momentum - SFSU Physics & Astronomy
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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