**Physics...Before/After**

jos360

Dwellingdweller wrote: »

Just had a quick look at that myself. It's l on the y axis and T^2 on the x axis, but it's not obvious at first, so I'll type out how I got to grips with it, hopefully it'll help ya understand it too. Those graphs can be confusing.

So you start with your formula:
T=(2pi)*(sqrt(l/g))
Square both sides
T^2=4(pi^2)*(l/g)
So now you have an equation in the form y=mx+c. c=0 because the graph goes through the origin, so the equation is just y=mx. You need to switch it around to get g.
g=4(pi^2)*(l/t^2)
As you can see l/t^2 is the slope, just multiply 4(pi^2)(slope) to get g. Hope that helped!

Its easier to do this in this case since slope will be usable in solving for g without having to use 1/slope.
But correct graphing practice says to put whatever variable you control on the x-axis, and the variable you record on the y-axis.
So really you should have length on the x-axis, since thats the variable you set yourself (Unless your'e a genius and predict the period).

But this is the leaving cert so I think you can just put whatever you wan't where ever you want as long as you get a straight line through O and know how to manipulate the value you get for slope.

jos360

And sure f**kit, if you get stuck on a mechanics question, just use E=mc^2 to covert all the energy to mass. That way acceleration, velocity and potential energy all equal 0. Much easier way to do calculations :P
An they never specify that energy-mass conversion doesn't happen in the question so...

D.murphy

hi i recently dropped down to ordinary level physics and i want to get a good grade in this. anyone have any tips for me? experiments, sections of the book etc that i should learn? thanks

cocopopsxx

I hope they don't ask us to draw stuff in modern physics, I can't draw properly in just a few mins, takes a lot of time

bluesky94

Does anyone know if there is a difference between first ionisation energy and work function? The definitions are basically the same in Physics and Chemistry..

Hiccuper

I realised that I'm not going to pick any of the electricity or magnetism questions so I'm pretty much ditching it, only learning a few definitions for Q5.

bladet1delta

is there a form for what kind of experiments come up? Like in chemistry, Q1 is always an organic experiment. Just curious, would cut my work load a bit if I could narrow the experiments down

Wanchor

bladet1delta wrote: »

is there a form for what kind of experiments come up? Like in chemistry, Q1 is always an organic experiment. Just curious, would cut my work load a bit if I could narrow the experiments down

Yep, I think the first one is always mechanics then the two in the middle are unpredictable and then the last one is something predictable. How useful was that?

Wesc.

bladet1delta wrote: »

is there a form for what kind of experiments come up? Like in chemistry, Q1 is always an organic experiment. Just curious, would cut my work load a bit if I could narrow the experiments down

This is what is most likely to happen this year:

1) Mechanics
2) Heat
3) Sound/Light
4) Electricity

bladet1delta

Thanks guys. I had a quick look through the years and the pattern seems to be that Q1 is always mechanics and Q4 is always electricity. Q2 and Q3 tend to vary between sound, light and heat, though Boyle's Law was question 2 in 2011 and my book classes that as mechanics? (I'd be more inclined to say it's heat/pressure, but whatever).

Anyways, take from it what you will, knowing Mechanics and Electricity well won't do any harm by the looks of things.

RedTexan

bluesky94 wrote: »

Does anyone know if there is a difference between first ionisation energy and threshold frequency? The definitions are basically the same in Physics and Chemistry..

Maybe you mean the work function? Well I would think that work function is specific to photo emission and all that dandy while ionisation energies are more relevant to chemical bonding, that's a difference. I wouldn't want to confuse the matter!

RedTexan

bladet1delta wrote: »

is there a form for what kind of experiments come up? Like in chemistry, Q1 is always an organic experiment. Just curious, would cut my work load a bit if I could narrow the experiments down

Q2 is always organic!

bluesky94

RedTexan wrote: »

Maybe you mean the work function? Well I would think that work function is specific to photo emission and all that dandy while ionisation energies are more relevant to chemical bonding, that's a difference. I wouldn't want to confuse the matter!

Yes, work function is what I meant.. :cool: Yeah, probably better that I just leave each course to itself.. Thanks

bladet1delta

RedTexan wrote: »

Q2 is always organic!

my bad! Q1 is always titration!

jos360

bluesky94 wrote: »

Does anyone know if there is a difference between first ionisation energy and threshold frequency? The definitions are basically the same in Physics and Chemistry..

The amount of energy a photon has depends on it's frequency so they are similar but not the same.

