On the meaning of the second derivative When we want to find the velocity of an object we use the derivative to find this. However, I just learned that when you find the acceleration of the object you find the second derivative. 
I'm confused on what is being defined as the parameters of acceleration. I always thought acceleration of an object is it's velocity (d/t). 
Furthermore, in the second derivative are we using the x value or the y value of interest. In the first derivative we were only concerned with the x value. Does this still hold true with the second derivative?
I would post pictures but apparently I'm still lacking 4 points. 

Slope of a Displacement vs Time graph gives you the velocity of the body. Also, the slope of the Velocity vs Time graph gives you the acceleration of the body.
So when you find the double derivate of the displacement vs time graph, you get acceleration. When you are finding the double derivative, what you are actually doing is finding the slope of the slope of displacement vs time graph, i.e, slope of the velocity vs time graph which is nothing but acceleration.
