Moodle  : Physics

blueline

Introduction

These lectures are written as tutorials for IB Physics (or equivalent). I myself is a former IB student and I try to write this in such a way that I would have found it as easy as possible to understand. These pages are not intended as a detailed reference, but more as an introductionary tutorial to help whoever is interested to explore this , in my opinion, very interesting subject. I must apologise for all selling mistakes. My mother tounge is Swedish and I therefore often make minor mistakes when I try to write in English. Hopefully this will not disturb the reader too much, and the site will be understandable. Except for that I wish you will find these pages usefull, and good luck with any examinations you may be taking in the future.

Best wishes: Jonatan Ring

(Note, I usually do not sign these pages, but this introduction sounded quite personal so I found it a comfortable way to end this page)


How to Study for Physics

Some people find Physics an easy subject, other find it terrible and difficult (Often the latter group tend to refer to the former as "nerds" ). The purpose of this lesson is to offer some advice for how to find studying Physics as easy as possible.

Firstly, dont try to memorise all formulas and hope it will enable you to solve the problems. Its much more important, and rewarding, to understand the concepts. Secondly, if you find something difficult to understand, dont give up and skip that part. Most of physics is a little hard to grasp the first time you face it. Many people fail to understand physics merely because they give up if they do not get it the first time they face it. Don't worry about those people who seem to know everything from before (theres one in every class) everyone learn at different rates and find differents ubjects to be difficult or easy.  Take your time, try to read teh explanations and examples a few more times. Try to draw a small sketch of teh situation if that helps and try to understand one step at a time. Usually physical theories can be very complex, but they tend to be built from many small and simple steps.

Acceptance is a good word when you try to learn a theory. Basicly, accept the assumptions of the theory eaven if you beleive them to be wrong. Its not that important if you agree with the argument until you have understood it. If a step in the argument seems wrong, then ignore the error and return to it once you understand the rest of the argument. Start with the things that you do understand and then build your way further from there. You will have plenty of time to refute or question the theory after you have understood it, but when you try to learn it just take it for granted that it is true. Im not saying you should not be sceptical (after all a lot of people do try to fraud others by claiming that their work is scientific when in fact it is not)  just wait with being sceptical until you have developed an understanding of the argument. It is often very difficult (if not impossible) to understand a theory if you are determined that it is wrong to begin with.

A very common error people make is to just ask for the solution to a problem. In Physics the solution is irrelevant. It doesnt matter what the final answer is, it is how you arrived at it that counts. If you understood the procedure of solving the problem, then it doesnt really matter what the actual figures were. This is why many problems will ask you to develop a formula for the answer rather than finding the answer itself. It is in order to force you into thinking about the problem rather than simply plug numbers into the formula they recognise. Sometimes pluging numbers into formulas will give you the right answer, but it woant help you understand the concepts, and that is what will determine how well you do on the final exam.

Finally, the secret to Master physics is the same as mastering Chess, playing the violin or becoming world champion in Tennis. Practice, practice and lots of practice. Often what appears difficult at first gets easier as you get used to it. In this respect the Brain is an organ remarkably similar to the Muscles. The more you use it, the stronger it gets. (Just don't push it too hard so that you stretch it and get a headache).  


Measurements

Note that this Lesson is mainly about formalities surrounding measurments and contains little Physical theory and therefore differ greatly from the octher lessons.  

All Physical theories are to some degree based on experiments. In some cases these are real experiments, or simply an theoretical experiment (I will come back to this). The purpose of a physical theory is to predict the outcome of a carefully controlled experiment. A good experiment is one which is clearly defined (there is no question as to how it shall be carried out) easily repeatable (others should be able to repeat it to confirm your findings) and the results of the experiments should be measurable with enough accuracy and precision to give the information required to draw a conclusion.

When a measurement is made, it is extremely important to specify how accurate the measurement is, and what degree of uncertainty is to be expected.  There is no such thing as a measurement without uncertainty. In some cases the uncertainty may be so small that it can be assumed neglichable, but it still exists and it should be recorded as negligible. Uncertainty information is crucial because it determines what conclusions you can draw from an experiment. If, as an example, you use your plain sight to measure a persons height, it may be sufficient to determine which family member is tallest, but at the same time it may not be accurate enough to determine which size you should request for a jacket bought on postal order. The same situation may arise in physics. Your measured density for Uranium may be accurate enough to determine if it can be safely loaded on a truck, but far to uncertain to be used in a nuclear powerplant without risking a meltdown. Here is an example of some correct ways of writing measured values, and some erroneous ones:

Correct ways:

Pi = 3.14  to three significant figures.

My height: 1.80m ± 2cm

Density of water: 1 kg dm-3 Uncertainty is negliiable when making coffee.

Note that the uncertainty has the same units as the measured quantity, and that it can be given either as the number of significant figures or as a deviation from the measurement.

Erroneous:

Pi = 3.14 

This is wrong because 3.14 is not an accurate value for pi.

My Height 180m ± 2

Here the units of the uncertainty has been omitted. Thus it is not possible to know whether they should be in cm, meter or eaven kilometers.

Density of water: 1 small uncertainty

That the uncertainty is small is useless information unless any reference is given as to what it is small compared to. Is it smalled compared to the uncertainty of a microscope? A standard ruler?

All experiments have variables. controlled, independent and dependent. Typically an experiment seeks to determine the relationship between one independent and on or several dependent variables when other variables which may affect the outcome is kept at a controlled value. As an example, if you try to determine how the pressure inside a sealed container increases with temperature, then the temperature is your independent variable (the one you force to different values) whereas the pressure is dependent. All other variables ( size and shape of the container, material of the container etc) are controlled. Note that a variable is controlled eaven if you do not have any means of controlling its value. If a variable shoudl ideally be controlled, it shoudl be listed as a controlled variable. That it is controlled merely means that it is not dependent on your independent variable, and that you do not deliberately alter its value throughout the course of the experiment.

Finally a word about thought experiments.

Some experiments are impossible or extremely difficult to carry out in practice. In these cases it can often be usefull to "pretend" that you carry out the experiment and use commonly accepted theories to predict the result. Typically a thought experiment uses one theory which is assumed to be true in order to argue why another theory is also correct. As an example Galileo argued that you would not feel if the earth was moving because everything else around you would move at the same time. He justified this through a thought experiment were he argued that if a horseman dropped an apple, the motion of the horseman would be transfered to the apple so that the apple would not fall straight down, but instead follow a path such that it would always be alongside the horseman provided the horseman continued at the same speed. Therefore, Galileo argued, as far as the apple was concerned, the horseman would not be able to tell the difference between droping it while standing still and while he was in motion. This is a thought experiment because it was not carried out. Galileo simply stated what would happen IF the experiment was carried out, and used this as evidence for his theory.