Transcript Kinetic Energy

Energy
Its lost, gained, found, stolen, etc. but does it even exist
Energy
• “Something” that enables an object to do
work.
• When you have “Energy,” you can do work
• The amount of energy that an object
possesses is related to the amount of work
the object can do.
• SI unit is Joules (J) – (N*m) – (kg*m2/s2)
• Comes in various forms like ……..
Five main forms of energy:
• Mechanical
• associated with motion.
• Thermal (Heat)
• internal motion of particles.
• Chemical
• bonds atoms and ions together.
• Electromagnetic
• moving electric charges.
• Nuclear
• in the atomic nucleus.
• Nuclear fission & Nuclear fusion
Mechanical Energy
• Energy due to the
position, or
movement, of
something
• Mass is, or will be,
displaced
• Kinetic or Potential
• Non-Mechanical
• Mass is not displaced,
no movement or
velocity gained or lost
• Examples
• Chemical Energy
• Gasoline
• Food
Potential Energy
• Gravitational
• Energy an object has Potential Energy
due to position
• due to gravity acting
• Stored Energy
on an object,
• Potential Energy
• Stored Work
• Examples
• Why do we not walk
under ladders
• Scared of Rubber
Bands
PE = mgh
PE = mass x gravity x height
Metric unit of energy is
Joules (same as for work).
Sample Problem
6 kg
Gravitational Potential energy is
PE = (mass) x (Gravity) x ( Height )
= (6 kg)x (10m/s/s ) x ( 20 m )
= 1200 Joules
What is potential energy at zero height?
At a height of zero meters PE is zero Joules.
20 m
What is the potential energy of a 6kg bowling
ball at a height of 20 meters?
An automobile is transported 7.0 m above the
dock. If its gravitational potential energy is
6.6 x 104 J, what’s its mass?
Kinetic Energy (KE)
• Energy associated
with an object due to
its motion.
• If something is in
motion it has the
ability to do work
• Examples
• How do you stop
something that is
moving
• A stationary object
has zero kinetic
energy
2
KE = ½mv
KE= ½ x Mass x Speed2
Notice that speed is
squared, If speed is
doubled what happens
to the KE?
Sample Problem
6 kg
What is the kinetic energy of a 6kg
bowling ball rolling at 20 m/s?
(Kinetic Energy) = ½ ( Mass ) x ( Speed ) x (Speed)
= ½ x( 6 kg ) x ( 20 m/s ) x ( 20 m/s )
= 1200 Joules
20 m
Kinetic energy is
20 m/s
Conservation of Energy
• Law of Conservation of Energy states that
energy can never be created or destroyed. It
can only be transformed from one form into
another.
• Example
• A book falls from a table
• Draw out and label in Notebook
• The total Mechanical Energy must always be
equal to the sum of Kinetic Energy and
Potential Energy
ME = KE + PE
Pendulum
“Like a child on a swing”
Energy exchange from PE to KE and back.
Maximum
Height
Maximum
Speed
Can predict speed from height
since PE+KE constant
Maximum
Height, again
Check for Knowledge
• One Arm Pete is playing with his son One
Legged Leroy on the slide. The slide is 3.0 m
high. It is (of course) frictionless. What is Little
Leroy’s speed, at the bottom of the slide if he
has a mass of 40 kg?
• What would One Arm Pete’s speed be at the
bottom if he has a mass of 100 kg?
• Would their speeds be different if they fell off
the back of the slide?
Energy Conversions
• When a match is struck, three of the five types
of energy are on display. On a blank sheet of
paper, write at least five lines (double spaced)
discussing.
• Ex. Before the match is struck, there exists
gravitational potential energy, heat energy, and
chemical potential. Then, mechanical kinetic
energy is used to convert the potential energy into
heat energy.
Try this one…
• A ball is thrown straight up in the air from the
ground. Jot down a quick picture of it and label
at least one point where it has only kinetic
energy, only gravitational potential energy, and
both.