Transcript Kinetic Energy

What do you think of when
you hear the words:
Work, Power & Energy?
Work = Force|| x Distance
W = Fd
The distance is the displacement.
Only the force parallel to the
displacement does work
Work
• Force MUST be applied in the
direction of motion
• The unit for work is the Newtonmeter which is also called a Joule.
• 1 joule is equal to a force of 1 N exerted
over a distance of 1 m
Work or No Work
m= 10kg
In this case, the weight
does positive work
d=2m
Work = mgd = (100N)(2m)
m= 10kg
mg = 100N
Work = 200 Nm =200J
FA
m= 10kg
In this case, the weight
does negative work
mg = 100N
d=2m
Fa
Work = -mgd = -(100N)(2m)
Work = -200 Nm = -200J
m= 10kg
mg = 100N
Power
• measured in
watts (W)
• One watt of
power is used
when one joule
of work is done
in one second.
http://www.cartoonstock.com/newscartoon
s/cartoonists/pto/lowres/pton114l.jpg
Power
• Power is equal to the amount of work done
per unit time.
work done
Power 
time interval
• The unit for power is the Joule/second
which is also called a Watt.
What is horsepower?
• 1 horsepower = 746 Watts
• A horse exerting 1
horsepower can raise 330
pounds of coal 100 feet in a
minute, or 33 pounds of coal
1,000 feet in one minute, or
1,000 pounds 33 feet in one
minute.
Energy
• Universe is made up of matter and energy.
• Energy is the mover of matter.
• Energy has several forms, some examples:
– Mechanical
• Kinetic
• Potential
– Electrical
– Heat
– Chemical
http://tiki.oneworld.net/energy/energy.html
Mechanical Energy
• Energy contained by an object
due to its motion or its stored
energy of position.
• It can either be potential energy
or kinetic energy.
• All forms of energy are
measured in joules (J).
• ME = KE + PE
Conservation of Energy
• Energy cannot be created nor
destroyed
• Energy may be transformed from one
form into another, but the total amount
of energy never changes.
• So why worry about conserving energy?
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Conservation of Mechanical Energy
• KE (before) + PE (before) = KE (after) + PE (after)
• Why does a dropped ball not return to its
original elevation?
http://www.science-projects.com/Drop/DropBalls.htm
Energy Transformation of a
Pendulum
http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/pe.html
Potential Energy
• (PE) stored chemical energy or
energy of position.
– An object’s ability (potential) to do work
by virtue of its position.
– Types: elastic, gravitational and
chemical.
• Examples:
–
–
–
–
–
Rubber bands
Springs
Bows
Batteries
Gravity?
Gravitational Potential
Energy
• PE = Weight  height = Fwh = mgh
• PE = m g h
• Question:
– How much potential energy does a 10kg
mass have relative to the ground if it is 5
meter above the ground?
Potential Energy
• The same work is
done on
each block. What
matters
is the final
elevation, not the
path followed
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Kinetic Energy
• The energy of motion.
• Kinetic Energy = ½ mass  velocity2
KE = ½ mv²
How much work does gravity do on the falling
mass?
PE = mgh
mg
h
h
Work = mgh
mg
How much energy does the mass have at the
bottom of its fall, just before it hits the ground?
PE = mgh
mg
h
h
Kinetic energy
mg
v
PE = mgh
mg
h
KE = ½ mv2
mg v
If potential energy at
top of path is 100 J,
how much kinetic
energy does the ball
have just before it
hits the ground and
comes to a stop?
Check for Understanding
• Determine the kinetic
energy of a 1000-kg roller
coaster car that is moving
with a speed of 20.0 m/s.
Determine the Unknown
Information:
More Energy Transfer
• High speed winds are used to do
work on the blades of a turbine at
the so-called wind farm.
• Mechinacal Energy from the air
gives the air particles the ability to
apply a force to the blades.
• As the blades spin, their energy is
converted into electrical energy (a
non-mechanical form of energy)
and supplied to homes and
industries in order to run electrical
appliances.
Energy Transfer...
• This diagram shows
that the potential
energy of the boy is
changing as he
swings.
• Explain what is
happening, in other
words how is the
energy changing?
Mechanical Energy Conversion
•
Explain the following diagram:
a. What is happening
b. Explain the conversion of KE to PE to KE to
PE…
c. http://www.funderstanding.com/coaster
Work/Energy Relationship
• If you want to move something,
you have to do work.
• The work done is equal (ideally) to
the change in kinetic energy.
– what is ideal? is this actual?
• W = DKE
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Work-Energy Relationship
• What happens to your stopping distance
when you…
• Double your speed?
• Triple your speed?
http://www.thegrid.co.uk/index_files/Mickel-crash-brands.jpg
Joule to Joule
http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/energy/cs.html
Truck Collision
• In a head-on collision:
• Which truck will experience the greatest force?
• Which truck will experience the greatest change in
velocity?
• Which truck will experience the greatest acceleration?
• Which truck would you rather be in during the collision?
WHY?
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c1
• Imagine standing on your desk…if you
jumped off, how fast would you be moving
just prior to reaching the floor?
• How much work would you do on the
floor?
• What information do you need to know?
–G
–U
–E
–S
–S
http://school.discoveryeducation.com/clipart/images/in-desk.gif