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 February 8th, 2008, 07:07 PM #1 Senior Member   Joined: Apr 2007 Posts: 2,140 Thanks: 0 Momentum A car brakes to a stop. 1. Was this an example of an elastic or a inelastic collision? Explain the reasoning. 2. For what system was the momentum conserved, in this case? Explain. 3. What happened to the car's kinetic energy?
 February 9th, 2008, 01:06 PM #2 Senior Member   Joined: Apr 2007 Posts: 2,140 Thanks: 0 Some help would be really helpful to me.
 February 9th, 2008, 01:13 PM #3 Senior Member   Joined: Oct 2007 From: Chicago Posts: 1,701 Thanks: 3 Inelastic: The total energy of the system decreases. The momentum from the car goes to the Earth, so if our system is Earth and car, momentum is conserved. The car's kinetic energy is being passed, through friction, to the ground. However, some of it is lost in the form of sound/heat.
February 9th, 2008, 01:17 PM   #4
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Re: Momentum

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 Originally Posted by johnny A car brakes to a stop. 1. Was this an example of an elastic or a inelastic collision? Explain the reasoning. 2. For what system was the momentum conserved, in this case? Explain. 3. What happened to the car's kinetic energy?
I presume there was no actual collision. The process is inelastic - the kinetic energy ends up as the heating of the brakes and the tires. Momentum is conserved in the system involving the car and the earth - however as you might expect the earth doesn't really notice.

 February 9th, 2008, 01:55 PM #5 Senior Member   Joined: Apr 2007 Posts: 2,140 Thanks: 0 So basically, elastic collision means that if two objects collide, then they sort of bounces off. But, in an inelastic collision, it does not bounces off? Also, since Kinetic Energy = 1/2mv^2, and velocity decreases as car stops, the Kinetic Energy decreases and approaches 0. Am I getting this right?
 February 9th, 2008, 05:28 PM #6 Senior Member   Joined: Oct 2007 From: Chicago Posts: 1,701 Thanks: 3 Elastic collision means the KEi=KEf, where i and f signify initial and final respectively. Since some of the energy is lost to heat/sound, KEiā KEf. You are correct about the car losing energy, but you have to keep in mind the whole system: the Earth gains a tiny bit (for the earth) of kinetic energy from the car stopping. Mathman: Any energy transfer between two objects can be modeled as a collision; I was actually initially taught that any energy transfer was a collision.

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