g =  9.8  # m/s^s 
a = -g
tM = 2

v0 = 10 # m/s
t  = 0  # s  
plot(t, v0, xlim=c(0, tM), ylim=c(-11, 11),  xaxs='i',  yaxs='i',
     xlab='t (s)', ylab='v (m/s)', pch=19, col='blue')
abline(h=0, lty=2)

Dt = 0.02 # s

while(1) {
  Sys.sleep(Dt)
  t = t + Dt
  points(t, v0 + a * t, pch=19, col='blue')
  if(t > tM) break
}
abline(h=v0, lty=3, col='blue')
abline(h=-v0, lty=3, col='blue')

#----------------------------------------------------------
Sys.sleep(1)
v0 = 5 # m/s
t  = 0  # s
points(t, v0 + a * t, pch=19, col='cyan')
while(1) {
  Sys.sleep(Dt)
  t = t + Dt
  points(t, v0 + a * t, pch=19, col='cyan')
  if(t > tM) break
}
abline(h=v0, lty=3, col='cyan')
abline(h=-v0, lty=3, col='cyan')


#----------------------------------------------------------
Sys.sleep(1)
v0 = 0 # m/s
t  = 0  # s
points(t, v0 + a * t, pch=19, col='orange')
while(1) {
  Sys.sleep(Dt)
  t = t + Dt
  points(t, v0 + a * t, pch=19, col='orange')
  if(t > tM) break
}

#----------------------------------------------------------
Sys.sleep(1)
v0 = -5 # m/s
t  = 0  # s
points(t, v0 + a * t, pch=19, col='green')
while(1) {
  Sys.sleep(Dt)
  t = t + Dt
  points(t, v0 + a * t, pch=19, col='green')
  if(t > tM) break
}