This problem is not modeled appropriately by linear regression - the path of the object will follow a parabola, which is typically of the form y = -(x^2) which is a second-degree polynomial....

That is why the R square values are so low.....

You should try to plot the data and determine a best second-degree polynomial fit, and then the first derivative of the equation of that best-fit line should help answer the questions of initial velocity, max height, time of flight, and terminal velocity.