1. 9.2 Conservation of Momentum
A collision is an interaction between two objects which have made contact with each other
In a collision, there is a force on both objects which causes an acceleration of both objects.

2. 9.2 Conservation of Momentum
The rightward moving seven-ball experiences a leftward force which causes it to slow down; the eight-ball experiences a rightward force which causes it to speed up

3. under certain circumstances momentum is a conserved quantity
9.2 Conservation of Momentum
under certain circumstances momentum is a conserved quantity
– a quantity that remains unchanged as a
system evolves.
System – a set of objects that interact with each other
Closed system – a system that does not gain or lose mass
Two types of forces can act on a system of objects
Internal – forces the objects in the system exert on each other
External – forces exerted on the objects from a source external to the system

4. 9.2 Conservation of Momentum
isolated system - the net external force acting on the system is zero (no system on Earth can be completely isolated)
The momentum of each ball may change as a result of the collision (each departing with a velocity different than what it started with)
The sum of their momenta is found to be the same before as after the collision
Sum of momenta before:
mAvAi + mBvBi
Sum of momenta after:
mAvAf + mBvBf =

5. momentum before = momentum after
9.2 Conservation of Momentum
Law of Conservation of Momentum:
The total linear momentum of a closed, isolated system remains constant (is conserved).
m1v1i + m2v2i = m1v1f + m2v2f
momentum before = momentum after

6. There is no net external force during the collision.
9.2 Conservation of Momentum
Frictionless Pool Table (Two-ball system)
There is no net external force during the collision.
The total momentum of this two-ball system IS conserved.

7. Frictionless Pool Table (Two-ball system)
9.2 Conservation of Momentum
Frictionless Pool Table (Two-ball system)
The instant before the balls collide, a hole opens in the table beneath them.
During the collision, an external force causes the balls to accelerate downward causing a change in the total momentum.
Total momentum of this two-ball system is NOT conserved.

8. The force applied by ball 2 causes a net external force.
9.2 Conservation of Momentum
Frictionless Pool Table (One-ball system)
The force applied by ball 2 causes a net external force.
The total momentum of the system is NOT conserved.

9. 9.2 Conservation of Momentum
Starting from rest, two skaters push off against each other on smooth level ice, where friction is negligible. As figure (a) shows, one is a woman (mw = 54 kg), and one is a man (mm = 88 kg). Part (b) of the drawing shows that the woman moves away with a velocity of vf1 = +2.5 m/s. Find the recoil velocity vf2 of the man.

10. A rocket, containing fuel at rest in some reference frame.
9.2 Conservation of Momentum
A rocket, containing fuel at rest in some reference frame.
In the same reference frame, the rocket fires and gases are expelled at high speed out the rear. The total vector momentum remains zero.

11. 9.2 Conservation of Momentum
A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun is pushed backward and the bullet is pushed forward. The acceleration of the recoiling gun is ...
a) greater than the acceleration of the bullet
b) smaller than the acceleration of the bullet
c) the same size as the acceleration of the bullet

12. 9.2 Conservation of Momentum
Calculate the recoil velocity of a 5.0 kg rifle that shoots a kg bullet at a speed of 120 m/s.

13. 9.2 Conservation of Momentum
Movies often show someone firing a gun loaded with blanks. In a blank cartridge the lead bullet is removed, and the end of the shell casing is crimped shut to prevent the gunpowder from spilling out. When a gun fires a blank, is the recoil greater than, the same as, or less than when the gun fires a standard bullet?

14. Momentum is conserved in two-dimensional collisions as well
9.2 Conservation of Momentum
Momentum is conserved in two-dimensional collisions as well
x-component momentum conserved
y-component momentum conserved

15. 9.2 Conservation of Momentum
A 1325 kg car, C, moving north at 27.0 m/s, collides with a 2165 kg car, D, moving east at m/s. The two cars are stuck together. In what direction and with what speed do they move after the collision?

16. 9.2 Conservation of Momentum
A 65.0 kg ice skater moving forward with a velocity of 2.50 m/s throws a kg snowball forward with a velocity of 32.0 m/s.
What is the velocity of the skater after throwing the snowball. Disregard friction.
A second skater initially at rest with a mass of 60.0 kg catches the snowball. What is the velocity of the second skater?

17. 9.2 Chapter 9 Vocabulary impulse momentum impulse-momentum theorem
closed system
isolated system
internal forces
external forces
law of conservation of momentum