什么是 Acceleration vector？

The acceleration vector of $M$ is the derivative of its velocity vector, $\vect a(t) = \bigl(v_x'(t),\, v_y'(t)\bigr) = \bigl(x''(t),\, y''(t)\bigr)$, in $\mathrm{m}/\mathrm{s}^{2}$: how fast, and in which direction, the velocity vector is currently changing.