library(deSolve)
ball <- function(t, y, parms) {
  dy1 <- y[2]
  dy2 <- -9.8
  list(c(velocity=dy1, acc=dy2))
}
#yini <- c(height = 0, velocity = 10)


#rootfunc <- function(t, y, parms) return (y[1])
#
#
#eventfunc <- function(t, y, parms) {
#  y[1] <- 0
#  y[2] <- -0.9*y[2]
#  return(y)
#}
#

times <- seq(from = 0, to = 20, by = 0.01)
out <- ode(times = times, y = c(0,10), func = ball,parms = NULL
#,rootfun = rootfunc,events = list(func = eventfunc, root = TRUE)
)

head(out)