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Physics Teacher Notes - Grothaus
Chapter 8 – Momentum
Title: Momentum
*** Momentum is conserved for all collisions as long as external
forces don’t interfere ***
8.1
Momentum
Which is more difficult to stop: small car or big truck
Truck has more momentum than the car. WHY?
Momentum is merely inertia in motion or “mass in motion”
Momentum = mass x speed (p = mv)
When direction doesn’t matter we can use speed:
mass x speed = mv
A moving object can have a large momentum if it has a large
mass, a high speed, or both.
Large truck vs. roller skate?
Does a stationary object have momentum?
No, it has inertia, but no momentum (zero velocity)
What happens when you double the speed? Double the mass?
8.2
Impulse Changes Momentum
If momentum changes, either mass or velocity changes
If the mass stays the same, then it is the velocity that
changes. What happens when velocity changes???
What causes acceleration? Force!
The greater the force, the greater its change in velocity
and the greater its change in momentum
Remember Newton’s second law:
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Chapter 8 – Momentum
Physics Teacher Notes - Grothaus
F = ma or Newton wrote it as F  m
If we multiply both sides by t, we get
v
t
Ft  mv
Impulse:
The change in momentum depends on the force that acts and
the length of time it acts.
Figure 8.2: pushing a car – brief push vs. push for long time
Both time and force are important in changing an object’s
momentum
force x time interval is called impulse
impulse = Ft
The greater the impulse exerted on something, the greater
will be the change in momentum
Impulse = change in momentum
Or
𝐹𝐹 = ∆(𝐹𝐹)
Increasing Momentum
What’s the best way to increase momentum of an object?
Baseball player, golfer,
As much force as possible, for as long as possible
“follow through”
Examples: pulling a bow all the way back, long
cannon barrel, long range rifle
Other examples?
Decreasing Momentum
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Physics Teacher Notes - Grothaus
Chapter 8 – Momentum
In a car that’s out of control, hit haystack or brick wall?
Carpet vs. hard floor; pillow or brick
Throw a glass or an egg?
Wall vs. haystack: momentum is decreased by same impulse
Product of force and time
Contact time – the time during which your
momentum is brought to zero
Haystack – small force, but long contact time
Wall – big force, but short contact time
Figures on p. 127
A longer contact time reduces the amount of force and
decreases the resulting deceleration
If time is extended 100 times, the force of impact is
reduced 100 times
When we want the force small, extend the time
Ex. Airbags, padded floors for gymnasts,
pulling back when catching a ball
Other examples?
Bungee Jumping – p. 128
Jump down from a chair – bending knees can extend the time
by 10 to 20 times
Boxer p. 128
Dish on carpet or floor? Why?
Egg in a sheet activity
Parkour fall tutorial
http://www.youtube.com/watch?v=RgPd29MlHpg
8.3
Bouncing
If a flower pot falls onto your head and breaks – ouch
If a flower pot falls and then bounces – bigger ouch
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Physics Teacher Notes - Grothaus
Chapter 8 – Momentum
The impulse required to bring an object to a stop and
then to “throw it back again” is greater than the
impulse required merely to bring the object to a stop
Think about if you catch it – impulse to bring momentum to
zero
Then throw it – have to provide more impulse
This is provided by your hand
Above example, it’s provided by your head!
Gold rush – paddle wheels (Lester A. Pelton)
Breaking bricks – science project
https://www.youtube.com/watch?v=aPNCNFeDJIw
Science of breaking bricks
https://www.youtube.com/watch?v=A4Qpu3M4aO8
Slo mo
https://www.youtube.com/watch?v=OwKO2PSphls
35 bricks
https://www.youtube.com/watch?v=5sFYgt-rkNk
8.4
Conservation of Momentum
Newton’s second law – a net force must be applied to an object
for it to accelerate (change in velocity)
Same thing – but different language: an impulse must be
applied to an object for there to be a change in momentum
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Physics Teacher Notes - Grothaus
Chapter 8 – Momentum
(remember that a change in velocity results in a
change in momentum and vice versa)
The force or the impulse has to be external, not internal
Dash board, molecular forces within baseball, etc
Cannon: 3rd law – force of cannonball is equal and opposite to the
forced causing the recoil of the cannon
This is internal to the system so the net force is still zero
Net momentum is also still zero
Figure 8.10, p. 131
Momentum is a vector quantity– magnitude and direction
p = mv velocity has direction and the momentum is in
the same direction as the velocity
If in opposite directions, will be different signs and can cancel
each other out
Cannon/cannonball – the net momentum of the system
is still zero
The Law of Conservation of Momentum – momentum of a system
The law of conservation of momentum states that, in the
absence of an external force, the momentum of a
system remains unchanged.
(always conserved if make the system big enough –
cannon/cannonball, etc.)
Newton’s Third Law
Fundamental!
8.5
Collisions
Conservation of Momentum
Whenever objects collide in the absence of external forces,
the net momentum of both objects before the collision equals
the net momentum of both objects after the collision.
net momentum before = net momentum after
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Physics Teacher Notes - Grothaus
Chapter 8 – Momentum
Elastic collisions (bouncy):
Billiard balls / golf balls
When objects collide without being permanently deformed
and without generating heat, the collision is said to be an
elastic collision.
Figure 8.11 p. 132
Inelastic Collisions (sticky):
A collision in which the colliding objects become distorted and
generate heat during the collision is an inelastic collision.
Head on car crash is one example
When two things collide and get tangled or coupled together,
it’s a totally inelastic collision. (even though there’s no
damage, they are now considered one unit and
their original form has changed)
Figure 8.12 p. 133
Still conservation of momentum -- just momentum is shared
between both objects
Problems:
Remember the following:
Change in momentum = Impulse
𝐹𝐹 = ∆(𝐹𝐹)
also:
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 8 – Momentum
(netmv)before = (netmv)after
If there are two objects before that combine to one object
after, the following happens:
m1v1  m2 v2  (m1  m2 )vafter
There is usually some external force like friction or gravity so
you want to look at the momentum right before and right
after the collision occurs
2 colliding trucks, momentum is same before and right
after – friction provides an impulse that will reduce
momentum afterward
Billiard balls, etc.
Later, we’ll discuss kinetic energy (energy of moving things).
Momentum is conserved, but in an inelastic collision, kinetic energy is
lost to sound, heat, damage, etc. In an elastic collision, almost all
kinetic energy is conserved.
Perfectly elastic collisions are rare, some loss of energy usually
occurs. This is different at the microscopic level.
Conceptual Physics
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