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# Where does the kinetic energy go in this collision?

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1. Where does the kinetic energy go in this collision?
You have two trucks, truck A with a mass of 5000kg which is moving at 3 m/s and truck B which has a mass of 10,000kg and is moving at 0 m/s
They collide and then move together at 1 m/s

The kinetic energy before the collision is 0.5 * 3 * 3 * 5000 = 22500 J
The kinetic energy after the collision is 0.5 * 1 * 1 * 15000 = 7500 J

Where does the lost kinetic energy go in such a collision? I don't believe it was lost to friction, heat, etc in the above example as the mathematics didn't take those into account and yet because energy cannot be destroyed it had to go somewhere.
2. Re: Where does the kinetic energy go in this collision?
It goes back in time, to the prehistoric ages and all of the kinetic energy together creates a massive explosion on the earth, which wiped out the dinosaurs as we know today.
3. Re: Where does the kinetic energy go in this collision?
The difference between the final and initial kinetic energies is because some of the initial kinetic energy will be converted to other forms of energy in the collision. For example some will be converted to heat and sound energy.
4. Re: Where does the kinetic energy go in this collision?
(Original post by Bobby132)
You have two trucks, truck A with a mass of 5000kg which is moving at 3 m/s and truck B which has a mass of 10,000kg and is moving at 0 m/s
They collide and then move together at 1 m/s

The kinetic energy before the collision is 0.5 * 3 * 3 * 5000 = 22500 J
The kinetic energy after the collision is 0.5 * 1 * 1 * 15000 = 7500 J

Where does the lost kinetic energy go in such a collision? I don't believe it was lost to friction, heat, etc in the above example as the mathematics didn't take those into account and yet because energy cannot be destroyed it had to go somewhere.
It is lost as heat, or ends up as internal energy (ie increased temperature), but not through macroscopic uniform friction but during the deformation that occurs when the trucks stick together.

Energy is conserved IF heat and internal energy are taken into account.

(Given the collision happens quickly, it is most likely to end up as internal energy ie increasing the temperature of the trucks near the collision point, rather than as heat which takes time to conduct away)
5. Re: Where does the kinetic energy go in this collision?
Surely the mathematics does take it into account? After all, the speed after the collision is given to you - you don't have to work it out.

You can work out the maximum speed after the collision assuming perfect energy transfer:

Kinetic energy = KE = 22500 = 0.5mv^2
New speed = v = sqrt(E/0.5m) = 22500/0.5*15000 = sqrt3 = 1.732 m/s

You can see this is higher than the speed of 1m/s they give you.

Hope I helped
6. Re: Where does the kinetic energy go in this collision?
(Original post by Bobby132)
You have two trucks, truck A with a mass of 5000kg which is moving at 3 m/s and truck B which has a mass of 10,000kg and is moving at 0 m/s
They collide and then move together at 1 m/s

The kinetic energy before the collision is 0.5 * 3 * 3 * 5000 = 22500 J
The kinetic energy after the collision is 0.5 * 1 * 1 * 15000 = 7500 J

Where does the lost kinetic energy go in such a collision? I don't believe it was lost to friction, heat, etc in the above example as the mathematics didn't take those into account and yet because energy cannot be destroyed it had to go somewhere.
The "lost" energy amounts to 15,000J
This is about enough to heat a kg of water by 3 or 4 degrees C. Not much really in the greater scheme of things.
If this lost energy were converted to heat and raised the temperature of the couplings in the two tricks, the temperature rise would be insignificant.

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Last updated: June 2, 2012
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