To state this formally, in general an equation of motion M is a function of the position r of the object, its velocity (the first time derivative of r, v = ⁠ d r / dt ⁠), and its acceleration (the second derivative of r, a = ⁠ d r / dt ⁠), and time t. Euclidean vectors in 3D are denoted throughout in bold. This is equivalent to saying an equation of motion in r is a second-order ordinary differential equation (ODE) in r,