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Conservation of momentum and loss of energy in inelastic collisions 
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#1
May109, 04:34 AM

P: 106

I just learned that, in elastic collisions, the total momentum in the system is preserved while a certain amount of kinetic energy is lost.
I know that kinetic is energy may be lost in the form of heat or sound. Isn't the same energy responsible for keeping the two objects in motion and preserving their momentum? How then does the momentum remain the same when the amount of energy in the system decreases? 


#2
May109, 04:37 AM

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Sci Advisor
PF Gold
P: 29,239

In elastic collision, BOTH momentum and KE are conserved. Zz. 


#3
May209, 01:03 AM

P: 106

Opps. My mistake. I made a small typing error. What I meant was "inelastic".



#4
May209, 01:46 AM

Sci Advisor
P: 8,307

Conservation of momentum and loss of energy in inelastic collisions
Momentum is a vector. Energy is a scalar. Say you have 2 vectors of equal magnitude pointing in opposite directions = 0. Now subtract lots of little vectors representing the energy loss as heat in all directions (ie. nondirectionally). Now the vector sum is still 0.



#5
May209, 07:58 AM

P: 106

So what you are saying is that the loss of heat energy in all directions adds up to zero. Therefore it can make no change to momentum? 


#6
May209, 09:11 AM

P: 261

Consider a motionless boulder poised at the top of a cliff. It has a lot of energy but no momentum. And conversely, an object doesn't consume energy to keep it moving. Such would be an Aristotelian, rather than a Newtonian, model of the world. 


#7
May209, 09:23 AM

P: 106

"You seem to believe that energy causes momentum."
That's exactly what I was thinking. Since it is true that an object with momentum has kinetic energy, I assumed that a loss of energy would be accompanied by a loss of momentum. But I still don't understand why a loss of energy has no effect on momentum. 


#8
May209, 09:33 AM

Mentor
P: 41,293




#9
May209, 09:40 AM

P: 261

Imagine two balls of putty speeding towards each other. The total momentum is zero, even though the total kinetic energy is not. When they collide, the kinetic energy converts into heat. But remember the momentum is already zero. How could it lessen? 


#10
May209, 10:11 AM

P: 106

First of all, thanks for your replies.
I understood the puttyball example but I didn't understand the how force being a scalar quantity and momentum being a vector quantity could explain the conservation of momentum 


#11
May209, 10:39 AM

Mentor
P: 41,293




#12
May209, 11:26 AM

P: 106

Oh. After a little thinking I understand what's happening,. Thanks for all your replies.



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