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Momentum:
a vector quantity that is the product of an object’s velocity and mass
p = mv kg m/s

If velocity is constant, momentum is constant.
A net external force applied constantly to an object for a certain time interval will cause a change in the object’s momentum equal to the product of the force and the time interval during which the force acts.

The product of the constant applied force and the time interval during which the force is applied is called the impulse of the force for the time interval.
F Δt = impulse (N · s)

The change in momentum
is impulse that acts on the object.

A system:
a defined collection of objects.

Closed isolated system:
objects do not enter or leave and no external force is exerted on it.

Law of Conservation of Momentum (expressed as Newton’s Third Law):
 the momentum of any closed, isolated system does not change; it is conserved.
 In every interaction between two isolated objects, the change in momentum of the first object is equal to and opposite the change in momentum of the second object.
 Therefore, momentum is conserved in collisions.

Internal forces:
forces between objects within a system.

External forces:
forces exerted from outside a system.

Momentum of a system
Momentum of a system changes when it is not isolated, i.e., when a net external force acts on it.

perfectly inelastic collision
In a perfectly inelastic collision, two objects stick together and move as one mass after the collision.

inelastic collision
In an inelastic collision, some KE is changed into other forms of energy. Some is converted to internal elastic PE when the objects deform, some to sound energy, heat energy. in an inelastic collision, momentum is conserved; total KE is decreased.

elastic collision
 In an elastic collision, two objects return to their original shapes and move away from the collision separately.
 In an elastic collision, the total momentum and KE of the system is the same before and after the collision. Both are conserved.
 Few collisions are elastic or perfectly inelastic.

