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One dimensional collisions between two objects can be studied using a linear air track (shown below). The mobiles "float" on a cushion of air to eliminate the force of friction. The mass of a mobile can be changed by adding an extra piece of metal.
An elastic collision is a collision in which kinetic energy is conserved. The springs on the mobiles ensure that the collisions are (very nearly) elastic.
Different collisions were observed but in each case mobile B was initially stationary.
These observations confirm Newton’s third law of motion.
It can easily be shown that for an elastic collision, the relative velocity of approach is equal to the relative velocity of separation.
During a non-elastic collision, some K.E. is converted to other forms of energy.
A totally non-elastic collision is one in which the relative velocity of separation is zero. In other words, the two bodies remain in contact (stick together) after the collision.
Detailed study of totally non-elastic collisions is quite easy because there is only one speed to measure after the collision.
The speed of A was measured before the collision. B was initially stationary. The mass of A was varied. The speed of A and B together was measured after the collision.
The results suggest that the quantity mass multiplied by speed is the same before and after the collision. Experiments involving interactions between bodies moving in two (or three) dimensions show that it is mass multiplied by velocity which is the useful quantity.
The product of mass and velocity is called the linear momentum of the body.
The units of momentum are kgms-1