11.1: Newtonian Motion

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Newton’s second law of motion is often written like this:

\[F = m a\notag\]

where \(F\) is the net force acting on an object, \(m\) is the mass of the object, and \(a\) is the acceleration of the object.

This equation suggests that if you know \(m\) and \(a\), you can compute the force. And that’s true, but in most physical simulations it’s the other way around: based on a physical model, you know \(F\) and \(m\), and you compute \(a\).

So if we know acceleration as a function of time, how do we find the position of the object, \(r\)? Well, we know that acceleration is the second derivative of position, so we can write the differential equation

\[\frac{d^2r}{dt^2} = a\notag\]

where \({d^2r}/{dt^2}\) is the second time derivative of \(r\).

Because this equation includes a second derivative, it’s a second-order ODE. We can’t solve the equation using ode45 in this form, but by introducing a new variable, v, for velocity, we can rewrite it as a system of first-order ODEs:

\[\begin{aligned} \frac{dr}{dt} = v \\ \frac{dv}{dt} = a\end{aligned}\]

The first equation says that the first derivative of \(r\) is \(v\); the second equation says that the first derivative of \(v\) is \(a\).