Chapter 8 Momentum
Chapter
objectives
8.1 Momentum
p. 138 #1-3
p. 139 #20
p. 140 #24, 25
Define Momentum (8.1)
Define impulse and describe how it affects
changes in momentum (8.2)
 Explain why an impulse is greater when an
object bounces than when the same object
comes to a sudden stop (8.3)
 State the Law of Conservation of Momentum
(8.4)
 Distinguish between an elastic and inelastic
collision (8.5)
 Describe how the vector nature of
momentum affects the law of conservation of
momentum. (8.6)
Momentum is the inertia of objects in motion.
Momentum is inertia in motion


Momentum = mass x velocity
More mass  more momentum
More velocity  more momentum
So both mass and velocity are directly proportional
to momentum.
** momentum is a vector because it has magnitude
and direction.
A moving object can have a large momentum if it
has a large mass, high speed, or both.
8.2 Impulse
changes
Momentum
p. 138 #4-11
p. 139 #21
Impulse is a change in momentum
For momentum to change there must be a change in
velocity or a change in mass.
For there to be a change in velocity (this is called
acceleration) there must be a force. The larger the
force, the greater the change.
Not only is the size of the force important, but how
long the force acts on the object is considered.
Impulse = force x change in time
Or
Impulse = Fxt
Or
Ft= (mv)
A batter in baseball swings through the ball to apply
force for the longest period of time. Same as a
golfer swinging through.
The change in momentum depends on the force tha
acts and the length of time it acts.
Egg in the sheet
demo
When decreasing impulse, the longer the time of
impact the smaller the force.
If
Then
impulse = Fxt
F = impulse
t
so the longer the time of impact, the less force. We
can see from this equation that time of the impact
is inversely proportional to Force
Think about this in terms of cushioned athletic
shoes. The padding increases the time of impact so
force is decreased.
See bungee
jumping on p. 128
consider Ali’s
“rope-a-dope”
move
These barrels are filled with increasingly heavy sand
to gradually slow a car speeding toward the
barricade. Since the impact will be spread out,
there will be less force.
A ceramic dish falling on a tile floor is more likely to
break than the same dish falling on a wooden floor.
When we jump down, it is safer to bend the knees.
8.3 Bouncing
Have students
catch a bowling
ball. Have them
catch the ball
then throw it
back. Which
requires more
force?
p. 138 #12-14
p. 141 #47
In a bungee jump, the cord stretches twice the
length of the cord to absorb the force.
“Impulses are greater when an object bounces.” P. 91
Stopping an object takes a certain amount of force.
But stopping the object and then throwing it back
again takes more force.
Imagine catching a ball in your hand. Imagine
catching it and throwing it back. Which involves
more impulse?
The impulse required to bring an object to a stop
and the to “throw it back again” is greater than the
impulse required merely to bring the object to a
stop.
8.4 Conservation
of momentum
p. 138 #15,16
Changing momentum requires an external force.
Momentum within a system is conserved.
The Law of Conservation of Momentum states:
In the absence of an external force the momentum of a
system remains unchanged.
Consider a cannon and cannonball system. Before
firing the system has a momentum of zero. After
firing, the cannon and cannonball each have the
same momentum but in opposite directions. So the
net is zero
8.5 Collisions
see diagram with
the colliding balls
on p. 132
The law of conservation of momentum states that,
in the absence of an external force, the momentum
of a system remains unchanged.
There are two types of collisions: Elastic & Inelastic
When objects collide without being permanently
deformed and without generating heat, it is said to
be an elastic collision.
Momentum is transferred from one object to
another.
i = F
see diagram of the
train cars on p. 133
p. 139 # 17,18
p. 140 #26,27,32
net momentum before = net momentum after
 represents momentum
i is “initial” momentum
F is “final” momentum
Whenever colliding objects get tangled or coupled
together, there is an inelastic collision.
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.
???? Why is p used for momentum? m is used for
mass, so P is used for the persistence of the object.
8.6 Momentum
Vectors
Collisions that occur at angles can be analyzed by
using vectors that we used earlier.
p. 139 #19
The vector sum of the momenta is the same before
and after a collision.