Physics

Sangre

Fine, Ill do it your way
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Darth_Jman

I have physics on monday, and spanish and chemistry on tuesday. You think you have it bad?
Admittidly, studying physics and chemistry is hard, because its so easy to fall asleep or stare into space for half an hour. The best method of attack is to quickly learn each experiment then sketch out the diagram and writedown the chemicals used, indicators and how the endpoint is reached. Then reach for the previous exam papers.

On a side note, has anyone noticed how the extra sample papers in the educational company papers can range from pathetically simple to absurdly technical? Not having decent papers is something the board of education (or whoevers in charge now) should have done something about.

Darth_Jman

****, rambled on about chemistry there.
Same thing applies to physics. Materials required, diagram, formulas etc. Dont bother writing out procedure, at this stage it takes too much precious time.

ella minnow pea

Originally posted by DSMe
holy sweet mother of christ i hate this subject with such a passion .... its the only subject i have left to study and i have to get an a in it ... but i cant force myself to look at this ****e , especially electricity , what a pile of borin crap! jesus im ****ed

im the EXACT same....someone lock me in a room wit just my physic book (and maybe colin farrell, just for like, moral support) NOW!!!!

pseudonym

wow, the "Physics" thread now has 4 pages. it must be really really popular

Bottle_of_Smoke

Just studying the light there,

When they give you a table of data for a concave mirror experiment and ask "how can we find a second set of data without doing calculations" or something along those lines, what are they looking for?

Also when they say "newtons universal law of gravitation is an example of an inverse-square law, what is meant by an inverse-square law?"

What are they looking for, if anything, other than gravitation is inversely proportional to the distance squared?

Sangre

Yeah inverse square law is just inversely proportional to the distance like Coloumb's law.

For the concave mirror, just let U equal the V values and vice-versa.

Or maybe you could extrapolate the graph? I think the first one is better though.

Delphi91

Originally posted by Sangre
Yeah inverse square law is just inversely proportional to the distance like Coloumb's law....

Not quite.

An inverse square law means that one variable is proportional to 1/(other variable^2). Coulombs law and Newtons law are both examples. In Netwons law, if you double to distance between, say, two planets, the force of attraction goes down to 1/4 of what it was. If you treble the distance, it goes down to 1/9, if you quadruple the distance, it goes down to 1/16, etc, etc.

If you look at Newtons law, it basically says F = (G x m1 x m2)/d^2. For any two planets, m1 and m2 are constant, therefore G x m1 x m2 is a constant, so F is proportional to 1/d^2, which is an inverse square relationship.

The same can be done for Coulombs law.

Mike

Delphi91

Whether you start the graph at (0,0) or not will have NO effect on the slope.

You should always chose a range of scales on the axes so that your points are suitably spread out on the graph paper and not all cramped at one end.

Can't quite follow the logic of the 1/u vs 1/v graph and why it should start at (0,0). If you rearrange the mirror/lens equation you get 1/v = -1/u + 1/f. This is in the form of y = mx + c. m is -1 (line slopes downward from left to right) and the intercept, c, is the point where the graph crosses the y-axis (1/f) from which you can calculate f.

Points for graphs:

1. USE graph paper!!!!!!! (you will lose marks if you were asked to use graph paper and you don't)

2. Always draw the graph in pencil (don't use a 6B pencil!!!)

3. Label each axis, giving the quantity and the unit e.g Time/seconds

4. Make sure that your graph takes up at least 2/3 to 3/4 of the graph page.

5. Label the graph with a suitable heading e.g. A graph of the Sin of the angle of incidence vs the sin of the angle of refraction for a glass block.

6. Chose a suitable scale so that your points are spread along the full length of each axis.

7. Draw an average line through the points - don't "join the dots"!!! If they're not all in a straight line, then the average line will generally have as many points above the line as there are below the line.

If you follow these, you should do ok with graphs.

Mike

PHB

No but since the majority of graphs you run a line of best fit through the origin, theres no point in changing your method for a couple of odd ones

Also it means that you never have to think about where to start, you just always start at 0.

Sangre

Score one point for me.....


Oh yeah, doh, I meant to say it was squared aswell

Boston

Originally posted by Delphi91
Not quite.

An inverse square law means that one variable is proportional to 1/(other variable^2). Coulombs law and Newtons law are both examples. In Netwons law, if you double to distance between, say, two planets, the force of attraction goes down to 1/4 of what it was. If you treble the distance, it goes down to 1/9, if you quadruple the distance, it goes down to 1/16, etc, etc.

If you look at Newtons law, it basically says F = (G x m1 x m2)/d^2. For any two planets, m1 and m2 are constant, therefore G x m1 x m2 is a constant, so F is proportional to 1/d^2, which is an inverse square relationship.

The same can be done for Coulombs law.

