By Newton's second law,
F=mamax.
The maximum acceleration for a SHM is given by
amax=ω2A...(i)
The time period is given by
T=ω2π=3.14s
⇒ω=T2π=3.142π=2rads−1
Thus, the maximum acceleration from equation (i)
amax=4×1=4ms−2
The maximum force is
F=5kg×4ms−2=20N