Try: ylab=expression(paste("Acceleration [ ",m/s^{2}," ]"))
par(mar=c(4,5,4,4))
x <- seq(1,10,.01)
plot(x, sin(pi/2*x), type = "l", xlab="Time [s]",
ylab=expression(paste("Acceleration [ ",m/s^{2}," ]")),
main="Example of signal of horizontal acceleration", cex.main = 0.9)