Basic Relativity  Turning Points  A2 Physics
 Created by: Stephanie Clarke
 Created on: 130411 19:14
Topic 5 – Relativity
In the exam you are expected to:

Describe the Principle of the MichelsonMorley interferometer;

Outline the experiment as a means of detecting absolute motion;

Describe significance of the failure to detect absolute motion;

Describe the invariance of the speed of light;
Basic Relativity
When we measure movement, we do so against a fixed reference point. A car travelling at 30 m/s is moving at 30 m/s relative to the road. Suppose we have two cars, A travelling at 30 m/s and B travelling at 20 m/s.

Relative to B, car A is travelling 10 m/s faster, i.e. +10 m/s;

Relative to A, car B is travelling 10 m/s slower, i.e. –10 m/s.
We can use any of these frames of reference:

The road;

Car A;

Car B.
Another example is an aeroplane flying at 90^{o} to the wind:
The plane is heading due North at 75 m/s and the wind is blowing from West to East at 15 m/s. We can easily work out the resultant velocity to be 76.5 m/s.
There can be three frames of reference on the ground:

The speed is 75 m/s heading due North;

Or 15 m/s due East

Or a resultant velocity of 76.5 m/s at 11.5^{o} east of north.
The question that bothered physicists was whether there was an absolute fixed point relative to which all speeds could be measured.
A Boat Race
Both boats, X and Y have a speed of 5 m/s. Boat X has to cross the river from A to B and back to A, while Boat Y has to travel from A to C and back again.
Inertial Frame of Reference
An inertial frame of reference is one in which Newton's first law is valid. If you are in a train travelling at constant speed, all objects behave as if they were stationary in the stationary train. The train is travelling at 60 m/s, the passengers and their luggage are all travelling at 60 m/s.
Suppose now that you are in an aeroplane. Against all airline regulations, there is a drinks trolley free (not secured) in the central aisle. The aeroplane accelerates…
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