y=0.5*g*t.^2; %Y position from initial y
ypos=y0-y; %y pos from ground
g is negative, so when you do the subtraction you end up going up.
plot(Y)
No, your Y values are calculated every 1/2 second, you should not be plotting them as if you had calculated them every second. You should be keeping track of the times and return them along with the Y positions and using the times as the x values in the plot.
I also recommend that you change your 0.5 time increment to something smaller.
Also, your current code has already done the calculus to get the time/distance formula directly. That is not exactly wrong but probably the spirit of the assignment is that you should be modelling step by step, keeping track of current velocity, altering it by acceleration applied for a time, altering position by velocity applied for a time. Your current code would not, for example, be easily adaptable to having drag added in.
