m <- 0.2; k <- 39.2; x0 <- 0.02
x <- function(t) x0*cos(sqrt(k/m)*t)
plot(x,0,2*pi*sqrt(m/k)*2)
abline(v=axTicks(1), h=axTicks(2), col="blue",lty=3)
abline(v=0, h=0, col="blue",lty=2)
i <- c(1,2,3,4)
abline(v=i*pi*sqrt(m/k), col="red",lty=3)
2*pi*sqrt(m/k)
# 0.448799
sqrt(k/m)
# 14

Come si risolverebbe l'equazione con WolframAlpha:
x" + 39.2/0.2*x = 0, x(0)=0.02, x'(0)=0

Otterremmo:   x(t) = 0.02*cos(14*t)