Transcript Kinetic Energy

What do you think
of when you hear
these words:
Work…
Energy…
Power…
Energy
• Universe is made up of matter and energy.
• Energy is the mover of matter.
• Energy has several forms:
– Kinetic
– Potential
– Electrical
– Heat
– etc.
http://tiki.oneworld.net/energy/energy.html
Energy
• The Universe is made up of matter
and energy.
• Energy is the “mover” of matter.
• There are many forms of energy.
• Conversions from one form of energy
to another continually occur.
• Energy cannot be created or
destroyed.
Work = Force|| x Distance
In this case, the distance is the
magnitude of the displacement.
Only the component of force
parallel to the displacement does
work
Work
• Force applied in the direction of
motion
• Work = Force||  Distance
• W = F||d
• 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

F

Fy

Fx
x
W  Fxdx
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
expended 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.
Mechanical Energy
• Energy possessed 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).
• Em = Ek + Ep
Conservation of Energy
• Energy cannot be created nor
destroyed; it 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
• Ek (before) + Ep (before) = Ek (after) + Ep (after)
– When no other type of energy is present.
• 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
• 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
Ek = 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.
• If the roller coaster car in the last
problem were moving with twice
the speed, then what would be its
new kinetic energy?
Calculate speed at positions
B,C and D.
More Energy Transfer
• High speed winds are used to do
work on the blades of a turbine at
the so-called wind farm.
• Mech. Energy from the air gives
the air particles the ability to apply
a force to the blades.
• As the blades spin, their energy is
subsequently converted into
electrical energy (a nonmechanical 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?
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
http://www.cartoonstock.com/newscartoons/cartoonists/jko/lowres/jkon533l.jpg
Forces in Car Crashes
•
http://hyperphysics.phy-astr.gsu.edu/hbase/carcr.html#cc3
Seatbelt Physics
http://hyperphysics.phy-astr.gsu.edu/hbase/seatb.html#cc1
• The formula for KE is _____?
• Kinetic Energy is dependent upon ___ & ____
• If an object’s velocity doubles, its kinetic energy
_____. If velocity triples, it will have _____
times the kinetic energy.
• If mass doubles, kinetic energy _____. If mass
triples, kinetic energy _____.
• …so what happens to KE if mass doubles and
velocity doubles? What if mass is cut in half
and velocity doubles?
• Think about 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?
http://school.discoveryeducation.com/clipart/images/in-desk.gif
Object Falling from Rest
http://hyperphysics.phy-astr.gsu.edu/hbase/flobj.html#c2
Bungee Jumping
•
•
•
•
•
•
You wish to bungee jump off of a platform.
Assuming that there is no air resistance and the
spring constant of the 40 m long bungee cord is 100
N/m. How high should your platform be?
Determine all the forces acting ON the body.
Draw a free body diagram.
Apply Newton's second law.
Think about elastic potential energy.
Solve.
Momentum
Momentum
• Moving inertia
• Symbol- p
• Equation p = mv
inertia
• Units: kg • m/s
• VECTOR
In motion
• A Thought Experiment:
•
Suppose that you were captured by an
evil physicist who gave you the following choice:
• You must either:
• Stand in front of a 1000 kg. truck moving at 1
m/s, or
• Stand in front of a 1 kg. frozen meatball moving
at 1000 m/s.
• …think…
• Truck:
• Truck's momentum = mv = (1000 kg)(1 m/s)
= 1000 kg m/s
• Truck's kinetic energy = 0.5 mv2 =
(0.5)(1000 kg)(1 m/s)2 = 500 Joules
• Meatball:
• Meatball's momentum = mv =
(1 kg)(1000 m/s) = 1000 kg m/s
• Meatball's kinetic energy = 0.5 mv2 =
(0.5)(1 kg)(1000 m/s)2 = 500 000 Joules
Diagramming:
• A 120 kg lineman moving west at 2 m/s
tackles an 80 kg football fullback moving
east at 8 m/s. After the collision, both
players move east at 2 m/s. Draw a vector
diagram in which the before- and aftercollision momenta of each player is
