Hey everyone! Time for another Physics guide. This one is going to cover relativity; and all the associated content, equations, explanations, as well as visiting some potential exam questions. Relativity is undoubtedly the most confusing and troubling part of HSC Physics, with some of the hardest concepts, most difficult formulae, and will generally prove a bit of a pain in the exam. The trick is to
understand the ideas of relativity; if you understand it, potentially confusing questions become easy marks.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.I'll first cover the idea of relativity in general. It is to do with
reference frames . These can be defined a number of ways, but in HSC Physics, simply consider them as a reference point for measurements, or a 'zero' point. For example, from my reference frame sitting at my desk, I consider myself stationary. However, from the reference frame of the sun, I am actually moving quite quickly in an orbit. The idea to understand is that all measurements are made with reference to some zero point. This was the initial idea of relativity, that all motion is relative to something else.
There are two kinds of reference frames;
inertial and
non inertial . Again, these can be defined in depth, but for this course you need only remember that non-inertial frames of reference are
accelerating . Inertial frames are not accelerating. In non inertial frames, we have to invent fictitious forces to account for the physical behaviour of objects.
Questions on these initial concepts of relativity are quite rare. However, if they do pop up, it is most likely to ask you something similar to one of the following:
- Differentiate between inertial and non inertial frames of reference
- Describe the idea of relativity.
- What is a frame of reference?
In these questions, marks will be lost in the
wording . Be very clear what you are trying to say, don't waffle or hint, be direct. The marker should know what question you are answering without seeing it. Relativity IS the idea that all measurements are made relative to something else. It relates to frames of reference, either inertial or non-inertial, which form criteria for measuring quantities. Bang. Two marks. Move on.
In the 1800's, the idea of an absolute rest frame developed, something against which all other motion could be measured. This frame was called
The Aether , an undetectable material which permeated all matter in the universe, and also acted as the medium by which light was transmitted. At this stage, it was thought all waves permeated through a medium; the light was thought to permeate through the Aether.
Michelson and Morley attempted to detect the Aether by detecting the earth's movement through it. To do this, they tried to detect changes in the interference patterns of two beams of light. A beam was split, and the effect of the Aether searched for in interference patterns between them. The apparatus is shown below. But this experiment yielded a null result, discrediting the Aether model and inspiring perhaps the greatest piece of mathematics/science literature since the Principia Mathematica by Isaac Newton.
Questions on this experiment are VERY common. Be sure you can replicate this diagram, and talk about the purpose correctly. They were NOT trying to measure the velocity of light. They were NOT trying to detect the Aether. They were attempting to
measure the velocity of the earth through the Aether . Be very careful there. Besides this, quick explanations of the logic behind the experiment, should form the basis of most questions here.
Einstein published his
Special Theory of Relativity in 1905. In it, he suggested that ALL inertial frames of reference are equally valid. Further, he suggested that the speed of light (
) was an absolute constant, the same in all frames of reference. His reasoning was based on thought experimentation. Consider yourself on a train travelling very close to the speed of light. You bounce a light off the ground and to the roof. You see it go straight up and down, but another observer sees it differently. Something has to change...
The astounding consequences of this theory are that time, mass and length all become relative quantities. Simulteneity becomes a relative statement also. Mass dilation, time dilation, and length contraction can be described by the following quantities.
The important thing to note here is that mass and time quantities increase as the velocity of a reference frame increases. Length decreases. This is an easy way to check answers to common questions like this (one pops up in almost every HSC exam):
Example One: The distance between the cathode and screen in a cathode ray tube is 40cm. If an electron travels through the tube at
, what is the apparent distance from the cathode to the screen in the electron’s frame of reference?
Don't be confused about the wording, this is simply a sub and go. Be careful with SI units and substituting in the correct places!
These questions are usually muddled in interpretation.
Re read the question. Be sure you understand what it is asking, and substitute in the right place.
The other consequence of this theory is that mass and energy are equivalent, a very interesting idea. This of course links to the infamous equation,
. Interestingly, this equation is not asked as a mathematical question very often at all. Probably because it is one of the easiest formula in the course. Much more common is an explanation of the equivalence of mass and energy.
Accounting for this is simple. Consider an object travelling at close to the speed of light. If we apply a force, kinetic energy increases. But the velocity cannot exceed the speed of light. Thus, mass must increase. Therefore, we can deduce that mass and energy must be equivalent, since we increase the mass of an object by doing work on it.
The trickiest questions in a HSC exam often come from this topic, and usually concern Einstein. There are even questions which link to later topics on his political opinions! Tricky questions here, however, often concern Einstein's thought experiments. Be sure you can explain them, and draw simple diagrams like the one above, to make your meaning clear. Further, be prepared to discuss (quite loosely):
- The advantages of thought experiments, in terms of limitations on current technologies.
- The link between theory and proof
- The proof of special relativity. This has been observed in atomic clocks, and in the lifespan of small, sub-atomic particles.
Questions on relativity in the HSC are punishing; they ask a HSC student to describe and explain one of the most complex ideas in modern physics. My biggest tips; be VERY careful with your wording, leave no room for interpretation. For example, do NOT write: "Special relativity means that the speed of light is constant, which means that mass, time and length are relative."... No mention of reference frames, no explanation, it screams "I MEMORISED THIS SENTENCE." Don't do this.
Understand why these quantities are relative. Understand the role of reference frames. It is worth the time investment.
That is actually about all for relativity. Not much in terms of content; but very demanding subject area. Invest time to truly understand what is going on, and questions in these areas will become easy marks for you! Be sure to ask any questions you have below. It doesn't take long to
register for an account , and it is an awesome way to pick the brains of the students around you. They are your best resource.
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