A collision takes place when two objects interact with each other (generally, though not always) through touch and transfer momentum and/or kinetic energy to each other. There are two types of collisions: inelastic and elastic collisions. In an elastic collision, both the total momentum and the total kinetic energy of the colliding objects is conserved. That is, the total momentum and the total kinetic energy is the same before and after the collision. In comparison, in an inelastic collision, the total momentum is conserved, however kinetic energy is not. Some of the kinetic energy of the colliding bodies is converted to other forms of energy, such as heat, sound, etc. No such conversion of energy takes place in the case of an elastic collision. During an elastic collision, conservative forces are at play while during an inelastic collision, non-conservative forces, such as friction, result in the loss of energy. Most of the collisions we observe in our daily lives, such as automobile accidents, etc. are inelastic collisions. The swinging balls in a device called Newton's cradle are very close to being elastic collisions, although in everyday life it is nearly impossible to witness truly elastic collisions.
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