How much force is required to keep a 2.0kg stone from falling?

19.6 N

Explanation:

When holding a stone, there are two forces acting on the stone:

- The weight of the stone, W = mg, in the downward direction, where

m = 2.0 kg is the mass of the stone

is the acceleration due to gravity

- The upward force applied by our hand on the stone, R, in the upward direction

According to Newton's second law of motion, the net  force on the stone is equal to the product between its mass (m) and its acceleration (a):

However, here we want to keep the stone at least at reast (not falling), so the acceleration must be zero:

This means that the force applied by our hand must be equal in magnitude to the weight of the stone, so: