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## Vector basics

I wanted to get back to explaining Maxwell’s equations, which I mentioned in this blog of the statue to James Clerk Maxwell that is in Edinburgh. Before I do that I thought I would cover some very basic mathematics, namely the basic idea of vectors.

In physics and mechanics, a vector is something which has both size (magnitude) and direction. If the quantity has only a size, we call it a scalar. So, for example, in physics we differentiate between speed and velocity, even though in everyday language they are used interchangeably.

If we say a car is moving at 50 km/h, that is a speed, and hence is a scalar. But, if we were to say it is moving at 50 km/h due North, that is a velocity, and hence a vector as we have given it both a size and direction.

Vectors are very important in physics, and one of the most important and widely used vectors is force. We measure force in Newtons (named after Sir Isaac Newton), but a force has both a size and a direction. We usually denote a vector by either putting a line or arrow above the symbol, or by using a bold font. I will use an arrow above the symbol, so for example $v$ would be a scalar, but $\vec{v}$ would be a vector.

Any vector in 3-dimensional space can be split into 3-components. This is often useful, as unlike scalars which add simply, when we add vectors we need to take into account their directions. As an example, suppose we have two forces $\vec{F_{1}}$ which has a size of 7N in the x-direction, and $\vec{F_{2}}$ which has a size of 5N at $25^{\circ}$ to the x-axis. The combined force is not 12N, as the two forces are not acting in exactly the same direction. A 7N and a 5N force at 25 degrees to each other, with the 7N force acting along the x-direction.

To find the combined (resultant) force, we need to split the two forces up into their x and y-components. To do this we simply use trigonometry, and note the unit vectors $\hat{x}$ and $\hat{y}$, which are vectors with a size of unity (1) in the x and y-directions respectively. We can then split each of the forces $\vec{F_{1}}$ and $\vec{F_{2}}$ into their respective x and y-components. $\vec{F_{1}} = 7\cos(0) \hat{x} + 7\sin(0) \hat{y} = 7\hat{x} + 0\hat{y}$ $\vec{F_{2}} = 5\cos(25) \hat{x} + 5\sin(25) \hat{y} = (5 \times 0.9063)\hat{x} + (5 \times 0.4226)\hat{y} = 4.53 \hat{x} + 2.11 \hat{y}$

From this we can write that the total force in the x-direction is $7 + 4.53 = 11.53 \hat{x}$ and the total force in the y-direction is $0 + 2.11 = 2.11 \hat{y}$. To then find the resultant force, we need to combine these two components as follows The size of the resultant force R can be calculated using Pythagoras’ theorem, its direction using trigonometry.

To calculate the size of the resultant vector we just use $R^{2} = x^{2} + y^{2}$ so here $R^{2} = (11.53)^{2} + 2.11^{2} = 157.0 + 4.45 = 137.39$ so $R=\sqrt{137.39}=11.72 N$. To calculate the angle $\theta$ we note that $\theta = \arctan \left( \frac {2.11} {11.72} \right) = 10.2^{\circ}$.

So, as we can see, the resultant of these two forces is a force of size 11.72N which is at an angle of $10.2^{\circ}$ to the x-axis.

In a series of future blogs I will go on from these basic ideas of vectors to talk about vector fields, which we need to understand in order to understand Maxwell’s equations. A vector field. The length of each arrow represents the strength of the field at that location, the direction is given by the orientation of the arrow.

### 6 Responses

1. on 17/01/2013 at 14:21 | Reply Phillip Helbig

The north star is a vector: it has a magnitude and a direction. 🙂

(I don’t know who was the first to tell this joke; I heard it from Edmund Bertschinger.)

• on 17/01/2013 at 14:34 | Reply RhEvans

Groan….

Is that what passes for a joke in Germany? 😛

• on 18/01/2013 at 08:05 Phillip Helbig

Bertschinger is from the US (professor at MIT—though when I first saw him (when he was already a professor) I mistook him for a student) and he said this in a talk in Denmark.

2. […] this blog, I explained some of the basics of vectors. I only dealt with 2-dimensional vectors, but of course […]

3. […] this previous blog I discussed some of the basics of vectors, including how to add them. As I discussed, because […]

4. […] my blog about vectors I mentioned that, to combine vectors which have different directions, we need to split the vectors […]