Mike

you seem very good at making very easy concepts look extremely difficult. Basically You square the yoke below the line

CuLT

Originally posted by Boston
you seem very good at making very easy concepts look extremely difficult.

The physics book is brilliant at that .

Boston

Originally posted by CuLT
The physics book is brilliant at that .

Is that the big black book from folens, i hate that ****ing thing, as soon as the leaving cert is over its getting burnt. Havent looked at it in a year, piece of **** would turn anybody off physics.

Sev

Originally posted by Delphi91
Whether you start the graph at (0,0) or not will have NO effect on the slope.

Well yes thats true, didnt mean to put it like that. But if you're advocating mickey mouse graphs that dont start at the origin then I'm sure my physics teacher would have series issues with you.

Originally posted by Delphi91
You should always chose a range of scales on the axes so that your points are suitably spread out on the graph paper and not all cramped at one end.

Again this goes against all I've been taught. The graph is about accuracy, not pretty aproximation.

Originally posted by Delphi91
Can't quite follow the logic of the 1/u vs 1/v graph and why it should start at (0,0).

Why not?

Read real world physics. The intention of drawing a graph for that experiment, and having it begin at the origin, is such that when youve drawn your approximation line through the points, it should cut the x and y axis at points such that 1/v = 0 => 1/f = 1/u and vice versa. This allows you to take the average of both points to be used as your value for 1/f.

I can understand that you could still derive a value for 1/f from your graph, other ways, as you have mentioned, its just not.. as correct. In a sense I agree with you, I hate the system with a fiery passion. But I've learned that if you want full marks in anything, no matter what subject it is, you have to give them back exactly what they want, no questions asked. Physics is no different. The leaving cert is about being force fed rigid procedure and regurgitating it on the day. You may look at some of my so-called 'micky-mouse' graphs, and find them acceptable. But I'm seriously doubting that other teachers will lack the same stringency.

CuLT

Originally posted by Boston
Is that the big black book from folens, i hate that ****ing thing, as soon as the leaving cert is over its getting burnt. Havent looked at it in a year, piece of **** would turn anybody off physics.

Yeah , I'm having serious problems keeping awake for more than 10 minutes reading it, no joke! This does not bode well for 2moro.

Boston

Having a graph thought the origin means you only need one point to work out the slope. what hes is saying about a scale, he just means that your should take it in regular intervals like 5 10 15 and then plot the points in between. the 1/u 1/v graph should not go through the origin, what the hell are you people talking about teh origin for? where it cuts the x or y axis is the reciprocal of the focal lenght

Sev

Mostly the principle reason for drawing a graph is to show (explicitly) that a particular value is dependent and directly proportional to another. If youre graph does not clearly show a line going through the origin, then you cannot be sure (visually) that there is not a constant in the equation.

Flick to page 262 of Real World Physics, notice the graph on the left hand side of the page. THIS IS THE KIND OF GRAPH YOU ARE PROMOTING. Notice what it says about it...

Note that the graph is a a straight line, but it does not pass through the origin. We say the resistance changes linearly with temperature

LINEARLY, not PROPORTIONALLY. Most experiments require you to demonstrate that values are directly (or inversely) proportional.

Sev

Originally posted by Boston
the 1/u 1/v graph should not go through the origin, what the hell are you people talking about teh origin for? where it cuts the x or y axis is the reciprocal of the focal lenght

I'm not saying that the line itself should go through the origin, I'm saying that the scales of the graph, the axes should be counted from zero. Always.

Real World Physics, page 22. If you're scales do not start at zero, then the point at which the graph cuts the axes will not be a representation for 1/focal length.

You may very well get a question on this tomorrow, and asked "From you're graph find a value for the focal length". And you will be expected to have drawn such a proper graph, and have taken the average two points of intersection.

Sev

If you want to take chances tomorrow, go ahead. But I'm going to place my trust in the teachings of my two wise and very experienced Physics/Applied Maths teachers. Either way, I cant be marked down.

Boston

Originally posted by Sev
I'm not saying that the line itself should go through the origin, I'm saying that the scales of the graph, the axes should be counted from zero. Always.

Real World Physics, page 22. If you're scales do not start at zero, then the point at which they cut the axes will not be a representation for 1/focal length.

You may very well get a question on this tomorrow, and asked "From you're graph find a value for the focal length". And you will be expected to have drawn such a proper graph, and have taken the average two points of intersection.

Another example of pointless complecations, where else would the the x and y axis intersect only at the origin, bit of a stupid point your making, gee i was going to start from the point (-9,-9) but since you cleared that one up for me. Do you really think anybody will not know this. seriously where talking honors physics here and you have banged on for ages about startign the scale from zero. do you really think that onces going to take the first reading an srat the scale from there? such people should be riding the special bus to play school, not sitting a physics exam

Sev

I've no idea what in God's name you're saying. I made a typing error in that last post, which I tend to do when im writing very goddmanfast. It should have read...

