A man lifts a 2.5kg stone vertically with his hand at a constant velocity of 1.2m/s. what is the magnitude of the total force of the man's hand on the stone.

Zero

Explanation:

According to Newton's second law of motion, the net force on an object is equal to the product of its mass and its acceleration:

(1)

where

F is the net force

m is the mass

a is the acceleration

For the stone in this problem, we have:

m = 2.5 kg is its mass

We are also told that the velocity of the stone is constant (1.2 m/s): we know that the acceleration is equal to the rate of change of velocity, therefore, this means that the acceleration (because the change in velocity is zero):

Therefore, from equation (1), we get that the total net force on the stone is zero:

Which means that the upward force on the stone balances the downward force of gravity.