Applied Science III - Finau
MOMENTUM
What is inertia?
 An object’s ability to
change its motion
 A measure of the amount
of matter within the object
Which has more inertia? A
car traveling at 20 mph or a
bus standing still? Why?
 The bus has more inertia
 It has more mass, thus harder to change its
motion
Which would have more momentum?
 The car would have more momentum
because it has more velocity (speed)
If a bus and a car are both
traveling at 20 mph, which
would have more momentum?
 The bus would have more momentum
because it has more mass
If two Mustangs w/the same
mass move at different
speeds (1st at 20mph & 2nd at
40 mph), which would have
more momentum?
 The 2nd Mustang would have more
momentum because it is moving with a faster
velocity
What does momentum depend on?
What are the Relationships?
 Momentum depends on both mass & velocity
of an object
 Both are directly proportional to Momentum
Can you define momentum?
Equation?
Formulated by French Scientists/Philosopher
René Descartes (1596-1650):
 The amount of motion of an object that
determines the amount of time needed to change
its motion when acted on by a force
 Momentum = mass X velocity
How much momentum does a
1000 kg car have traveling
at 5 m/s?
 p = mv
 Mass = 1000 kg
 Velocity = 5 m/s
 p = (1000)(5) = 5000 kgm/s
Differences in Momentum: Sumo vs Punch
In order to change an
object’s momentum, what must
happen?
 Change its velocity
 Create an
acceleration
 Apply a force
What is the difference
between applying a force for
2 seconds and applying a
force for 4 seconds?
 Applying a force for more time changes an
object’s speed more
 ie. - The longer you push, the faster it goes
What is impulse? Equation?
 Impulse is a force applied over a time interval
that changes the momentum of an object
 Impulse = Force X time
How big of an impulse would
pushing on a couch with 50 N
of force over 2 seconds
create?
 I = Ft
 Force = 50 N
 Time = 2 s
 I = (50)(2) = 100 Ns
How are Impulse and change in
momentum related?
 Directly Proportional – increase Impulse,
increase change in momentum
What is the Impulse-momentum
theorem?
 The change in an object’s momentum is equal to
the impulse acting on the object
 I = Δp
 *note – Δ means “change in”
 If we break into variables  Ft = mΔv
When a boxer hits his
opponent, an Impulse
is created that will
change the motion of
the opponent’s head.
In each situation, we
assume the change
in momentum to be
the same: Ft = Δp
The boxer throws a punch and his opponent has 1 of 3
choices. He can either 1) (accidentally) move into
the punch, 2) stand still 3) roll with the punch.
Let’s try to understand how much force is applied to
his opponent’s face.
Rocky IV Final Fight
If the opponent moves into the punch,
what will happen to the amount of time
the boxer’s fist interacts with his
opponent’s face? What happens to the
force?
 Time decreases because it spends
less time changing his face’s
momentum
 The force increases as time
decreases
If the opponent stands still, what
will happen to the amount of time
compared with if he moves into the
punch? What happens to the force that
hits the boxer’s face?
 The boxer’s fist takes
more time interacting
with his face
 Less Force acting on his
face
If the opponent rolls with the punch,
what will happen to the amount of time
compared with if he stands still? What
happens to the force that hits the
boxer’s face?
 The boxer’s fist takes the most time
interacting with his face
 Least amount of force acts on the opponent’s
face
So assuming that the change in
momentum of an object remains the
same, what’s the relationship between
the force acting on the object and the
time it acts on the object?
 Inversely Proportional
 As the time of interaction increases, the force
acting on the object decreases
What is the Impulse-momentum
theorem?
 The change in an object’s momentum is equal to
the impulse acting on the object
 I = Δp
 If we break into variables  Ft = mΔv
 *note – Δ means “change in”
How does an air bag help prevent
a person from serious injury?
 The bag increases
the amount of
time to slow your
head down
 Reduces the force
acting on your
head
Can you name some situations
where we use this concept
everyday?
 You give some examples…
 Airbags
 Seatbelts
 Padding (in football, gym equipment)
 Water barrels at corner of highway exits
Do you remember what Newton’s
Third Law of Motion is? (it’s
important!)
 For every action (or force), there is an equal
and opposite reaction (or force)
Imagine two pool balls rolling
towards each other. How does
Newton’s 3rd Law apply?
 Each ball applies an equal and opposite force
on each other
 F1 = F2
What about the amount of
time the two balls interact
with each other?
 The collisions takes place over one set
amount of time; there is no change.
 Time is constant
What can we say about the
impulse acting on each ball?
 Remember…Impulse = Force X Time
 The impulse from each ball is equal and
opposite
What can we say about the
change in momentum of each
ball?
 Remember…Impulse-Momentum Theorem?
 The changes in momentum are equal and
opposite
What is the law of Conservation
of Momentum?
 The total momentum before a collision is
equal to the total momentum after the
collision
 Momentum is ALWAYS CONSERVED!!!!
Astronaut Richard Garriott
discussing Momentum
Two skaters initially at rest push
against each other so that they move in
opposite directions. What is the total
momentum before they push off of each
other?
 Total momentum is
zero, since there is
no velocity.
What is the total momentum
after they push off? Explain
this even if they’re moving.
 Total momentum is
zero, according to
conservation of
momentum.
 The velocities are
equal and opposite.
After a gun is shot, explain what
happens. Why does this occur?
 The gun recoils…shoves backward
 Because the bullet is shot outward at high
speed, the gun is shoved backwards
What is the law of Conservation
of Momentum?
 The total momentum before a collision is
equal to the total momentum after the
collision
 Momentum is ALWAYS CONSERVED!!!!
What’s wrong with this clip?
After watching the video clip, explain
using Conservation of Momentum why
this can’t happen.
 The total momentum of the bullet should equal the
total momentum of the person.
 Approximate bullet speed – 400 m/s
Bullet mass – about .01 kg
 Thus momentum of bullet is 4 kgm/s
 If man has a mass of 90 kg, what is his calculated
speed?
An astronaut working in space finds himself
drifting away from the shuttle. He forgets his
zip cord and has no propulsion device. The
only object he has is a large wrench. Using
the concept of Conservation of Momentum, what
could he do in order to safely get back to the
shuttle?
 Throw the wrench
away from the shuttle
 Conservation of
momentum says he
will be propelled in the
opposite direction of
the wrench
What is the law of Conservation
of Momentum?
 The total momentum before a collision is
equal to the total momentum after the
collision
 Momentum is ALWAYS CONSERVED!!!!
Explain why can’t this happen?
What are the 3 types of
Collisions
 Elastic Collision
 Inelastic Collision
 Perfectly Inelastic Collision
Elastic Collision
 Objects collide and separate w/
NO deformation
 Both momentum & kinetic energy
are conserved
 Approximate
Examples:
 Playing pool
 Marbles
 Exact Examples:
 Atoms colliding
Inelastic Collision
 Objects collide and split
apart w/ SOME
deformation
 Momentum is conserved
 Some Kinetic Energy is
lost
 Examples:
 Bumper cars
 Boxing punch
 Kicking Soccer ball
Perfectly Inelastic Collision
 Objects collide and
stick together
 Momentum is
conserved
 Some Kinetic Energy
is lost
 Examples:
 Football tackle
 Bug on a windshield
 Hockey puck and glove
Bill Nye on Momentum