If you're scales do not start at zero, then the point at which the graph cuts the axes will not be a representation for 1/focal length

It's corrected now, anyway, try to read between the lines. I'm also not sure if you're agreeing with me or not. If you think what I'm saying is pointless, and obvious, then read up the thread a little.

Sev

Originally posted by Boston
Another example of pointless complecations, where else would the the x and y axis intersect only at the origin, bit of a stupid point your making, gee i was going to start from the point (-9,-9) but since you cleared that one up for me.

Can you not understand what I'm saying? Disagree? Agree? Find its obvious anyway, or have you missed my point entirely? Either way sarcasm gets you nowhere, just makes you look like an idiot.

If you dont start the scales from zero, then they clearly will not intersect at the origin.

Delphi91

Originally posted by Sev
...But if you're advocating mickey mouse graphs that dont start at the origin then I'm sure my physics teacher would have series issues with you...

I would never advocate "mickey mouse" graphs.

The only reason why you are using the origin in the experiment that you picked is so that it is "easier" to get an answer. My problem with that is that most of the graph will be "empty space". Your data points will tend to be collected at one end of the axes. This, as a result, will lead to innaccuracy in plotting the points and also reading off any further points.

Again this goes against all I've been taught. The graph is about accuracy, not pretty aproximation.

You've missed my point completely!!!! By spreading the points out, you increase the accuracy because you can accurately plot smaller variations in the points. The only type of graph that I would approximate is one which I would draw on normal paper, with no scales and no points, just a curve, if for example I wanted to show the shape of a curve.

Read real world physics.

You've advocated this book a number of times. Don't refer to it like it's the bible. It isn't, and it has mistakes.

The intention of drawing a graph for that experiment, and having it begin at the origin, is such that when youve drawn your approximation line through the points, it should cut the x and y axis at points such that 1/v = 0 => 1/f = 1/u and vice versa. This allows you to take the average of both points to be used as your value for 1/f.

Fair enough, but you then extrapolate that to imply that ALL graphs should include the origin. Not true.

You may look at some of my so-called 'micky-mouse' graphs, and find them acceptable.

I never referred to them as "mickey mouse". I can see your point and your teacher's reasoning for drawing the graph using the origin. My problem with this is that it then becomes a "special case". You have to remember this particular "tweak" if that particular graph comes up.

Have a look at this site for graph drawing rules: http://www.saburchill.com/physics/mpi/graphs/002.html and especially http://www.saburchill.com/physics/mpi/graphs/0051.html


Mike

Boston

Sev, i can start my scale from any point and the x and y axis with still intersect at zero. Juast say your dong the veriation of resistance with temperature for a metallic conductor experiment and you us cuso4 sultion instead of water, then you can start from a minus temperature, and work your way up and your graph would still be right. This is clearly an example os someone who ahs learnt off the book with out understanding the why.

Delphi91, i agree about the folens book, as a teacher what book do you use? turthfully its a piece of crap, and its heavy.

Delphi91

Despite what I said in my last post, I actually use "Real World Physics". However, I always treat the book carefully. I've used books before which were riddled with mistakes. The problem with the new physics books is that they were produced for a new course. I am always a bit dubious about "first editions" - they nearly always contain mistakes - like the examples given already of graphs drawn the "wrong way around"

However, having said that, its not too bad as books go - there are worse out there.

Sev

Originally posted by Boston
Juast say your dong the veriation of resistance with temperature for a metallic conductor experiment

If you read through what I've said, you'll see that was one of the experiments that was an exception to the "through the origin rule".

Delphi91

Here's an excellent resource for graph drawing:

http://www.physicsweb.com/schechter/physics580/topicguides/c1_motion/c1a_20_24_drawing_graphs.pdf


Ok guys, go study your physics - I'm gonna eat lunch!

Mike

Sev

Originally posted by Delphi91
You've missed my point completely!!!! By spreading the points out, you increase the accuracy because you can accurately plot smaller variations in the points. The only type of graph that I would approximate is one which I would draw on normal paper, with no scales and no points, just a curve, if for example I wanted to show the shape of a curve.

I can understand that, but thats not the same sense in the word accuracy that I'm trying to communicate, I mean rigour. You're showing a microcosmic picture of whats going on, on a scale that is tailored to fit the confines of the results.

Tell me, what is the most frequent question to follow up a Section A graph of an experiment?

Explain why your graph verifies X's law.

Because the graph produces a straight line which passes through the origin

Sev

Ok Delphi, im gonna drop it now. But I'll be making very sure I have zero marked clearly on my scales tomorrow. You might want to ask around and get opinions from other physics teachers, but I know for certain that the two teachers in our school would have a serious exception to your methods (for experiments like boyle's law, focal length, fundamental frequency of stretched string, especially)