Let x be the displacement at time t of an object in motion.

Given, x = kt^{3}, where k is a constant of proportionality.

Let u be the initial velocity of projection of body and v be the velocity of the same body while passing downwards through point of projection.

The displacement of body s = 0.

It means that the final speed is independent of mass of the body.

Hence, both the bodies will acquire the same speed while passing through point of projection.