A heavy object and a light object are dropped from the same height. If we neglect air resistance, which will hit the ground first?

None, both objects will hit ground at the same time.

Explanation:

Assuming no air resistance present, and that both objects start from rest, we can apply the following kinematic equation for the vertical displacement:

       

As the left side in (1) is the same for both objects, the right side will be the same also.Since g is constant close to the surface of the Earth, it's also the same for both objects.So, the time t must be the same for both objects also.

You get 6 for mechanical