Two train tracks run parallel to each other, except for a short distance where they meet and become one track over a narrow bridge. One morning, a train speeds onto the bridge. Another train coming from the opposite direction, also speeds onto the bridge. Neither train can stop on the short bridge, yet there is no collision. How is this possible?

It is possible if the two trains don't cross the bridge simultaneously on the given morning.

The question is framed to confuse the reader by thinking that both the trains arrive at the bridge simultaneously from the opposite directions and then cross the bridge. But the question actually states that the two trains cross the bridge from opposite directions, the same morning. But it is possible that one train was closer to the bridge and crossed the bridge first and then the second one arrived.

So it is possible if there is a small interval in time in the arrival of the two trains at the bridge.

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This situation is possible.

Explanation:

It is said in the problem that one morning two trains are speeding up from opposite directions onto the bridge. It may seem from this statement that the trains are arriving at the same time. But this should be a wrong assumption.

Because it is only said in the problem that the trains are passing the bridge on same morning. No explicit timing is given.

So it may so happen that both the trains pass the narrow bridge on the same morning but at two difeerent times, making no collision with each other.

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s