Linear Momentum
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What we will cover.
Momentum and Its Relation to Force
Collisions and Impulse
Conservation of Momentum
Conservation of Energy and Momentum in Collisions
Elastic & Inelastic Collisions in One Dimension
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Mr. Baldwin
PHYSICS
IMPULSE & MOMENTUM
31-Oct-13
Aim: Why is a follow through so important?
Do Now: If a toddler and a sumo wrestler are
both running toward you, which one would be
easier to stop? Why?
Homework: ESSAY is Due tomorrow
Let’s start with everyday language.
What do you say when a sports
team is on a roll?
They may not have the lead, but
they sure may have ___________
MOMENTUM
A team that has momentum is hard
to stop.
An object with a lot of
momentum is also hard to stop
Momentum and Its Relation to Force
Momentum is defined as the product between an
object’s mass and its velocity. It is a vector &
symbolized by the symbol p.
p  mv
CHECK: Can you derive the units for momentum?
The unit is: kg∙m/s
The rate of change of momentum is equal to the net
force:
p
F
t
m  v 
v

m
 ma
t
t
This is nothing but Newton’s second law.
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Let’s practice
A 1200 kg car drives West at 25 m/s for 3 hours.
What is the car’s momentum?
• Identify the variables:
– m = 1200 kg
– v = 25m/s, West
– t = 3 hrs
p = mv = 1200 kg x 25 m/s= 30,000 kg∙m/s, West
How hard is it to stop a moving object?
To stop an object, we have to apply a force over
a period of time.
This is called Impulse
The definition of impulse J: the change in
momentum of an object OR force times time of
contact.

p
F
t
F  t  p
p  F  t  J
CHECK: What is the unit of Impulse?
The unit is: N∙s
Collisions and Impulse
During a collision, objects are
deformed due to the large forces
involved.
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Why does an egg break or not break?
• An egg dropped on a tile floor breaks, but an egg
dropped on a pillow does not. Why?
F  t  p
In both cases, m and Δv are the same, so Δp is the same.
If Δt goes up, what happens to F, the force?
Right! Force goes down. When dropped on a pillow,
the egg starts to slow down as soon as it touches it. A
pillow increases the time the egg takes to stops.
CHECK: Why are dashboards padded?
Practice Problem
A 57 gram tennis ball falls on a tile floor. The ball changes
velocity from -1.2 m/s to +1.2 m/s in 0.02 s. What is the
average force on the ball?
Identify the variables:
m = 57 g = 0.057 kg
Δv = +1.2m/s – (-1.2m/s) = 2.4 m/s
t = 0.02 s
using FΔt= mΔv
F x (0.02 s) = (0.057 kg)(2.4 m/s)
F= 6.8 N
Sample Regents Question
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Mr. Baldwin
PHYSICS
IMPULSE & MOMENTUM
1-Nov-13
Aim: What happens when objects collide?
Do Now: What would happen if a 2 tonne truck
rear ends a 1 tonne pick up? How would the
wreckage move afterwards?
Homework: DO the first five problems on
Momentum and Impulse worksheets.
Conservation of Momentum
During a collision, measurements show that the total
momentum does not change. The total momentum in
any collision is conserved.
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Conservation of Momentum
More formally, the law of conservation of momentum
states:
The total momentum of an isolated system of objects
remains constant.
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Mr. Baldwin
PHYSICS
IMPULSE & MOMENTUM
4-Nov-13
Aim: How does a rocket (space shuttle) take off?
Do Now: QUIZ
Suppose you are an astronaut that became stranded
in space when your tether line breaks. You begin to
float away from your space station with only a
wrench in hand. Explain how would you save your
life? How would you return to the space station?
(Use the terminology you have learned.)
Homework: Design an Egg Drop Experiment
PHY 205 BALDWIN
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Conservation of Momentum
Momentum conservation works for a rocket. WHY?
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Conservation of Momentum
Momentum conservation works for a rocket. WHY?
What about Newton’s 3rd law? Does it work here?
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What are the equations associated with
momentum and impulse?
p  mv
p  F  t  J
p
F
t
PHY 205 BALDWIN
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A 0.2 kg baseball is travelling at 40 m/s. After being hit
by a bat, the ball's velocity is 50 m/s in the opposite
direction. Find
a) the change in momentum
b) the impulse , J
c) the average force, F, exerted by the bat if the ball and
bat are in contact for 0.002 s.
d) In which direction is the force acting?
m  0.2 kg
vi  40m / s
v f  50m / s
J  p  mv f  mvi   0.2 kg  50m / s    0.2 kg  40m / s 
J  10 kg  m / s  8 kg  m / s  18kg  m / s
t  0.002s
p 18kg  m / s
F