represented by a momentum vector. Label
the magnitude of each momentum vector.
Answer to previous slide
question
Impulse
• Force applied over a period of time
creates impulse
• Impulse equals the change in momentum
impulse = ∆momentum
Total pbefore collision = Total pafter collision
pT before = pT after
(p1 + p2) before = (p1 + p2) after
(m1v1 + m2v2 )before = (m1v1 + m2v2)after
Bouncing increases impulse
explain...
Impulse= change in momentum
Impulse
• Rhonda, who has a mass of 60.0 kg, is
riding at 25.0 m/s in her sports car
when she must suddenly slam on the
brakes to avoid hitting a dog crossing
the road. She strikes the air bag, which
brings her body to a stop in 0.400 s.
What average force does the seat belt
exert on her?
Why wear seatbelts?
• If Rhonda (from the previous question)
had not been wearing her seat belt and
not had an air bag, then the windshield
would have stopped her head in 0.001
s. What average force would the
windshield have exerted on her?
Elastic Collision
• An elastic collision occurs when the two
objects "bounce" apart when they collide.
Two rubber balls are a good example.
– Both p and Ek are conserved
http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/momentum/cthoe.html
• Elastic
collisions:
momentum is
redistributed or
transferred without
loss.
m1v1i + m2v2i = m1 v1f +m2v2f
Inelastic Collision
• An inelastic collision is one in which part of
the kinetic energy is changed to some
other form of energy in the collision.
– p is conserved, Ek is not conserved
• Some of the kinetic energy is converted into
sound, heat, and deformation of the objects.
– A high speed car collision is an inelastic collision.
http://www.glenbrook.k12.il.us/gbssci/Phys/mmedia/momentum/cthoi.html
• Inelastic
collisions:
when two objects
become entangled
m1v1i + m2 v2i = (m1+m2)vf
v = 10
v=0
M
M
Before Collision
p = mv
v’ = 5
M
mv = 2mv
v’ = ½ v
M
After Collision
p = 2mv’
Granny & Little Timmy:
calculate their final velocity.
Law of Conservation
of Momentum
• In the absence of an external
force, the momentum of a system
does not change.
http://www.glenbrook.k1
2.il.us/gbssci/Phys/Class
/momentum/u4l2e.html
A 15-kg medicine ball is thrown at a velocity of 20
km/hr to a 60-kg person who is at rest on ice. The
person catches the ball and subsequently slides with
the ball across the ice. Determine the velocity of the
person and the ball after the collision.
• A large fish is in motion
at 2 m/s when it
encounters a smaller
fish which is at rest. The
large fish swallows the
smaller fish and
continues in motion at a
reduced speed. If the
large fish has three
times the mass of the
smaller fish, then what is
the speed of the large
fish (and the smaller
fish) after the collision?
SOLVE:
• A 9.2 g bullet is fired into block of wood
that is hanging motionless from a string,
and becomes embedded in the wood. The
combined mass of the wood and string is
3.1 kg. If the wood rises a vertical distance
of 6.3 cm, calculate the initial speed of the
bullet.
Truck Collision
• In a head-on collision:
Which truck will experience the greatest force?
Which truck will experience the greatest impulse?
Which truck will experience the greatest change in
momentum?
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?
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c1
Ride in
the BIG
truck! 
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c2
Stopping a Truck
• Suppose a large truck
and a small truck are
moving with equal
momentum. Which will
require more work to
stop? Which will require
more force to stop in a
given distance?
http://hyperphysics.phy-astr.gsu.edu/hbase/truckc2.html#c1
• A large truck has more
momentum than a car moving at
the same speed because it has
___________.
• Which is more difficult to slow
down? The car or the large truck?
More Force to Stop the Little
Truck!??
• Since the little
truck is moving
substantially faster
it will have more
Kinetic Energy!
Impulse
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
YIKES!
• If the boulder and
the boy
have the same
momentum,
will the boulder
crush the
boy?
• …do not try this at
home!
http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html