Verified answer

Assuming they have equal power, they will both stay on the ground. This follows trivially from Newton's third law (action = - reaction).

Denote the forklifts by A and B.

If A lifts the weight of B, then B pushes A down with B's weight. But the weights are equal, so if A lifts B, B pushes down on A with A's weight. What then is the total downward force on A:

Its own weight + its own weight from lifting B - the force with which B lifts A (which is A's weight)

= its own weight.

So A stays put. Likewise B will stay put, from symmetry.

If you get this, the shortcut answer is: A experiences an upward force from B lifting him, but an equal downward force from himself lifting B. They cancel.

That is the same as you grabbing your hair and lifting yourself....

Your arm exerts an upward force on the rest of your body, but the rest of your body a downward force on your arm (Newton's third law). And because yor arm is part of you, these forces are "internal" and therfore cancel on the body as a whole.

Two Forklifts Lifting Each Other

They lift each other into the air. Duh!

Seriously though, they will probably tip over. Each lift is pulling the others wheels off the ground. But gravity keeps them grounded. If the forks of the lift are raising higher and higher under hydraulics, but staying on the ground at the same time, then what can you conclude about the machine the forks are attached to? Obviously the tops of the fork lifts (where the forks are) are going to get closer to the ground.

They tip over