Momentum is Conserved Without External Forces
Elastic collisions are typically visualized by two perfectly elastic rubber balls bouncing off each other. Energy and momentum are conserved in elastic collisions.
In an elastic collision, total kinetic energy is conserved. Two perfectly elastic rubber balls bouncing off each other lose no energy in the process. This means that they continue with the same kinetic energy but are likely moving in a different direction than they originally were.
Partially inelastic collisions describe the vast majority of collisions that occur. In these collisions, two objects do not bounce off each other perfectly. They do lose some energy in the process, but momentum is still conserved.
Partially inelastic collisions are defined by the fact that the total initial kinetic energy of the objects involved is greater than the total final kinetic energy. This is because some of the energy was released.
A fully inelastic collision occurs when two objects stick together after a collision. In this case, while momentum is still conserved the equation for final momentum changes. The final momentum is the velocity of the combined objects multiplied by the added mass of the two objects.
Energy given off is typically in the form of sound or heat. The fact that car crashes are audible indicates that some energy is lost. If two people "high-five", the kinetic energy of the moving hands is converted to sound and heat.
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