- The magnitude of the force is zero at the instant right before it contacts the object.

- During contact, the net force rises to a maximum, and then returns to zero when the force leaves the object.

- The product of the average force and the time of contact is called the impulse of the force.

- Linear momentum is a quantity that points in the same direction as the velocity.

- When a net force acts on an object, the impulse of the net force is equal to the change in momentum of the object.

- When two objects approach each other in mid-air, their collision is known as a "system".

- Internal forces are forces that the objects within the system exert on each other.

- External forces are those exerted on objects by agents that are external to the system.

- The internal forces can be ignored.

- The principle of conservation of linear momentum states that the total linear momentum of an isolated system remains constant.

- If the sum of the average external forces is zero, the final and initial total momenta are equal.

- When objects are atoms or subatomic particles, the total kinetic energy of the system is often conserved.

- The kinetic energy gained by one particle is lost by another.

- Elastic collisions- those in which the total kinetic energy of the system after the collision is equal to the total kinetic energy before the collision.

- Inelastic collisions- those in which the total kinetic energy of the system is not the same before and after the collision.

- If the objects stick together, the collision is said to be inelastic.

- For a system consisting of two objects, external forces would include the weights of the objects, and the corresponding normal forces.

- Since a normal force balances each weight, the sum of the external forces is zero, and the total momentum of the system is conserved.

- Momentum is a vector quantity, and in two dimensions, the x and y components are conserved separately.