An express train is running behind a goods train on the same line and in the same direction, their velocities being u1 and u2 (u1 > u2) respectively. When there is a distance x between them, each is seen from the other. At which point it is just possible to avoid a collision f1 is the greatest retardation and f2 is the greatest acceleration which can be produced in the two trains respectively?

(u 1 + u 2 ) 2 = 2x(f 1 + f 2 )

An express train is running behind a goods train on the same line and

1.	An express train is running behind a goods train on the same line and in the same direction, their velocities being u₁ and u₂ (u₁ > u₂) respectively. When there is a distance x between them, each is seen from the other. At which point it is just possible to avoid a collision f₁ is the greatest retardation and f₂ is the greatest acceleration which can be produced in the two trains respectively?
A.	(u<sub>1</sub> u<sub>2</sub>)<sup>2</sup> = 2x(f<sub>1</sub> + f<sub>2</sub>)
B.	(u<sub>1</sub> + u<sub>2</sub>)<sup>2</sup> = 2x(f<sub>1</sub> f<sub>2</sub>)
C.	(u<sub>1</sub> u<sub>2</sub>)<sup>2</sup> = 2x(f<sub>1</sub> f<sub>2</sub>)
D.	(u<sub>1</sub> + u<sub>2</sub>)<sup>2</sup> = 2x(f<sub>1</sub> + f<sub>2</sub>)
Answer» D. (u<sub>1</sub> + u<sub>2</sub>)<sup>2</sup> = 2x(f<sub>1</sub> + f<sub>2</sub>)

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