✎ Newton's Second Law [The Physics Travel Guide]

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<WRAP lag>$\color{blue}{\vec F}=\color{olive}{m}\color{magenta}{\vec a}$</WRAP>

====== Newton's Second Law  ======

Newton's second law tells us that how an object $\color{magenta}{\text{gets faster}}$ depends on its $\color{olive}{\text{mass}}$ and the $\color{blue}{\text{total force }}$ acting on it.

Formulated a bit differently it tells us that the $\color{magenta}{\text{acceleration}}$ of an object is given by the ratio of the $\color{blue}{\text{force }}$ acting on it divided by its $\color{olive}{\text{mass}}$.

However, take note that it is not sufficient to describe a physical system. Additionally, to describe a system we need to know what forces act on the object and what equations describe them. Famous examples of such force laws are

* [[equations:newtons_law|Newton's law of gravity]]
  * [[equations:lorentz_force_law|Lorentz' force law]]
  * [[equations:coulombs_law|Coulomb's force law]]

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So, for example, when we want to describe the movement of a planet around the sun we need to think about what forces act on the plant. For this system gravity is the most important force since both objects - the sun and the planet - a superheavy. Therefore, to calculate the movement of the planet, all we have to do is use [[equations:newtons_law|Newton's law of gravity]] to calculate the force acting on it. Then, when we have calculated the force we can use Newton's second law to calculate the acceleration of the object. Then, given some starting point and starting velocity of the planet we can calculate where the planet will be at every point in time in the future.

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**Example: The movement of an asteroid towards the sun**

As a first step, we use [[equations:newtons_law|Newton's law of gravity]] to calculate the force acting on the asteroid

$$ F = G \frac{m_s m_a}{r^2},$$

where $G$ is the gravitational constant, $m_s$ the mass of the sun, $m_a$ the mass of the asteroid and $r$ the distance between them. We assume that the asteroid is much lighter than the sun and therefore can neglect the effect the asteroid has on the sun. In other words, we assume the sun remains stationary, although of course an equal gravitational force also acts on the sun.

We can then put this equation into Newton's second law

$$ F = ma \quad \rightarrow \quad  G \frac{m_s m_a}{r^2} = ma . $$

Next, we recall that the acceleration $a$ is just the rate of change of the velocity and subsequently that the velocity is just the rate of change of the location:

$$ a = \dot v = \ddot r .$$

Putting this into our equation yields a differential equation for the location $r$ that we need to solve

$$ G \frac{m_s m_p}{r^2} = m \ddot r . $$

The equation is not easy to solve, and a nice discussion can be found [[https://math.stackexchange.com/questions/1781/can-this-gravitational-field-differential-equation-be-solved-or-does-it-not-sho|here]].

Take note that before we can fully solve the equation, we need to specify the [[basic_notions:boundary_conditions|boundary conditions]].

<note tip>
The motto in this section is: //the higher the level of abstraction, the better//.
</note>

Newton's second law is the most fundamental equation of [[theories:classical_mechanics|classical mechanics]]. It is still used nowadays, for example, by engineers.

* $\vec F$ is the force that acts on the object in question.
  * $\vec a$ is the acceleration of the object. The acceleration is the rate of change of the velocity $\vec v$ of the object, which in turn is the rate of change of the location $\vec r$ of the object: $a = \ddot{r} = \dot{v}$.

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