Fundamental Physics

You seem to have a basic grasp already. Simply, each fundamental force: strong, weak, electrostatic and gravity each have their own force carrying particle that is transferred whenever the interaction happens (gravities is only theoretical at the moment). So the strong force uses the gluon, which means inside a nucleus there is a constant exchange of gluons which hold it all together. Which concept in particular are you struggling with?

Hi thanks for the reply. Taking the example of the strong nuclear force, are the gluons always present? How does the constant exchanging provide the force to hold the nucleus together, what are they exchanging??

The problem with exchange particles is we don't know everything yet, and what we do know can often be counter intuitive. I'll try to help though:

The gluons are constantly being exchanged, but it takes time for them to travel. As there are a bunch of conservation laws only one can be emitted between another being received. So there is technically a time where no gluons are being given out, but the particles are so close together and the gluons so fast that this is impossible to actually see and so for practical purposes the gluons are constantly there. Gluons carry energy, which they must have in order to be able to move, and colour - a property that quarks have. So gluons change the energy and colour of the quarks, in other words, the nature of the quarks in for example a proton is constantly switching, but by conservation laws it is always a proton (that is 2 up quarks and a down quark)

Thanks this is probably going above what the syllabus requires yet its still interesting. Thanks again for explaining that.

No problem - I plan on becoming a physics teacher so the practice is useful. If you're A level it is definitely way over and above what you're expected to know - for me it was first year degree but I know some that don't encounter it until their second or possibly even final year. At A level you just need to know that it deals with the strong force, which holds atoms and particles together, is stronger than the weak force and has a longer range than the W and Z bosons (for the weak force - which is necessary otherwise particles would decay on creation).

Drag Force = (ñ*A*CD/2)v^n


where CD is the drag coefficient,
A is the frontal area of the object,
ñ is the density of the fluid in the wind tunnel,
v is the speed of the fluid relative to the object,
n is a constant.

Sorry to bother you, but you explained the last question so well. When working out the units of n how do you know that it has no units? Is it because the speed is raised to the power of it? thanks

As far as I know, an exponent is always dimensionless (no units). This is because if you think about an equation x=y^n and take logs of both sides this gives n=(logx)/(logy). A logarithm never has units as it is just a ratio (which by definition have no units). If there are no units on the RHS of the equation then there can't be on the left hand side, and so any power will always be dimensionless. If you want something interesting to think about or look into, consider some exponential decay with time so there is an e^(-t) term. Does this '-t' still have units or is it now dimensionless? (I have an idea as to the answer, but I'm not certain).

As far as I know, an exponent is always dimensionless (no units). This is because if you think about an equation x=y^n and take logs of both sides this gives n=(logx)/(logy). A logarithm never has units as it is just a ratio (which by definition have no units). If there are no units on the RHS of the equation then there can't be on the left hand side, and so any power will always be dimensionless. If you want something interesting to think about or look into, consider some exponential decay with time so there is an e^(-t) term. Does this '-t' still have units or is it now dimensionless? (I have an idea as to the answer, but I'm not certain).

EDIT: It is dimensionless, it's more the reason why I was referencing (this goes way beyond the A level syllabus and really beyond most areas of physics as well).

Taking the exponential decay equation A=A0e^- λt
Then by taking natural logs the graph lnA on y axis and t on the x-axis can be plotted. The y-axis has no units, however the x-axis still has the units /s. So that would suggest that the t would still have a dimension. Unless the textbooks wrong....which it has been a few times.

No. Both sides are dimensionless. If you take logs of both sides it's Log on one side and λt on the other
λt is dimensionless as λ has unit s^-1

Yeah i can see that no problem, the units cancel each other out. The initial question is Drag Force = (ñ*A*CD/2)v^n

The bit i dont understand is that it is speed to the power of an unknown constant. Does that mean than any power has no units?

where CD is the drag coefficient,
A is the frontal area of the object,
ñ is the density of the fluid in the wind tunnel,
v is the speed of the fluid relative to the object,
n is a constant.

Yes. An exponent (or the expression in the exponent) must be dimensionless.
Exponents are normally just numbers anyway, but if there is an expression there the expression as a whole must be dimensionless.

Do you have any knowledge as to why this is the case?

It's true by definition.
Ask yourself...What is an exponent? What is it saying?
x³ means multiply x by itself 3 times.
3 is a pure number as it expresses how many times.

So what about for as an example e^v where v represents velocity. If v now becomes just a number, why is this the case? Is there some mathematical reason for this?

You can't have e^v if v is velocity.
There is no such real formula. It is not dimensionally valid.
If you have something that varies with e^v then there would of necessity have to be some other term in the exponent (kv or v/k for example) where k is a constant with appropriate units leaving the exponent dimensionless. v doesn't "become a number". It stays as velocity and keeps its units.
The mathematical reason is that an exponent is a pure number.

Is the fact that an exponent is dimensionless an axiom then?