Bullets
Two bullets of equal mass are shot at equal
speeds at blocks of wood on a smooth ice
rink. One bullet, made of rubber bounces
off of the wood. The other bullet, made of
aluminum, burrows into the wood. In which
case does the block of wood move faster?
Explain.
MOMENTUM
What is a collision?
A collision is a short duration interaction
between two objects.
An example of a collision includes a train car rolling toward an
identical car at rest. The two cars couple together upon impact
and then roll down the tracks together.
ANOTHER COLLISION
The collision between a tennis ball and racket may seem
instantaneous to your eye, but that is a limitation of your
perception. A careful look at this picture reveals that the right side
of the ball is flattened and pressed up against the strings of the
racket.
The magnitude of the momentum is the
product of the object’s mass and speed
p=mv
The momentum has the same sign as the
object’s velocity.
Ways to Increase Momentum
• Increase velocity:
– A fast moving object will have more
momentum than a slow moving/still object.
– Compare the momentum of a 40kg
skateboarder moving 10m/s and a 150kg
parked car.
• Increase mass:
– A larger massed object will have more
momentum than a small massed object.
– Compare the momentum of a 5kg dog and a
15kg dog both moving at 5 m/s.
Summary
– A heavy, fast moving object will have a lot of
momentum
– A light, slow moving object will have very little
momentum.
Practice Problem #1
• Describe a situation when two objects with
very different masses can have similar
momenta.(plural for momentum)
– Yes…they must be moving….
Impulse
• A change in momentum is called an
impulse or force applied over a time
I = ΔFt
• What is impulse’s derived unit?
Practice Problem #2
• A professional boxer can punch with a
force of 150N. When he uses gloves, the
time of impact is .9s. However, when he
does not use gloves the time of impact is
.7s. Calculate the impulse for both
punches.
• Impulse is also known as a change in
momentum.
THE IMPULSE-MOMENTUM THEOREM
The stick
exerts an
impulse
on the
puck,
changing
its speed.
The Law of Conservation of
Momentum
State: Total momentum of a system remains
the same before an interaction (a collision
or explosion) and after an interaction (a
collision or explosion)
Mathematically:
Momentum before = momentum after
pbefore = pafter
(mv)before = (mv)after
Collisions
• Two types of collisions:
– Elastic
• Ex:
– Inelastic
• Ex:
– Momentum is still conserved in collisions.
• Baseball
• http://www.youtube.com/watch?v=QFlEIybC7rU
• Pool Ball
• http://www.youtube.com/watch?v=MV28z
AJKGtg
Practice Problem #3
• A billiards ball, ball A, rolls towards a
stationary billiards ball, ball B, with a
velocity of 5 m/s. Both balls have a mass
of 2kg.
– Classify this collision.
– Calculate the momentum before the collision.
– Calculate the velocity of each ball after the
collision.
– What direction is each ball traveling in?
Practice Problem #4
• A 50kg lineman running at 10m/s tackles a
stationary 40kg quarterback.
– Classify this collision.
– Calculate the momentum before the collision.
– Calculate the velocity of the players after the
collision.
– In what direction are the player traveling in
after the collision?
Explosions
• Opposite of collisions
• The particles of the system move apart
after a brief, intense interaction
• Exploding forces are usually internal
7.1 Momentum
• Newton’s 1st Law is also called the Law of Inertia.
• Inertia can be applied to objects in motion and at rest.
• This chapter inertia is concerned with objects in motion, also
called momentum.
• Momentum = mass x velocity
• An object can have a large momentum if it has a large mass,
velocity, or both.
• Which as more momentum a car or truck traveling at the same
velocity? Why? Truck b/c it has a larger mass
• Which has more momentum a car traveling at a faster velocity
than a truck? Car b/c it has more velocity
• How much momentum does an object at rest have? zero
7.2 Momentum
• What has to change if the momentum of an object changes? Mass or
velocity
• A change is velocity means there is an acceleration.
• What is the cause for acceleration? Force
• The greater the force applied, the great the acceleration, which
means a greater change in velocity, which mean a greater change in
momentum.
• A force sustained over a great period of time produces more change
in momentum.
• Both force and time are important in changing momentum.
• Impulse = Force x time
• The greater the impulse the greater the change in momentum.
• You could write another equation:
• Force x time = Δ (mass x velocity)
18. Case #1
• Example given: golfer or baseball player
• Major Concepts: swinging through allows for more time and an applied
force- change in force from instant to instant
• Impact= Force (N)
• Impulse= impact force x time (Ns)
• Case #2
• Example given: hitting haystack instead of wall in a car
• Major Concepts: extend impact time and reduce force
• When you increase the time of impact, you decrease the force of impact.
7.3 Bouncing
• Impulse are great when an object bounces.
• The impulse required to bring an object to a stop
and then to “throw it back again” is greater than
the impulse required to bring the object to a
stop.
• Explain the physics behind Lester A. Pelton’s
improved paddle wheel.
- U- turn of paddle wheel
- Water “bounce,” so impulse increased
7.4 Conservation of Momentum
• Based on Newton’s 2nd Law: a net force produces an
acceleration.
• The chapter says the same thing only with different key terms.
A change in momentum produces a impulse.
• Both impulse and force must be exerted on the object by
something outside the object, internal forces will not work.
• Internal forces come in action/reaction pairs that are equal and
opposite to each other in an object.
• Newton’s 3rd Law: is about action and reaction forces.
• Use Newton’s 3rd Law to analyze a cannonball being fired from
a cannon.
• A. The cannon and cannonball are considered to be internal
forces.
• B. The momentum before and the momentum after are equal
and opposite therefore they cancel each other.
• C. The system does not gain ___________ ___________,
therefore it does not gain momentum.
• If no net force acts on a system, the momentum of that system
cannot change.
• The Law of Conservation of Momentum states: In the absence
of an external force, the momentum of a system remains
unchanged.
7.5 Momentum
• Net momentum before = net momentum after.
• Elastic Collisions = collision without
permanently deforming or generating heat
• Example: billiards
• Inelastic Collisions = collision with deformation
and generating heat
Ex: a toy car hitting play dough
7.6 Momentum Vectors
• Momentum is a vector quality.
• Example:
Review Questions
1. Skater has a faster velocity.
2. Same impulse because same
momentum. Impact would be different.
3. Force and time are inversely related to
time.
4. Yes, no acceleration means no net force
which means no momentum.
5. Yes, because impulse is Ft, or change in
momentum
6. Travel away from each other with the
same velocity
7. Stick together and zero acceleration
8. The velocity is in the direction of motion, a
decreasing to zero