Andy Duff, you're forgetting that if a car travelling at 120mph hits a stationary car, both with probably be moving after the crash & so all the energy won't be used.

In fact, if there was no friction, both cars would be moving at a combined speed of 60mph assuming they weigh the same! (Conservation of momentum)

MV= M(V/2) + M(V/2)

 

M=mass of car, V=120mph!

 

That means that total energy in 120mph car before crash =

1/2* M *(V)^2

=

4 * 1/2* M *(V/2)^2

 

after crash, combined energy of both cars would be =

1/2*(2*M)*(V/2)^2

 

that means that the energy used in the crash =

4* 1/2* M (V/2)^2 - 1/2*(2*M)*(V/2)^2

 

=

2*M(V/2)^2 - M*(V/2)^2

 

=

M * (V/2)^2

 

Which is the same energy used if two cars travelling at 60mph were to crash into each other.

i.e.

1/2 * M * (V/2)^2 + 1/2 * M * (V/2)^2

 

=

M * (V/2)^2

 

Sorry, just had to make my point:p

:D

Oh sh!t, this was supposed to go into that thread re 'hypothetical crash!

WTF!!!:mad:

Almost worthy of a new thread ;) .

 

My degree in mathematics means I'm immune to algebra and unable to read it. Looks right tho :) Marks for effort anyway.

 

Pete

looks OK to me too! very good, straight out of text book!