Module 2
1. Module 2
1.16. Page 7
Module 2—The Conservation of Momentum in Isolated Systems
Lesson Summary
At the start of this lesson, you were asked the following essential questions:
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How can the kinetic energy of the objects in a system before and after a collision be used to categorize the type of collision?
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What is the fundamental difference between an elastic and an inelastic collision?
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In a collision between two automobiles, is it better for the occupants if the collision is elastic or inelastic?
The kinetic energy values of all the objects in a system can be used to categorize a collision as elastic, inelastic, or perfectly (completely) inelastic. In the case of elastic collisions, the total kinetic energy of the system before the collision equals the total of the kinetic energy of the system after the collision. In other words, kinetic energy is conserved. These collisions are rare with regard to the collision of everyday objects because some of the kinetic energy is usually converted to sound, thermal energy, or deformation. This is the fundamental difference between an elastic collision and an inelastic collision.
In an inelastic collision, the total kinetic energy of the system after the collision is less than the total kinetic energy of the system before the collision. In other words, kinetic energy is not conserved. In some cases, the kinetic energy is completely converted to other forms of energy after the collision. In these cases, the collision is described as being perfectly, or completely, inelastic.
In collisions between two automobiles, it is better for the occupants if the collision is inelastic rather than elastic. Crumple zones of vehicles are designed to transform some of the initial kinetic energy into work. This work changes the shapes of crushable parts of the vehicle. Since the vehicles have less kinetic energy after the collision, there tends to be less reversal of direction. The change in velocity of each vehicle is less, so the change in momentum for each vehicle is less. Thus, the impulse acting on each vehicle is reduced. Since inelastic collisions between vehicles involve the crumpling of crushable parts, the time interval for the interaction is increased, which reduces the forces acting on each vehicle.