# Time and velocity

Suppose two cars are traveling at a velocity of $c/2$ ($c$=speed of light) in two straight lines parallel to each other but in opposite direction in vacuum. Then what will a person sitting in one of those cars see when they approach each other?

## migrated from astronomy.stackexchange.comJan 15 '16 at 18:12

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She will feel motionless, seeing the other car approach with the combined velocity of the two cars: $$v_\mathrm{tot} = \frac{v_\mathrm{\scriptsize{1}} + v_\mathrm{\scriptsize{2}}}{1 + v_\mathrm{\scriptsize{1}} v_\mathrm{\scriptsize{2}}/c^2} = 0.8c,$$ with all the usual implications of observing something moving at $0.8c^\dagger$.
$^\dagger$E.g. an epic car crash: The kinetic energy of an oncoming car of 1 ton is equivalent to 2 million Nagasaki bombs.