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MOMENTUM
What is momentum?
What do you think about when you hear
that word?
What examples can you give of large or
small momentum (plural is momenta).
MOMENTUM:
Inertia in motion
Mass X Velocity
More mass, more momentum
More velocity, more momentum
No mass, NO momentum
No velocity, NO momentum
MOMENTUM
Momentum is
mass X velocity
p = mv
The unit is kg m/s
The Law of Conservation
of Momentum
Momentum in a closed
system is ALWAYS
CONSERVED
Momentum “before”
an event is equal to
momentum “after” an event
Classic examples are explosions, car crashes,
freight trains, pool balls, & shooting a gun.
Momentum conserved
In a collision, if one pool ball collides into
another one that is at rest, pool ball 1 “shares”
some momentum with pool ball 2
The TOTAL momentum of both pool balls (or
cars in a crash, etc.) added together is THE
SAME before and after the collision
p = p’ or mBvB = mAvA or
m1v1 + m2v2 = mTvT
The Impulse Momentum
Theorem
CHANGE in momentum is
EQUAL to Impulse
IMPULSE is equal to IMPACT (or force)
times the TIME INTERVAL of the impact
Δp = FΔt
or
Δ (mv) = F Δt
Applications
Why is it better to bend your
knees when you jump off a table?
Why do you move your hand
backward when catching a fast pitch?
Why do air bags help?
Change in (mv) = Impulse = Ft
Why does a
karate expert
often try to have
a SHORT time
of impact?
More applications
If you only want maximum velocity, such
as trying to achieve maximum range of a
golf ball, you should hit the
ball with
a) a short time of impact
b) a long time of impact
c) it makes no difference
Applications continued
If a building is on fire and you want to
minimize the force of impact on your bones
when you jump from the 2nd story window,
you should
a) land with straight legs
b) land on your feet but bend knees
c) drop and roll to maximize time of impact
PELTON WHEEL
Why did the PELTON wheel work BETTER
than other kinds of water wheels?
When was it invented?
Who invented it?
How were the paddles different?
What does this have to do with anything?
Pelton wheel—what’s so
special?
Which arrow—A or B—imparts a
greater IMPULSE to the target?
A. You shoot an arrow with 25 kg m/s of
momentum and it sticks into the target.
What is the CHANGE in the momentum (and
therefore the impulse)?
B. You shoot an arrow with 25 kg m/s of
momentum and it bounces back off the
target with a momentum of 10 kg m/s.
What is the CHANGE in the momentum (and
therefore the impulse) of this arrow?
ELASTIC COLLISIONS
Bouncing, springy (like elastic !)
The only PERFECTLY elastic collisions in
real life are INSIDE ATOMS (and these are
weird quantum collisions—not hitting each
other in the regular sense)
A perfectly elastic collision means there is
NO friction and NO energy lost as heat
INELASTIC COLLISIONS
Sticky
Does not bounce back
Not springy or elastic
More energy lost as heat due to the force
of friction
In real life, all collisions are partly elastic
and partly inelastic
A 0.5 kg ball of putty is at rest. Another 0.5 kg ball
of putty comes along at 2 m/s and hits the first in a
perfectly inelastic collision.
A) What is the total momentum before the
collision?
B) What is the total momentum after the
collision?
C) As they slide off together, what is their
velocity?
D) What is conserved among the following:
momentum; kinetic energy; energy; speed
A group of astronauts floating around in
outer space are in a circle. They throw a
ball back and forth to each other.
What happens?
You are on a skateboard (not moving) and
throw a basketball to a friend on a
skateboard (also not moving).
What happens?
You are standing on a frozen pond with ice
skates on. Your friend who is twice your
mass is standing in front of you. You push
off of each other.
What happens?
Who (if either of you) moves faster?
The conservation of momentum is
really derived from Newton’s _______
Law.
Now you are in the middle of a big frozen
pond where there is ZERO friction. No
ropes, nobody to help, no helicopter, etc.
 How can you get off the pond?
Given all variables equal except mass of
the gun, which will “kick” more when
fired—a gun with
a) more mass, or b) less mass?
If you had a space age extremely light
gun so that the gun was just as light as
the bullet, what would be the result?
Before the “big bang,” the total momentum in
the universe was _______.
How about after? Explain!
Before you shoot a gun, the total momentum
of the system is __________.
After you shoot the gun, the total momentum
is _______________ .
This works by adding up the [positive]
momentum of the bullet in one direction + ?
________________________________________