Work, Energy, Power, Momentum
Impulse and
Momentum
Egg drop
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 Drop an egg in a beaker
 Drop an egg in a beaker with a sponge in the bottom
 What do you observe?
Similarities
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Differences________
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Impulse
 Impulse is a force applied over time
 To stop such an object, it is necessary to
apply a force against its motion for a
given period of time
 Impulse = F (t)
In terms of impact and impulse, why
are airbags in car a great invention?
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Impulse
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Which activity would require more impulse
1.) Accelerating a soccer ball from rest to 10m/s OR
accelerating a medicine ball from rest to 10 m/s?
2.) Slowing a car from 60mph to 40 mph OR slowing
the same car from 40mph to 10mph?
3.) Landing from a jump while flexing the legs (bending
at the knees) OR landing from a jump while keeping the
legs straight (locking knees)?
4.) What can we conclude about IMPULSE?
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Bowling ball
What happens if…
 I swing a bowling ball at you?
Possibility #1
Possibility #2
Possibility #3
(Circle the one that does happen)
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Newton’s Cradle
What happens if
 I lift and release one ball
Possibility#1
Possibility #2
Possibility #3
What if I lift and release more than 1 ball?
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Newton’s Cradle Physics
There’s an app for that!
 The same principle applies to the suspended-ball desk toy,
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which eerily “knows” how many balls you let go…
Only way to simultaneously satisfy energy and momentum
conservation
Relies on balls to all have same mass
Momentum depends on speed/velocity and mass
Giant Newton’s Cradle video
Discover for yourself. Record in your notebook and on a
whiteboard to share out with the class.
 Place 5 marbles in the center groove of a ruler. Launch a sixth
marble toward the 5 stationary marbles. Note and record what
happens.
 Now launch two marbles at four stationary marbles. Then launch
three marbles at three stationary marbles and so on. Note and
record what happens each time.
 Remove all but two marbles from the groove. Roll these two
marbles at each other with equal speeds. Note and record what
happens.
1.) How did the approximate speed of the marbles before each
collision compare to after each collision?
2.) What factors determine how the speed of the marbles changes in
a collision?
3.) What do you think would happen if three marbles rolling to the
right and two marbles rolling to the left with the same speed were
to collide?
4.) What factors affect an object’s momentum?
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What is momentum???
 Discuss with your partner and come up
with an example to share with the class
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What happens if
 I drop one super ball?
Slow motion ball bounce
 I drop two balls stacked on each other?
Basketball and tennis ball
Describe the motion on your paper
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Superball Physics
 During bounce, if force on/from floor is purely vertical,
expect constant horizontal velocity
 constant velocity in absence of forces
 like in picture to upper right
 BUT, superballs often behave contrary to intuition
 back-and-forth motion
 boomerang effect
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Real-World Collisions
 Is a superball elastic or inelastic?
 It bounces, so it’s not completely inelastic
 It doesn’t return to original height after bounce, so some energy
must be lost
 Superball often bounces 80% original height
 Golf ball  65%
 Tennis ball  55%
 Baseball  30%
 Depends also on surface, which can absorb some of the ball’s
energy
 down comforter/mattress or thick mud would absorb
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Momentum
 Momentum can be defined as "mass in motion." All objects
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have mass; so if an object is moving, then it has momentum
Momentum depends upon the variables mass and velocity
Momentum = mass * velocity
p=m*v
where m = mass and v=velocity
 Momentum is conserved.
Momentum can pass from one
object to another (like the super balls)
Momentum is a vector quantity
 To fully describe the momentum of a 5-kg
bowling ball moving westward at 2 m/s,
you must include information about both
the magnitude and the direction of the
bowling ball
p = m * v
 p = 5 kg * 2 m/s west
 p = 10 kg * m / s west
Check Your Understanding
 Determine the momentum of a ...
1.) 60-kg halfback moving eastward at 9 m/s.
p = mv =
2.) 1000-kg car moving northward at 20
m/s.
p = mv =
Momentum and Impulse Connection
 To stop such an object, it is necessary to apply a force against its
motion for a given period of time
Impulse = F (t) = m D v
Check Your Understanding
 If the halfback experienced a force of 800 N
for 0.9 seconds to the north, determine the
impulse
 Impulse = F ( t ) = m D v
Impulse Question #2
 A 0.10 Kg model rocket’s engine is designed
to deliver an impulse of 6.0 N*s. If the rocket
engine burns for 0.75 s, what is the average
force does the engine produce?
 Impulse = F ( t ) = m D v
Impulse Question # 3
 A Bullet traveling at 500 m/s is brought to
rest by an impulse of 50 N*s. What is the
mass of the bullet?
 Impulse = F ( t ) = m D v
To finish…
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 CU (p307) 1-3
 PtoGo (p309) 1-2
Get your work stamped before you leave
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When is a collision elastic or inelastic?
 Phet: Collision Lab
http://phet.colorado.edu/en/simulation/collision-lab
 Collisions and conservation of momentum
 Click on advanced tab for more settings
 Green arrows = velocity
 Yellow arrows = momentum
 Total momentum displayed in the chart
Finish both sides and get a stamp before
you leave today
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When is a collision elastic or inelastic?
