We can use the formula:
force = pressure × area
We can rearrange the formula to solve for force:
force = pressure × area
The pressure on the piston is equal to the force divided by the area:
pressure = force ÷ area
We can substitute the given values:
pressure = 21000 N ÷ 3 m^2 = 7000 Pa
Now we can solve for the force needed on the piston:
force = pressure × area = 7000 Pa × 0.06 m^2 = 420 N
Therefore, a force of 420 N is needed on the piston to lift the object weighing 21000 N on a 3 m^2 platform.