Fence post machine

This machine is a wonder. It holds the posts in place and then hammers them in. The hammering action involves hoisting a metal cylinder and letting it fall. I am estimating the cylinder to be 50cm high with a diameter of 20cm. That gives a volume of about 15000 cm^2. If it is steel, the mass is 125 kg. If raised to 50cm above the post, it will hit it with a theoretical velocity from the v^2=u^2 + 2as equation, giving about 3m/s. Suppose it stops in 0.1 second on hitting the post. Force = change in momentum/time taken = 125 x 3 / 0.1 = 4kN to 1 sf.