Quote:
Originally Posted by codprawn
No no no!!!
Think about it  you hit a concrete block at 70mph  you stop dead. You hit another car both doing 70mph  you stop dead. Now if you are doing 70mph and the car you hit is doing 80mph then you go backwards at 10mph.
It's called an inelestic collision. Lets assume a car has 20,000 units of energy at 70mph. When it stops it has 0 units of energy. It loses 20,000 units very rapidly. The other car has 20,000 units of energy  it also loses all it's energy. 20,000 hitting 20,000 = 0. If the car was doing 140mph it would have 40,000 units of energy.

Yes yes yes!!!
Actually if it was doing twice the speed it would have FOUR times the energy as kinetic energy is proportional to the square of the velocity, however... I think we need some proofs!... If you don't like physics you should look away now...
Let's make a few assumptions:
1) Car 1 is identical to Car 2 and each does 10m per second and weighs 1000kg...
2) The collision is perfectly inelastic, i.e. they do not 'bounce' apart, rather they crumple and 'stick' together
Scenario 1) Car 1 hits Car 2, both are doing 10 m/s toward each other
Total kinetic energy = 2 x (0.5 x 1000 x 10^2) = 100kJ
As the cars are identical and the forces in any collision are equal and opposite (Newton III) the energy absorbed by each car will be the same. We can also summise that due to conservation of momentum that the two cars will be stationary after the collision. Hence
each car will have to absorb 50kJ of energy.
Scenario 2) Car 1 is doing 20m/s and car 2 is stationary
Total kinetic energy = 0.5 x 1000 x 20^2 = 200kJ
This is double the kinetic energy of scenario 1.... BUT conservation of momentum means that after the collision the mangled ball of wreckage of the two cars will still be moving!
Momentum before = momentum after
1000 x 20 = 2000 x velocity_after :. velocity_after = 10m/s
So the kinetic energy of the wreckage = 0.5 x 2000 x 10^2 = 100kJ
So, the energy absorbed by both vehicles in the crash = total kinetic energy before  kinetic energy of the wreckage
200kJ  100kJ = 100kJ, so the
energy absorbed by each car is still 50kJ
Scenario 3) Car 1 is doing 10m/s and hits a brick wall
This situation is entirely different as the wall will absorb no energy and, like scenario 1, the car is going to come to a complete halt so it will have to absorb all the energy.
Total energy = 0.5 x 1000 x 10^2 = 50kJ
The car must still absorb 50kJ of energy
So I say again... two cars heading towards each other at 70mph each will create the same impact as one car travelling at 140mph hitting the other car when it is stationary.