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## Newton’s 2nd law of motion, force and acceleration

Last week, I blogged about Newton’s 1st law of motion, and the concept of inertia. At the end I said that this week I would discuss what happens to an object if a force is applied. Or, to put it more correctly, an “external resultant force” is applied. This is what Newton’s 2nd law of motion is all about – the effect on a body of an applied force.

Newton’s three laws of motion appear in his masterpiece, The Principia, which was published in 1687.

If we apply a force to an object it will change its velocity, which means it will accelerate. As I have mentioned before, in physics acceleration has a more precise meaning than it does in everyday life. It doesn’t just mean an object is changing its speed, it can also be keeping a constant speed but changing its direction, such as an object moving at a constant speed in a circle. But, whether an object is changing its speed or changing its direction, it has to accelerate to do this, and so a force needs to be applied.

## The most important equation in physics

The relationship between force and acceleration is given by Newton’s 2nd law of motion, which states that

$\boxed{ F = m a }$

where $F$ is the force, $a$ is the acceleration, and $m$ is the mass of the body. From this equation, along with Newton’s 3rd law (which I will discuss next week), nearly all of mechanics can be derived. For example, this equation tells us that for the Moon to orbit the Earth, it must have a force acting upon it. That force is gravity, and Newton was also the first person to produce an equation to describe gravity. For example, in this blog I showed how we can derive the acceleration felt by a body travelling in a circle, which we call the centripetal acceleration.

Using calculus, this equation also allows us to derive the three equations of motion, equations like $v = u + at$ and $s=ut + \frac{1}{2}at^{2}$, as I did in this blog. It tells us that it is more difficult to accelerate a more massive object than it is a less massive one, which is why you need a more powerful engine in a large truck than you do in e.g. a small car. Along with Newton’s 3rd law, it explains why a bullet comes out at such a high speed from the nozzle of a rifle, but why the recoil of the gun moves much more slowly. As I said, the most important equation in physics.

Next week I will discuss the last of Newton’s laws of motion, his 3rd law.

## Derivation of the centripetal force

I have mentioned a few times in previous blogs that an object moving in a circle at a constant speed does so because of a force acting towards the centre. We call this force the centripetal force. The force is given by the equation

$F = \frac{ mv^{2} }{ r }$

where $m$ is the mass of the object moving in the circle, $v$ is its speed, and $r$ is the radius of the circle. More correctly, remembering that force is a vector, it should be written

$\boxed{ \vec{F} = - \frac{ mv^{2} }{ |\,\vec{r}\,| } \hat{r} }$

where $|\,\vec{r}\,|$ is the magnitude (size) of the radial vector $\vec{r}$, and $\hat{r}$ is the unit vector in the direction of $\vec{r}$.

But, from where does this formula come?

## The acceleration of an object moving in a circle

The acceleration of any object is defined at the change in its velocity divided by the change in time. Both acceleration and velocity are vectors, so mathematically we can write this as

$\vec{a} = \frac{ \Delta \vec{v} }{ \Delta t }$

We are going to consider an object moving at a constant speed in a circle, as illustrated in the diagram below. In a time $\Delta t$ the object has moved through an angle $\theta$, and its initial velocity $v_{1}$ has changed to $v_{2}$, where the only change in the velocity is its direction. Remember, the velocity is tangential to the radius, so makes a right angle with the radius vector $\vec{r}$. The direction of the radius vector is, by definition, radially outwards.

We will consider an object moving in a circle with a constant speed. It moves through an angle $\theta$ in time $t$.

In my blog about vectors I mentioned that, to combine vectors which have different directions, we need to split the vectors into components, add the components and then recombine the resultant components. The components need to be at right-angles to each other, and usually (but not always) we choose the x and y-directions when the vectors are in two dimensions.

To find the acceleration of our object in this example, we want to find the change or difference in the velocity, that is $\Delta \vec{ v } = \vec{ v_{2} } - \vec{ v_{1} }$. We start by splitting the two vectors into their x and y-components.

$\vec{ v_{1} } = v_{1}\hat{x} + v_{1}\hat{y} \; \; \text{ and } \; \; \vec{ v_{2} } = v_{2}\hat{x} + v_{2}\hat{y}$

Looking at our diagram, we can write

$\vec{ v_{1} } = 0\hat{x} + v\hat{y} \; \; \text{ and } \; \; \vec{ v_{2} } = v\sin(\theta)\hat{x} + v\cos(\theta)\hat{y}$

where $v$ is the speed, the size of the vectors $\vec{ v_{1} } \text{ and } \vec{ v_{2} }$.

The change in the velocity in the x-direction, which we will call $(\Delta v)\hat{x}$ is then just

$(\Delta v)\hat{x} = v\sin(\theta) - 0 = v\sin(\theta)$

Similarly, the change in the velocity in the y-direction, which we will call $(\Delta v)\hat{y}$ is given by

$(\Delta v)\hat{y} = v\cos(\theta) - v = v(\cos(\theta) - 1)$

To correctly calculate the acceleration, we need to find the change in velocity with time as the time interval tends to zero. This means the angle $\theta$ tends towards zero also, and when $\theta$ is very small (and expressed in radians) we can write

$\cos(\theta) \rightarrow 1 \; \; \text{ and } \sin(\theta) \rightarrow \theta$

So we can then write

$(\Delta v)\hat{x} \rightarrow v\theta \; \; \text{ and } (\Delta v)\hat{y} \rightarrow v(1 - 1) =0$

The overall change in velocity, $\Delta \vec{v}$ is then just the change in velocity in the x-direction, $\Delta \vec{v} = v\theta$. The direction of the change in velocity is in the positive x-direction, which as $\theta \rightarrow 0$ is along the radial vector towards the centre of the circle (that is, in the $- \vec{r}$ direction).

However, we can do a substation for the angle $\theta$. Remember, the arc-length, which we shall call $l$ is related to the angle $\theta$ via our definition of the radian (see this blog here), $\theta = l/r$, so we can write

$\Delta \vec{v} = \frac{ vl }{ |\,\vec{r}\,| }\hat{r} \; \text{ (Equation 1)}$

The acceleration $\vec{a} = \Delta \vec{v} / \Delta t$. But the speed $v$ and time $t$ are related via $v = l/t$, so we can write that $l = vt$. Substituting this into equation 1 above gives

$\frac{ vl }{ r } = \frac{ v^{2}t }{ r } \; \text{ (Equation 2)}$

and so the acceleration $\vec{a}$ is given by

$\vec{a} = \frac{ \Delta \vec{v} }{ t } = \frac{ v^{2}t }{ |\,\vec{r}\,| t } = \boxed{ - \frac{ v^{2} }{ |\,\vec{r}\,| } \hat{r} }$

where we have added the minus sign to remind us that the change in velocity, and hence the acceleration, is directed towards the centre of the circle.

If you prefer to think of vectors pictorially, then the direction of $\Delta\vec{v} = \vec{v_{2}} - \vec{v_{1}}$ can be seen from this diagram:

This shows the direction of $\Delta \vec{v} = \vec{v_{2}} - \vec{v_{1}}$, and it is along the radius vector towards the centre of the circle, in the opposite direction to the radius vector $\vec{r}$.

The centripetal force is found by remembering that $\vec{F} = m\vec{a}$ (Newton’s 2nd law), so finally we can write that the centripetal force is

$\boxed{ \vec{F} = - \frac{ mv^{2} }{ |\,\vec{r}\,| } \hat{r} }$