What Determines the Force Exerted by a Hammerhead in a Pile Driver?

[Heat]

Homework Statement

In a pile driver , a steel hammerhead with mass 200kg is lifted 3m above the top of a vertical I beam being driven into the ground. the hammer is then dropes driving the Ibeam7.4cm farther into the ground. The vertical rails that guide the hammerhead exert a constant 60N friction force on the hammerhead. Use the work energy theorem to find a) speed of the hammerhead just as it hits the I beam and b) the average force the hammerhead exerts on the I-Beam Ignore the air effects of the air.

The Attempt at a Solution

I managed to understand part a. Part b is where I am lost.

The book shows the following work, and I need help understanding on of the steps.

Wtotal = (w-f-n)s23

Wtotal = (w-f-n)s23 = k3-k2

*** n is the same as g right? and we solve for n? Since when is n the average force?***

n = w-f - k3-k2/s23

=1960N - 60N - 0J - 5700J / 0.074m

=7900N

the part underlined I understand.
The part in bold is where I can't explain why.

 

[mgb_phys]

Consider the force exerted by the pile on the hammer.
You are slowing the hammer from the speed found in part A to 0 in 7.4cm.
Just work out the accelearation ( v^2=u^2+2as) and use f=ma to get the force, assuming the deceleration is uniform.