 Phet: Collision Lab (finish both sides and get a
stamp before you leave today)
 Click on advanced tab for more settings
 Green arrows = velocity
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 Yellow arrows = momentum
 Total momentum displayed in the chart
 CU (p315) 1-3
 PtoGo (p319) 1-2
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PtoGo (p319)
Problem #2
Two 1 kg carts are each moving towards each other at 2 m/s.
They collide and each reverses direction, moving in the
opposite direction at 2 m/s. Draw a diagram showing the carts
before and after the collision.
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Before the collision
After the collision
P = mv
P = mv
P = (1)(2)
P = (1)(-2)
P = 2 kgm/s P = -2 kgm/s
Total p = 2-2 = 0 kgm/s
P = mv
P = (1)(-2)
P = (1)(2)
P = -2 kgm/s P = 2 kgm/s
Total p = 2-2 = 0 kgm/s
What did we learn about collisions and
conservation of momentum and impulse
from this week? (at least 5 required)
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This is our MODEL for MOMENTUM
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Elastic and inelastic Collisions
 When a Ball hits the ground and sticks,
the collision would be totally inelastic
 When a Ball hits the ground and
bounces to the same height, the
collision is elastic
 All other collisions are partially elastic
collision
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Collisions
 Two types of collisions
 Elastic: Energy not dissipated
out of kinetic energy
 Bouncy
 Inelastic: Some energy dissipated to other
forms
 Sticky
 Perfect elasticity unattainable (perpetual motion)
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HAPPY SAD SUPER BALLS
Which ball is elastic? How do you
know?
Which ball is inelastic? How do you
know?
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Things that go “bump”
 Record what you see
 Write a possible explanation on your
whiteboard
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Momentum modelDQ: what can objects do with momentum?
1.) Impulse is a force applied over time
impulse = force*time
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2.) Momentum is mass in motion
3.) the impulse equals the momentum change
4.) Momentum is mass x velocity p=mv
5.) Momentum can be transferred to other objects
when they collide
6.) Momentum is conserved (none is lost or gained
during collisions) Law of Conservation of Momentum
 Momentum video
Momentum modelDQ: what can objects do with momentum?
7.) an inelastic collision is when two objects
collide and stick together
8.) an elastic collision is when two objects
collide and then bounce apart
9.) an explosion is a type of elastic collision
Math review
Review (song)
Review video
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Finish
CDP 8-1
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CU(p315) 1-3/PtoGo(p319) 1-2
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Get each page stamped when finished
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CDP 8-1 Problem #8
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p = mv
240 = 120v
v= 2 m/s
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Collisions and conservation of
momentum
Number puzzles
CU (p329)1-4
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Warm-up Questions
1. Twin trouble-makers rig a pair of swings to hang from
the same hooks, facing each other. They get friends to
pull them back (the same distance from the bottom of
the swing) and let them go. When they collide in the
center, which way do they swing (as a heap), if any?
What if Fred was pulled higher than George before
release?
2. A 100 kg ogre clobbers a dainty 50 kg figure skater
while trying to learn to ice-skate. If the ogre is moving
at 6 m/s before the collision, at what speed will the
tangled pile be sliding afterwards?
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3. Summarize the steps needed to solve question #2.
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Elastic Collision: Billiard Balls
• Whack stationary ball with identical ball moving at velocity vcue
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To conserve both energy and momentum, cue ball stops dead,
and 8-ball takes off with vcue
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Momentum conservation: mvcue = mvcue, after + mv8-ball
Energy conservation: ½mv2cue = ½mv2cue, after + ½mv28-ball
The only way v0 = v1 + v2 and v20 = v21 + v22 is if either v1 or v2 is 0.
Since cue ball can’t move through 8-ball, cue ball gets stopped.
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Inelastic Collision
 Energy not conserved (absorbed into other paths)
 Non-bouncy: hacky sack, velcro ball, ball of clay
Momentum before = m1vinitial
Momentum after = (m1 + m2)vfinal = m1vinitial (because conserved)
Energy before = ½m1v2initial
Energy after = ½ (m1 + m2)v2final + heat energy
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Collisions and conservation of
momentum
Number puzzles part 2
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Momentum Quiz tomorrow
 You may use your notebook on all parts of the
test
 Bill Nye Momentum (5 min)
Momentum is ???
 Momentum tutorial
 Conservation of Linear Momentum (4 min)
 How is LM conserved?
 What is an elastic collision? Inelastic?
 Use simbucket.com to prove it!
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Angular Momentum
 Another conserved quantity is angular momentum, relating to
rotational inertia:
 Spinning wheel wants to keep on spinning, stationary wheel
wants to keep still (unless acted upon by an external
rotational force, or torque)
 Newton’s laws for linear (straight-line) motion have direct
analogs in rotational motion
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Angular Momentum
 Angular momentum is proportional to rotation speed ()
times rotational inertia (I)
 Rotational inertia characterized by (mass)(radius)2
distribution in object
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Angular Momentum Conservation
 Speed up rotation by tucking
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in
Slow down rotation by
stretching out
Seen in diving all the time
Figure skaters demonstrate
impressively
Effect amplified by moving
large masses to vastly different
radii
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Do cats violate physical law?
 Cats can quickly flip
themselves to land on their
feet
 If not rotating before, where
do they get their angular
momentum?
 There are ways to accomplish
this, by a combination of
contortion and varying
rotational inertia
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