1. Physics 218
Lecture 15: Momentum
Alexei Safonov

2. Chapter 8: Momentum Want to deal with more complicated systems
Collisions
Explosions
Newton’s laws still work, but using them directly gets harder:
New tricks similar to energy conservation could help

3. Center of mass definition
Today’s Lecture
Begin with a definition of Linear Momentum
Conservation of momentum helps us solve certain types of problems
Things colliding
Things exploding
Center of mass definition

4. Definition of Linear Momentum
Vector equation!

5. Restating Newton’s Second Law
“The rate of change of momentum of an object is equal to the net force applied to it”
If we exert a net force on a body, the momentum of the body changes
All we do is re-writing Newton’s Law to make many classes of problems easy to solve, nothing fundamentally new

6. What if SF=0? Momentum doesn’t change momentum before = momentum after
If SF=0, then dp/dt = 0,  p = constant
Momentum doesn’t change
momentum before = momentum after

7. Conservation of Momentum
For a system, by Newton’s laws, SF=0
 Conservation of Momentum
Sum of all Sum of all
momentum before = momentum after
True in X and Y directions separately!

8. Ball of mass m is dropped from a height h:
Example
Ball of mass m is dropped from a height h:
What is the momentum before release?
What is the momentum before it hits the ground?
Is momentum conserved?

9. SF=0, then dp/dt = 0, → p = constant
What if we add the Earth?
What is the force on the ball?
What is the force on the earth?
Is there any net force in this system?
Is momentum conserved?
SF=0, then dp/dt = 0, → p = constant

10. Momentum for a System is Conserved
Momentum is ALWAYS conserved for a COMPLETE SYSTEM, you just have to look at a big enough system to see it correctly.
Not conserved for a single ball in the field of gravity
A ball falling is not a big enough system. You need to consider what is making it fall.
Momentum is conserved if the system is closed, i.e. either “large enough” or no external forces
Internal forces do not break momentum conservation

11. Energy and Momentum in Collisions
Definitions:
Elastic collision = kinetic energy is conserved
Inelastic collision = kinetic energy is not conserved.
Momentum conserved?
Total Energy conserved?

12. Head On Collision
A ball of mass m1 collides head on (elastically) with a second ball at rest and rebounds (goes in the opposite direction) with speed equal to ¼ of its original speed.
What is the mass of the second ball m2?

13. Inelastic Collisions By definition, in inelastic collision
Mechanical energy is not conserved
Kinetic energy not conserved
Inelastic Example: Two trains which collide and stick together

14. Colliding Trains: 1 Dimension
The train car on the left, mass m1, is moving with speed Vo when it collides with a stationary car of mass m2. The two stick together.
What is their speed after the collision?
Show that this is inelastic

15. Ballistic Pendulum
A bullet of mass m and velocity Vo plows into a block of wood with mass M which is part of a pendulum.
How high, h, does the block of wood go?
Is the collision elastic or inelastic?

16. Two Balls in Two Dimensions
Before a collision, ball 1 moves with speed v1 in the x direction, while ball 2 is at rest. Both have the same mass. After the collision, the balls go off at angles Q and –Q. What are v’1 and v’2 after the collision? Q -Q