Calculate Acceleration, Velocity and Force of a Piston

[Pseudopro]

Homework Statement

Piston mass: 0.25kg
Stroke (distance between extreme positions): 9cm
4500rev min^-1

(a)Calculate acceleration of piston at maximum displacement
(b)Calculate velocity as piston moves past its equilibrium point
(c)What is the net force exerted on the piston at maximum displacement

Homework Equations

a_max=-w²A
v_max=wA
F_netmax=ma_max

The Attempt at a Solution

(a)a_max=-(2\pi4500/60)²9\times10^-2
(b)similar to (a) using equation
(c)m times answer to (a)

The problem is that the book answer is consistently half my answers. I think this is because they used 4.5cm as their amplitude instead of 9cm. One stroke is 9cm and in one revolution, the piston needs to travel twice that distance, right? So shouldn't amplitude be 9cm?

 

[Delphi51]

The amplitude is half the difference between the high and low.
In a water wave, the water goes up one amplitude, back down to the equilibrium, down one amplitude, then back up to equilibrium.

The piston does the same thing.

 

[Pseudopro]

Thanks, that example makes it clear