One body is dropped and a second body is thrown downwards with an initial velocity of 1m/s simultaneously. The separation between these two bodies is 18m after a time……………….. ans:18 sec

Yes, 18 seconds is correct.

Could you explain how?

Both have the same initial starting point and are subject to the same acceleration of -9.8m/s^2. The only difference is that one has a 1m/s initial velocity. That initial velocity times 18 seconds gives a distance of 18m due solely to the advantage of that 1m/s start.

Mathematically you can see it from the equation:


x(t) = x_0 + v_0t + \frac 1 2 a t^2


For two different objects that have the same value for x_0 and a, and one has initial velocity v_0 and the other has initial velocity 0, the difference between them is simply v_0 t.