MOMENTUM and COLLISIONS

"What one man can invent another can discover"
Arthur Conan Doyle

Momentum is a vector quantity, the product of the mass and velocity of an object

p = mv
Units: kg.m/s (SI); slug.ft/s (British)
Impulse is a vector quantity, the product of the force acting on an object and the time for which the force acts.

Units: N.s (SI); lb.s (British)
Newton's second law of motion can be written

Impulse = Momentum change

or
Force = Rate of change of momentum
Conservation of Momentum

Important basic principle of Science

"Total momentum of a system remains constant,
when the net external force acting on the system is zero"

Within the system objects may collide with each other, thus exerting forces on each other. However, Newton's 3rd law states that these forces are equal in magnitude, but opposite in direction, therefore there is no net force. The system conserves momentum. In order to ensure momentum conservation we must choose our "system" such that the net force is zero, in which case;

"Total momentum of system before collision = Total momentum of system after collision"

Note that since momentum is a vector quantity, this equation is actually three scalar equations, one for mometum along each of the three cartesian axes, x,y,z.
Energy and collisions

Energy is conserved in ALL collisions. However, in most cases some of the initial kinetic energy of the objects is converted into heat, sound or elastic potential energy during the collision. Such a collision is known as an inelastic collision and does not conserve kinetic energy. In certain special cases, kinetic energy is conserved, these are known as elastic collisions.

"To mark the opening of a sports hall for juvenile offenders the Prime Minister up rooted a tree."
Ronnie Barker

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu