These pages contain some thoughts and activities that might be useful when looking at physics from a TOK perspective or looking at TOK from a physics perspective. Neither the physics nor the TOK is rigorous but it might make you think.

Physics is concerned with the physical universe, if something does not obey the laws of physics then it either impossible or not part of the physical universe.

We model the universe by working out how the quantities that define it are related, we then use those models to make predictions but the first step is to define the quantities. The universe is probably infinite in size but we can only see the part of it "the observable universe", how can we be sure that the parts we can't see behave the same way as the parts we see? We can't but we have to assume they do, if this wasn't the case then we couldn't model the universe. We are not special, our bit of the universe is the same as all the others and it looks the same in all directions, this is the cosmological principle.

First things first, what are the fundamental quantities that we need to model the universe? We basically have two views, the one looking out and the one looking in, stars and atoms. The things close to use must obey the same laws as the things far away but lets start with stars. This is Orion by the way, but that has nothing to do with physics.

Distance

We can see that there are some differences, firstly they have different positions, to define the position we use distance. e.g. the distance between the brightest blue star and the red one. This universe is apparently 2 dimensional so we need 2 lengths to define the position of any star, we also need some reference point like the bottom left hand corner. We now have a coordinate system. For this to be useful our fixed distance mustn't change so it must be based on something constant, this is more difficult than it seems. The meter used to be based on the markings on a wall in Paris now it's based on the distance travelled by light in a certain time.

Brightness

Another difference is the brightness of the stars. This observation is based on our sense of sight. The Greeks put the stars in order of brightness in an attempt to quantify this difference. The brightest was 0 and a star half as bright was 1, 2 was half as bright again etc. You could try putting these stars in order. All stars, apart from the sun are points of light, the ones that look biggest are actually not bigger but brighter. What makes a star look bright?

Time

Something is happening, we can say an event is taking place, one of the stars is flashing. This difference can not be defined by position since it happens in one place, we need another quantity, time but how can we measure it? If the flashing star was the only thing that changed with time we couldn't however we can compare with something closer like a pendulum. The pendulum always swings 2 times in between every flash. Again we need to be sure that our pendulum  has a constant time period. If there wasn't a flashing star would there be time?

Mass

It would not be easy to realise that we needed the quantity mass to define our universe by looking at the stars. If we made closer observations over a long period of time we would notice that a lot of the stars rotate around each other, the time period of the rotation is different for different pairs of stars that are the same distance apart, mass is the quantity that is responsible for this difference. There is an easier way to define mass if we consider objects closer to us. Some objects are easy to pick up others difficult. Interestingly the objects that are difficult to pick up are also difficult to move, two completely different things but governed by the same property, mass.

Fundamental units

So, we can define our simple universe using 4 quantities mass, distance, time and brightness. Mass, length and time are fundamental units, they can not be simplified, brightness is actually a combination of mass and time. In physics we attempt to break things down into their simplest parts. Energy, power, momentum, density, volume, acceleration, intensity and pressure can all be expressed in terms of mass length and time. Observing the universe more closely reveals further fundamental quantities but 3 is enough for now.

SI units

It is important that physicists share their knowledge so we must all use the same units. In the past there were two main systems, pounds, feet, seconds and kilograms, meters, seconds. The second option is now adopted internationally.

Motion

The idea of defining quantities was to see how they are related, if it wasn't for movement they wouldn't be. We can see that the moving object changes position, as this happens time progresses. We never observe a change of position without change of time, they are related, the quantity that relates them is velocity.

Uncertainty

If two people measure the same thing they will probably get different results, this may be due to the instrument or the way it is used. Try measuring the length of a room with a 30 cm ruler. The uncertainty is approximately half the maximum value minus the minimum value. It is important to know the uncertainty in a measurement so you know how far you can trust it. Would you buy a door whose width had an uncertainty of ± 1 cm? What would be an acceptable uncertainty?

Graphs

Graphs are a visual way to represent relationships by looking at a graph you can see how the points are related.

Are graphs useful to blind people? How might you represent data in an alternative way?

Maths

Our number system was developed to count objects and mathematics was developed to model the way those objects combine.

Combining the apples on the left is the same as the apples on the right. We could simply draw the apples but it's simpler to use symbols

A + B = C

We then make up rules that model how the symbols relate to the apples.

Work out the following:

A + A =

B - C =

C - D + A =

We could use a different symbol for each number of apples but it would be extremely complicated it is easier to group the apples.

What do you get if you add one more apple to CCC?

We of course don't use the symbols A, B and C we use 1, 2, 3 and they are grouped in 10's not 3's. The numbers and the rules relating to them take on a life of their own. If we say 3 x 2 apples = 6 apples, the 3 is just a number not a number of apples. We have to be careful though.

3 x 3 = 9 but what would 3 apples x 3 apples represent?

In physics the numbers must mean something, they must have physical significance.

Numbers can be used for any quantity that adds up in the same way as apples but some quantities don't. If you walk 3 km west followed by 4 km North you'll be 5km from the start. In examples like this we use arrows not numbers.

Charge

When you rub a balloon on your jumper it will attract to your jumper but repel from another balloon, this is called the electric force and there are 2 types repulsion and attraction, the effect is caused by a property of matter called charge. Attraction and repulsion are opposite are opposite effects so we use + and - numbers to represent the two possible charges. Everything in the universe has either +, - or 0 charge, how convenient that our number system reflected this.

Quarks

Quarks are one of the fundamental particles of our universe they don't exist alone but only in groups, the way they combine doesn't fit in with our number system so we use colours, red, blue, green, cyan, magenta and yellow.

All combinations are white, can you explain why quarks combine in 3's and 2's but not 5's?

You now know how quarks combine but do you know anything about quarks? Is this knowledge useful?

Definitions

In physics quantities have very specific definitions e.g. Work is done when the point of application of a force moves in the direction of the force. We can use this definition to decide if work is done or not. Is work done when a heavy bag is carried across a room?

Laws

Apart from the actual subject law, physics probably has more laws than any other subject. Newton's, Snell's, Ohm's, Boyle's, Faraday's, Kepler's and Kirkhhoff's (all named after European physicists). For something to become a law you have to be pretty certain about it so all of its implications must be tested by experiment. Laws can be considered to be true and used to solve problems. When judges sentence a criminals they apply the law not their personal opinion, it's the same in physics. I know that the force required to accelerate 5 kg at a rate of 2 ms^-2 is 10 N because Newton's second law tells me so, it's not just me who thinks this is the answer.

Problem solving

This is what physicists are supposed to be good at but what does solving a physics problem entail?

Calculate the acceleration caused by an unbalanced force of 20 N applied to a body of mass 5 kg.

Step 1

Understand the question. To understand the question you must know the definitions of force, acceleration and mass.

Step 2

Draw a diagram to help visualise the problem.

Step 3

Choose the law that gives the relationship. In this case Newton's 2nd law.

F = ma

Step 4

Do some maths

a = F/m

Step 5

Substitute numbers

a = 20/5

Step 6

Calculate

a = 4 ms^-2

Not all problems are this easy though.

Physics in a nutshell

Define quantities > understand relationships > solve problems.

Like a nut, it looks simple but not easy to crack.

Which of the 12 concepts are related to this page?

• evidence
• certainty
• truth
• interpretation
• power
• justification
• explanation
• objectivity
• perspective
• culture
• values
• responsibility