Mike Hord wrote:

>> Something I once had a frustrating argument with a traffic cop about.
>> The speed of two objects (eg vehicles) in a head-on collision isn't the
>> sum of the speeds

> In all seriousness, how do you mean?  You're taking two vectors and
> adding them...ooooohhhhh, waitaminute:  it's zero, right?  

The speed of one relative to the other is not the addition, it is the
difference. Hence, with non-relativistic speeds and 180 degrees between the
vectors, it is the sum of the absolute values of the speeds.

Of course, once we get into speed ranges where the Lorentz factor diverges
significantly from 1, it is not anymore the simple sum. I'm not sure,
however, whether that is (yet) relevant to the professional life of a
traffic cop (photon traffic? :)

> Plus, there's the whole m*v^2 thing, which means that the kinetic energy
> of two cars, same mass, same speed, is lower than the kinetic energy
> of one of those cars hitting the other, parked, at twice the speed.

Not if you are sitting in one of the cars. In that case, you still see the
other coming towards you at twice the speed, with the associated energy --
no matter which way and which car :)

Gerhard

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