Threshold F only applies to photoelectric emission (obviously)

Threshold F is generally for a large body of metal or something whereas ionisation energy is per mole of independent atoms/molecules.
This is significant since we just assume one amount of energy is needed to isolate an electron when were talking about threshold f. but with ionisation energy we refer to what specific electron were removing (eg. first, second...).
What I mean by this is that at LC level, were expected to know that ionisation energy changes depending on previous ionisations, but with threshold f we just assume it's always the same.

Also then obviously threshold f is to remove one electron, where as ionisation energy is to remove a mole of electrons.

Their concepts are similar but they are slightly different.

jos360

If were asked to give the principal of conservation of mass or energy,
I think I might just put down E=MC^2 and write "your argument is invalid"...
#physicsnerd...

Dwellingdweller

jos360 wrote: »

Its easier to do this in this case since slope will be usable in solving for g without having to use 1/slope.
But correct graphing practice says to put whatever variable you control on the x-axis, and the variable you record on the y-axis.
So really you should have length on the x-axis, since thats the variable you set yourself (Unless your'e a genius and predict the period).

But this is the leaving cert so I think you can just put whatever you wan't where ever you want as long as you get a straight line through O and know how to manipulate the value you get for slope.

Yeah I just realised that length should be put on the x-axis in the case of this experiment. :P Which makes all the calculations pretty much irrelevant. Oh well. :pac: The principle (manipulate the slope to get a value) is correct so

Irish94

How do you know when to use the Kinetic/Potential energy formula in mechanics?

Dwellingdweller

Irish94 wrote: »

How do you know when to use the Kinetic/Potential energy formula in mechanics?

Generally potential energy will be asked for objects which are in the air, or falling (Ep=mgh, you need a height term). Kinetic energy is usually used for objects which are accelerating, have velocity, are moving etc (Ek=1/2mv^2).

Sometimes you have to equate the two, like in questions about the electron. The principle of conversation of energy states that Ep=Ek, i.e potential energy = kinetic energy when the electron is in equilibrium. The potential energy is maximum when the kinetic energy is minimum and vice versa

Incompetent

Could someone quickly outline when to use the minus in F=-ks for Hooke's law?

Never got that

Dwellingdweller

Incompetent wrote: »

Could someone quickly outline when to use the minus in F=-ks for Hooke's law?

Never got that

The minus sign stands for the direction of the restoring force that the spring experiences after deformation. Here's a little diagram:

|---x~~<

OK, so the straight line is the base of the spring. The 'x' is the equilibrium position of the spring at rest when no force is applied. The curly lines stand for the deformed spring, so it's been displaced by s, in metres. The < stands for the direction of the restoring force.

The reason there is a minus in the formula is because the spring moves in the direction of MINUS x when the force stretching it is removed. Think of it like a graph, x increases from left to right, and decreases from right to left. If the spring is compressed towards x=0, then k is positive: if the spring is stretched, then k is negative (because it moves back towards x=0). Hope that helps!

Wanchor

Incompetent wrote: »

Could someone quickly outline when to use the minus in F=-ks for Hooke's law?

Never got that

Force = (Elastic Constant)(Displacement)

It used in Mechanics with questions on springs.

D.murphy

anyone tell me what to learn for pass physics?

DepoProvera

Sure, it'll be graaand. We have loads of time to study

Nitsuj

DepoProvera wrote: »

Sure, it'll be graaand. We have loads of time to study

I just started an hour ago, and tbh, I'm pretty confident I'll get everything done today. I'm gonna do OL though.

D.murphy wrote: »

anyone tell me what to learn for pass physics?

this should give you some guidance.
www.youtube.com/watch?v=VFP4abTtjcU

fizzyorange

Still have to revise all of optics, mechanics, modern and heat. (hoping each will take me like 45 minutes tops) and five experiments.

oh god

ei.sderob

I was just looking at S.H.M and it's grand like but I was just wondering, in the derivation of proving that Hooke's law executes S.H.M how does -K/m = -omega squared? I mean it's an easy derivation but does anybody know WHY this is the case? It's not relevant to the exam but I just kinda want to know for myself:P

Nitsuj

do derivations come up in OL?

Wesc.

Ara fuck it I'll get up at 6 tomorrow and have a 2 and 1/2 hour cramming sesh on everything apart from electricity! It's taken up half my day doing it today ffs

Is anyone doing HL here going for an A1?

FinnD

Wesc. wrote: »

Ara fuck it I'll get up at 6 tomorrow and have a 2 and 1/2 hour cramming sesh on everything apart from electricity! It's taken up half my day doing it today ffs

Is anyone doing HL here going for an A1?

Hoping for a C1 but you won't be alone in getting up at 6! Last exam! Last early morning!