be present in objects even when
nothing is happening
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Work is measured in Joules
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This is easy to understand when you think
about how you feel after carrying a heavy
box up a flight of stairs.
You transferred energy from your moving
muscles to the box and increased its
gravitational potential energy by increasing
its height
Energy is always transferred from the
object that is doing the work to the object
on which the work is done
Gravitational Potential Energy Eg
Energy of position, or stored energy
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Results from the relative positions of
objects in a system
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Results from the gravitational attraction
between objects
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Equals the work done by gravity
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Depends on mass (m) , height (h), and the
acceleration due to gravity (g)
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Gravitational Potential Energy
Example 1. An apple at the top of a tree has
more Eg than an apple of the same size on a
lower branch. Why?
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Example 2. A large apple on the same branch
as a small apple has more Eg. Why?
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Gravitational Potential Energy
(continued)
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Eg = mass x gravity x height
Eg = m g h
m = mass (in kilograms)
h = height (in meters)
g = 9.8 m/s2
NOTE: m x g is the weight of the object
in Newtons
- This is equal to the force on the
object due to gravity, so like work,
gravitational potential energy can be
calculated by multiplying force times
distance
Gravitational Potential Energy
Examples
1. What is the potential energy of a 0.15
kg apple located 6.5m from the ground?
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2. A diver has 3400 Joules of gravitational
potential energy after climbing up onto a
diving platform that is 6.0 m above the
water. What is the divers mass?
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3. A science student holds a 0.055 kg egg
out a window. Just before the student
releases the egg, the egg has 8.0 J of
gravitational potential energy with
respect to the ground. How high is the
students arm above the ground?
Kinetic Energy (Ek)
Referred to as the energy of motion (all
moving objects have kinetic energy)
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Depends on the objects mass and velocity
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Example 1. A bowling ball can do more
work than a ping pong ball if they are
moving at the same speed. Why?
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Example 2. An apple falling at 15 m/s can
do more work than an apple falling at 1
m/s. Why?
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Kinetic Energy (continued)
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Ek = ½ mass x (velocity)2
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Ek = ½ m v2
m = mass
v = velocity
Velocity is squared, so a small change in
speed causes a large change in kinetic energy
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This is why car crashes are more dangerous
at speeds above the speed limit!!
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Kinetic Energy Examples
1. Calculate the kinetic energy of a 1500
kg car that is moving at a speed of 11.67
m/s.
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2. 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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3. If a 44 kg cheetah has 21,142 J of
kinetic energy, at what speed is the
cheetah running?
Law of Conservation of Energy
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Energy cannot be created nor destroyed,
but can change forms
The total energy in the universe never
changes.
If energy appears to increase or decrease
in a system, it is due to energy entering
from an external force (work has added or
subtracted energy from the system)
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Energy Transformations
Energy readily changes from one form
to another
Energy is constantly transforming
between potential and kinetic
W- Eg highest; Ek lowest
X – Eg lowest; Ek highest
Y – Eg increases; Ek decreases
Z - Eg decreases; Ek increases
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A moving roller coaster “loses” energy
because of friction and air resistance
The energy does not disappear
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Some increases the temperature of
the track, wheels and air
Some compresses the air causing a
roaring sound
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Initial energy of the system = final energy
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Ei = Ef
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Egi + Eki + W = Egf + Ekf
The initial energy equals the initial
gravitational energy plus the initial kinetic
energy plus work.
The final energy equals the final
gravitational energy plus the final kinetic
energy.
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Whenever energy of a system decreases, it
has leaked into the environment (usually in
the form of heat, light, or sound)
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Ex.; a ball bouncing – the ball shifts from
kinetic energy to gravitational potential
energy as it bounces. Each time it
bounces, it vibrates the surface (sound),
transferring some energy each time.
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Examples
1. Starting from rest, a child zooms
down a frictionless slide with an initial
height of 3.0 m. What is her speed at
the bottom of the slide?
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2. A 2100kg car starts from rest at
the top of a 2.5 m high hill. If the car
encounters 150N of friction over a
distance of 7.2m, how fast is the car
moving at the bottom of the hill?