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Chapter 7
Momentum
Remember:
Inertia is the resistance of
any moving or nonmoving
object to change its state
of motion.
Inertia in motion
(moving objects) is
known as MOMENTUM.
7.1 Momentum
Momentum is the product of
the mass and the velocity of
an object.
Momentum = mass X velocity
(momentum = mv)
Momentum has both
direction and magnitude
Therefore, momentum is a
vector quantity.
As mass increases,
momentum increases.
For example: getting hit
by a tennis ball vs. getting
hit by a bowling ball
As velocity increases,
momentum increases
For example: getting
touched on the face vs.
getting punched in the face
Could a roller blade and a
1-ton truck ever have the
same momentum?
Yes – if the velocity of the
skate was substantially more
than the truck since the mass
is so much less.
7.2 What changes momentum?
If the velocity or mass
change, the momentum
changes, since
momentum = velocity X mass
Since mass remains
constant, momentum
generally changes because
of changes in velocity.
What is this called?
Acceleration
What causes acceleration?
Forces!
Therefore…
The greater the force
acting on an object, the
greater the change in
velocity, and the greater
the change in momentum.
Time is also important…
The same force over more
time creates a greater
change in momentum.
Therefore…
Time and force are both
important in changing
momentum.
Force of impact X Time of impact =
Impulse
(f X t)
Impulse = any change in momentum
Impact = force (N)
Why do golfers “follow through”
when they hit the golf ball?
They are increasing the time of
impact which will increase the
momentum.
Your car is out of control. You
are about to crash.
You have a choice:
Do you crash into a brick wall
or a nice, soft haystack?
Why do you choose the haystack?
Why will the haystack cause
less damage?
Because the time of impact is
greater.
Remember: f X t = impulse
As time , force
.
It takes longer for the haystack
to bring your momentum to
zero than it would if you hit the
wall.
Why do we have airbags in cars?
You are forced to jump off a
building.
You can land on a trampoline
or the cement ground.
Which do you choose?
Why?
The trampoline has more
“give”.
This means that the time of
impact is greater, and,
therefore, the force of
impact is less.
As time of impact , the force
of impact
For example:
A car hitting a brick wall
Breaking blocks of wood with
your hands.
The action is sudden and short,
and the force is high.
7.3 Bouncing
Impulses are greater when
bounces occur.
Why?
The force needed to cause
an object to bounce back is
greater than the force
needed to merely stop it.
For example:
A glass bottle falls on your head.
If it breaks when it hits your head,
the impulse has ended.
If it bounces off your head, it takes
more force for your head to bounce
it away.
7.4 Conservation of Momentum
Remember Newton’s 2nd Law?
F=ma
If you want to accelerate
something, you exert a net
force on it
If you want to change
momentum, you exert an
impulse on it.
Internal forces – forces that
act inside a system
External forces – forces that
act on the entire system
For example:
You are in a car. If you push on
the dashboard, the car does not
move. You are an internal
force.
If you get out of the car and
push on the trunk, you can now
move the car; you are now an
external force.
A rifle recoils as it shoots a
bullet.
The bullet gains momentum
as it shoots out.
The rifle gains momentum as
it recoils.
BUT…
The system does not gain or
lose momentum.
Momentum of a system does
not change unless outside
forces act on it.
If momentum does not
change, we say it is
conserved.
Law of Conservation of Momentum:
In the absence of an
external force, the
momentum of a system
remains unchanged.
Law of Conservation of Momentum
Momentum of A + Momentum of B
=
(mass A + mass B)
X
new velocity together
For example:
(write this one down)
A 70000g man is running with a
velocity of 6m/s. A 7000g baby
sits in the road moving at 0m/s.
What is the velocity of the two
after the man picks the baby
up?
7.5 Collisions
Elastic collisions – collisions
that occur when the two
colliding objects do not have
a lasting deformation or the
generation of heat or sound.
Elastic Collisions
10 m/s
0 m/s
10 m/s
5 m/s
0 m/s
5 m/s
10 m/s
10 m/s
Inelastic Collisions –
Collisions which occur
between two objects that
become distorted and
generate heat.
Inelastic objects become
entangled or couple together
after the moment of impact.
Inelastic Collisions
4 m/s
0 m/s
2 m/s
7.6 Momentum Vectors
Since momentum is a vector
quantity, you can represent
collisions with vectors – nonlinear as well as linear
vectors.
Chapter 7 Key Terms
• Conserved
• Elastic Collision
• Impulse
• Inelastic collision
• Law of Conservation of
Momentum
• Momentum