If you are ontop of a giant heavy object that is falling from the sky, is it possible to jump and roll at the perfect time to survive? I don't know if gravity or the speed of the drop would prevent us from jumping, but would something like this be possible?
If it's not possible on the ground, is it possible in the ocean?
I'm sorry in advance if I used the wrong tag. I had a hard time figuring out which tag out of the physics tag to choose. If someone can kindly correct me, that would be awesome.
 A: Notice that if you are on a giant-rock, assuming it’s still not a size comparable to the earth’s radius, you’ll see that both the rock and you are accelerating downwards at about the same rate which is the acceleration due to gravity due to the pull of the earth. Hence, there will essentially be no contact force between you and the rock’s floor. You’d be in free-fall and it would be very easy to jump away from the rock as you float away a bit from its surface and even walk normally and roll-across. But even then you’d not survive as once you hit the ground/ocean you’re traveling at a very high speed as you’ve accelerated all this way. There is no way to reduce your speed relative to the ocean(although you seemed to be at rest with respect to the rock as you were falling at the same rate the rock was). Thus you will most inevitably fall to your death as at high speeds water acts more like a solid(recall bellyflopping). 
This death-causing force is due to the sudden deceleration from a very high velocity when falling into the ocean surface which causes a very large force(force is proportional to change in speed which is acceleration). 
A: Damage to a living being with bones, organs, tissues, in a collision
is a transfer of energy into breaking of materials.   The more
energy transferred, the more breakage is possible.
So, a large (large volume) heavy object will determine the
terminal velocity of the fall (it acts as a parachute would),
but a parachute is a large light object; a large heavy
object is no parachute, it will have gravity energy 
proportional to mass (volume, roughly), and lose energy to
atmospheric drag proportional to area.   'Large' implies the
area is smaller scale than volume (terminal velocity is very high).
So, if the ricocheting fragments of the 'object' and its
target aren't energetic enough to hurt you, if the impact
heating doesn't scorch you, your own personal kinetic energy
will be proportional to the square of the terminal velocity of
the object.   No, a leg spasm won't transfer enough energy
to help  much. 
A grasshopper, on a baseball, would have an excellent chance (but
better if it hopped off the baseball while still at high altitude).
