Basic Relativity - Turning Points - A2 Physics


Topic 5 – Relativity

In the exam you are expected to:

  • Describe the Principle of the Michelson-Morley 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 90o 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 11.5o 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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