1. Momentum

2. Conservation of Momentum
Like energy, momentum is conserved in nature. It is not created or destroyed; so in a closed system we will always have the same amount of momentum.
In a closed system, such as this flask with a stopper, the gas particles are in constant, random motion at all temperatures above absolute 0. They are constantly colliding with one another and the flask wall, but the momentum lost by one particle is gained by another.

3. A key difference between momentum and energy is that while energy is a scalar, momentum is a vector.
Another key difference is that there is only one formula for momentum.
The momentum (p) of a single object is the product of its mass and its velocity:
p = mv

4. Elastic Collisions
During a collision, measurements show that the total momentum does not change:
mAvA + mBvB = mAvA' + mBvB'

5. Sample Problem
An astronaut at rest in space fires a thruster pistol that expels 35 g of hot gas at 875 m/s. The combined mass of the astronaut and the pistol is 84 kg. How fast is the astronaut moving after firing the pistol?

6. Elastic Collisions With Identical Masses
A special case is when two objects with the same mass have an elastic collision.
In that case, they exchange velocities:
v1' = v2 and v2' = v1

7. Inelastic Collisions
A perfectly inelastic collision is one where the objects stick together afterwards, so there is only one final velocity.
m1v1 + m2v2 = (m1+ m2)v3

8. Sample Problem
A 2275-kg car going 28 m/s rear-ends a 875-kg compact car stopped on an icy road. The two cars stick together. How fast does the wreckage move after the collision?

9. Impulse
Impulse equals the force applied multiplied by the change in time.
Impulse is also equal to the change in momentum.
= p2 - p1
= m2v2 - m1v1

10. Sample Problem
A 1200 kg SUV traveling at 26 m/s can be stopped by applying the brakes for 5.5 seconds in a panic stop. What is the average force exerted on the SUV during this stop?