 9000 N
t
0.002s
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CHECK…YOUR TURN!!!
A 1000 kg car travelling 22 m/s (about 50 mi/hr) hits a
concrete bridge support and comes to a stop in .5 s.
a) Find the average force acting on the car
b) if the bridge support had been cushioned so that the
stopping time was increased to 3 s, what would have
been the average force.
c) In which direction is the force acting and why?
m  1000kg
vi  22m / s
v f  0m / s
t  0.5s
p  mv f  mvi  1000 kg  0m / s   1000kg  22m / s 
p  22000kg  m / s
p 22000kg  m / s
F

 44000 N
t
0.5s
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Mr. Baldwin
PHYSICS
Conservation of Momentum
7-Nov-13
Aim: How is momentum conserved in elastic and inelastic
collisions?
Do Now: QUIZ (10 minutes)
A 2000 kg car is travelling at 20 m/s. The driver hits the gas
and increases his speed to 50 m/s. Find
a) the change in momentum of the car
b) the impulse, J, imparted to the car
c) the average force, F, exerted by the engine if his foot was
on the gas for 5.0 s
d) In which direction is the force acting? Can you explain?
Homework: Design an Egg Drop Experiment
What can you tell about your impulse when you
jump off from a high place? Why do you bend
your knees?
The impulse tells us that we can
achieve the SAME change in
momentum with a large force
acting for a short time OR a
small force acting for a longer
time.
This is why you should bend
your knees when you land
after jumping; it increases
time; it reduces force.
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What can you tell about your impulse when you
impact your airbags?
How is this similar to when you bend your knees
after landing from a jump?
Why do airbags work?
Why does landing on a
pillow hurts less than
landing on concrete.
IT’S ALL ABOUT THE
FORCE!
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Impulse/Impact
Decreasing momentum in a short time
Car hitting concrete wall-- great damage by
quick change
Karate master-- great momentum of hand
causes breaking of bricks
Impulse
Can you tell me what a collision is?
Collisions are the interactions of two or more objects.
Momentum is conserved in
ALL collisions.
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Elastic Collisions
Collisions in which
kinetic energy is
conserved.
Momentum is
perfectly
transferred from
one object to the
next.
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Inelastic Collisions
With inelastic collisions,
some of the initial kinetic
energy is lost to thermal,
sound or potential energy.
A completely inelastic
collision is one where the
objects stick together
afterwards, so there is only
one final velocity.
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CHECK: CAN YOU GIVE ME SOME EXAMPLES
OF ELASTIC & INELASTIC COLLISIONS?
OK…How would you classify these types?
• Two lumps of clay colliding
• Rubber ball on hard surface
• Two billiard balls colliding
• Two-Car collision
• Two bumper cars collision
• Wad of gum tossed onto a rotating cd
• Meteor impacting the Earth
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Design an Experiment & Build a Device
You are to design and build a
device that will allow a
chicken egg to survive the
impact of dropping from
various heights. In the
process you will make a
hypothesis about what you
think will happen to the egg
by explaining its inertia,
motion, gravity, momentum
and impulse.
BE CREATIVE!!!
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http://weirdsciencekids.com/EggDropExperiment.html