Transcript momentum - SFSU Physics & Astronomy

Momentum
Momentum
inertia and motion
Newton’s “quantity of motion”
helps us understand collisions and
explosions
Momentum and Collisions
Elastic Collisions
– objects rebound
– e.g. superball
Inelastic Collisions
– object stick together an usually
become distorted and generate heat
– e.g. clay ball
Momentum
Momentum = mass  velocity
p = mv
Momentum is a vector quantity.
A large truck has more momentum
than a car moving at the same
speed because it has a greater
mass.
Which is more difficult to slow
down? The car or the large truck?
Large Momentum Examples:
– Huge ship moving at a small velocity
P = mv
– High velocity bullet
P = mv
Impulse
In order to change the momentum of
an object you must apply a force
over some time interval.
Impulse = Force  time interval

= Ft
Impulse and Momentum
Impulse = Change in Momentum

= Final (mv) - Initial (mv)
F t = D(mv)
Case 1: Increasing Momentum
– Apply a force for a long time.
– Examples:
• Follow through on a golf swing.
– http://www.youtube.com/watch?v=Xy9GTAENiJA
• Pushing a car.
F
Dt
Case 2: Decreasing Momentum
– Apply a force for a long time.
– Examples:
• Air bags in cars.
–
–
–
http://www.youtube.com/watch?v=nlWJayKWvw0
http://www.youtube.com/watch?v=_Av0WGrlTGY
http://www.youtube.com/watch?v=D-ogrMr4lWc
• Catching an egg.
• Boxing, Figure 6.6
• Soft collisions, Figure 6.3.
F
Dt
Case 3: Decreasing Momentum
– Apply a force for a short time.
– Examples:
• Boxing
• Karate
–
http://www.youtube.com/watch?v=jttqJZ9sTBk
F
Dt
Conservation of Momentum
This means that the momentum
doesn’t change.
Recall that F t = D(mv)
In this equation, F is the "external
force".
Internal forces cannot cause a
change in momentum.
Examples
Conservation of Momentum: If
there are no external forces, the
total momentum for a system
remains unchanged.
Example 1: a person sitting inside a
car pushing against the dashboard
Example 2: a bullet fired from a rifle
Example 3: a rocket is space
Conservation of momentum
 The total momentum of a group of
interacting objects remains the same in
the absence of external forces
 Applications: Collisions, analyzing
action/reaction interactions
Question
Which has more momentum?
– (A) A parked cruise ship
– (B) You in your vehicle driving home
Question
1. A force times time is most
appropriately called
(a) weight
(b) impulse
(c) velocity
(d) momentum
Question
2. Suppose two freight cars of equal
mass are coupled by a collision.
This is an example of
(a) an elastic collision
(b) an inelastic collision
(c) none of the above
Question
3. Which of the following is true about
momentum?
 (a) it is a vector
 (b) it is a product of mass times velocity
 (c) impulses are required to change it
 (d) all of the above