Transcript Potential and Kinetic Energy (176-178)

Potential and Kinetic Energy
• Energy: is the ability
to do work
• Work is being done
whenever some
physical force is
being used to move
an object some
distance
Energy means that
• birds can fly,
• tigers can roar,
• wind can blow,
• sun can shine,
• cars can go fast,
• factories can make things,
• light bulbs can glow and
• your computer can work.
Without energy, there would be nothing: no life, no
movement, no light, … nothing
Energy
• All objects contain energy in one form or another
• Can take the form of
–
–
–
–
Motion
Position
Heat
Light
-Sound
- Electricity
• It can never be destroyed
• It can only be converted from one form to
another
Kinetic Energy
= the energy an object possesses
because of its motion
• The amount of Kinetic energy is dependent on
the mass of the object in motion and it’s
velocity.
• Objects "tend to keep on doing what they're
doing"
• In fact, it is the natural tendency of objects to
resist changes in their state of motion.
• Without some outside influence, an object in
motion will remain in motion and an object at
rest will remain at rest.
This tendency to resist changes in their state of
motion is described as inertia
Cardboard and a coin placed on an
empty glass
Flick the cardboard with the finger. What do
you observe? The coin drops into the tumbler.
When we flick the cardboard the cardboard
moves fast whereas the coin continues in its
state of rest and hence drops into the glass.
So it is clear that objects continue to remain in their state of rest or of uniform motion until
an external force is applied.
Examples of Inertia of Rest
A passenger standing in a bus leans backwards when the bus starts all of a sudden
Fruits fall down when the branches of a tree are shaken
Dust particles on a carpet falls when we beat the carpet with a stick
Examples of Inertia of Motion
A passenger standing in a moving bus leans forward when the bus stops all of a sudden
A man carelessly stepping out of a moving train onto the ground leans forward
Example of Inertia of Direction
The water particles sticking to the cycle tire are found to fly off tangentially
Inertia of a body depends upon its mass. That is, massive objects
possess more inertia than lighter ones.
Galileo’s idea of inertia 1 - A balled rolled down an incline
would continue up the second incline
at equal angle until the force of
gravity forced it back down
2 – A ball rolled down the first incline
would roll farther up the second
incline since the angle is not as steep.
3 – A balled rolled down the inline
would continue to roll indefinitely on
the second plane since it would not
be affected by gravity
Calculating kinetic energy
If we know the mass of an object and its velocity we
can determine the amount of kinetic energy
possessed by using the following formula:
kinetic energy = 1/2 (mass of object)(velocity of
object)2
or KE = 1/2 mv2
or KE = 0.5mv2
The SI unit for kinetic energy is the Joule (J).
A Joule is kg • m2/s2
A bicycle with a mass of 14 kg traveling
at a velocity of 3.0 m/s east has how
much kinetic energy?
KE = 0.5mv2
= 0.5(14 kg)(3.0 m/s)2
= 0.5(14 kg)(9.0 m2/s2)
= 63 kg• m2/s2 = 63 J
A 1200 kg automobile is traveling at a
velocity of 100 m/s northwest.
What is the kinetic energy of the
automobile?
KE = 0.5 mv2 = 0.5(1200kg)(100 m/s)2
= 0.5(1200 kg)(104 m2/s2)
= 6 x 106 kg  m2/s2 = 6 x 106 J
VELOCITY = the measure of how fast
on object in traveling in a certain
direction!
Velocity = distance ÷ time
Distance = meters
Time = seconds
Potential Energy
• The energy of position
• The amount of energy contained in an object
at rest
Determining Potential Energy
By its position and its weight
(mass X gravity)
PE = (mass)(gravity)(height) = mgh
• where m is mass in kg
• g is the force of gravity = 9.8 m/s2
• h is the height
• The SI unit that represents potential energy is
the Joule (J) (kg m2/s2).
Examine an example of potential
energy
A flower pot with a mass of 15 kg is sitting on a
window sill 15 meters above the ground. How
much potential energy does the flower pot contain?
• PE = (mass)(gravity)(height)
• = (15 kg)(9.8 m/s2)(15 m)
• = 2205 kg • m2/s2
• = 2205 J = 2000 J
• = 2.2 x 103J
Examine an example of potential
energy
• If the flower pot is lowered to a window sill
that is 10. m from the ground. Does this
change the potential energy of the flower
pot?
PE = (mass)(gravity)(height)
= (15 kg)(9.8 m/s2)(10. m)
= 1470 kg  m2/
= 1470 J
= 1.5 x 103J
Potential or Kinetic Energy?
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•
Moving car
Tree branch
Bent car fender
Balloon filled with air
Balloon squirting around room
Person inside a moving car
• What type of
energy does the
space shuttle have
at lift off?
Conversion of Potential to Kinetic
Energy
• In this picture both
kinds of energy are
evident. Can you
point them out?
• The water at the
top has potential
energy
• When water falls
to a lower level,
the potential
energy is
converted to
kinetic energy.
Velocity & Acceleration
Some Review
Defining Velocity
Kinetic energy was
– KE=1/2 (mass) (velocity)2
•Describes both the rate and direction of
the motion
•If an object speeds up or slows down in
the given direction we say there is a
change in velocity
VELOCITY AND SPEED
Velocity is a measure of how fast an
object is traveling in a certain direction.
•Example: A bus traveling 15 m/s increases
its speed to 20 m/s
•The speed changed so the velocity
changed
•The bus changes direction and goes east.
Since the direction changed, so did the
velocity
Car on a circular track = may have constant
speed, but cannot maintain a constant velocity
as it’s direction is always changing.
VELOCITY AND SPEED
– Speed is a measure of how fast something is
moving, but there is not a directional element to it
– Is the distance on object moves per time or how
fast something is moving without direction
– Speed = Distance ÷ Time (S=D ÷ T)
– If speed changes, so does the velocity
VELOCITY
Velocity = distance ÷ time
The units we use are m/s and d is distance.
ACCELERATION
• Acceleration is the change in velocity per unit
of time.
• An example of this is when you travel in your
car.
• Your velocity is not constant throughout the
entire trip as you slow down and speed up as
necessary.
• A positive acceleration means that you are
speeding up and a negative acceleration
means that you are slowing down.
ACCELERATION
• Acceleration has the formula:
Acceleration = (Final Velocity) – (initial velocity)
(Final time) – (Initial time)
OR
(time it takes to change velocity)
A = vf – vi = ∆v
∆ means “change in”
tf – ti ∆t
Acceleration has the units of (distance unit)/(time unit)
Ex: m/s2 or mi/h2
ACCELERATION
• Example acceleration problems
• Calculate the acceleration of an object with:
» Initial Velocity : 0.0m/s
» Final Velocity: 14m/s
» Time 4.0s
» A = 14m/s – 0.0m/s
4.0s
A = 3.5m/s2
ACCELERATION
• A car stops from a velocity of 55m/s in 15
seconds. What is the cars acceleration? Is the
car speeding up or slowing down?
• A = 0 – 55m/s
15 s
A = -3.7m/s2
Car is slowing down
-55m/s
15s