What should I know?

1. When a force is applied to an object, it resists the effects of that force due to it inertia (i.e., its mass.)
2. This resistance causes a reaction force, applied to the pushing object by the object being pushed. Both the action and reaction forces are the same size (magnitude) and exist simultaneously.
3. NEWTON'S THIRD LAW: For every force applied by object A on object B, there is an equal magnitude and opposite direction force applied back on object A by object B.
4. Internal action-reaction force pairs always cancel (e.g., if your car has run out of gas, you cannot push your car to the next gas station from inside the car.)
5. When an object causes itself to change its motion -- i.e., it causes a net force to be applied to itself -- it often does so using an action-reaction force pair (e.g., when a human jumps off the ground by pushing down on the floor, the floor applies a reaction force to the human.)
6. As forces are rarely constant, the average net force and the time over which it is applied can be used to find the object's change in velocity.
7. The product of the average net force and the time over which it is applied is called the impulse. When F = ma is rearranged, it describes the impulse mathematically: F Δt = m Δv, where the F is the average net force, Δt is the time it is applied, F Δt represents the impulse (sometimes represented with a J,) m is the mass of the object, and Δv is the change in its velocity.
8. The units for impulse are Newtons x seconds, or N·s.
9. As a consequence of Newton's third law of motion, the impulse applied by object A on object B requires that object B applies the same impulse back on object A.
10. When considering how much impulse it will take to cause a certain change in an object's motion, both the inertia (mass) of the object and its initial velocity are important, as seen in the impulse formula above. The product of an object's mass and its velocity is called its momentum.
11. Momentum is a vector quantity that encapsulates both the ideas of the current motion of the object and also how difficult it is to change that motion. The formula is p = m v.
12. The units for momentum are kilograms x meters/second, or kg·m/s.
13. When momentum does change, it is typically because the velocity of the object has changed, not its mass. The formula for the change in momentum is Δp = m Δv.
14. If the impulse formula is analyzed, it can be seen that the quantity on the right is actually the change in momentum. That makes the true name of that formula the impulse-momentum theorem.
15. In a closed system of objects -- in which no forces or energy can enter or leave -- any change in momentum for one object must correspond to a change in the momentum (or momenta) of other objects. This principle is called the LAW OF CONSERVATION OF MOMENTUM.
16. Typically objects in systems apply impulses on each other during collisions.
17. Because the same sized impulse is applied to both objects during a collision, each object experiences the same change in momentum.
18. If little to no motion energy is lost to heat during a collision and the objects bounce off each other, it is an elastic collision.
19. If some motion energy is lost to heat during a collision and there is some deformation of the objects, or if they stick together, it is an inelastic collision.