IB Physics – Relativity Relativity Lesson 1 1.Galilean Transformations (one frame moving relative to another) Michelson Morley experiment– ether. 2.Speed of light constant 3.Simultaneity. 4.Inertial Observers. Frames of Reference. 5.2 Postulates of Special relativity – c is constant and all laws are the same for all inertial observers. 6.Light Clocks and how they work; Pythagorean treatment Click for good background site

IB Physics – Relativity Inertial frames of reference An inertial frame of reference moves at a constant velocity. It is not accelerated Lightning strikes at x = 60 m and t = 3 s x = 0x = 10x = 20x = 30x = 40x = 50x = 60x = 70 y x Tsokos, 2005, p553 An Event

IB Physics – Relativity Absolute Rest There is no such thing as absolute rest. In an inertial frame of reference there are no experiments you can do which prove you are moving. (Unless you look outside!) Newton’s Laws consider zero and constant velocity to be identical

IB Physics – Relativity Galilean Transformations Relatively easy? Consider a stationary frame and a frame moving at velocity v in the x direction.

IB Physics – Relativity Use of the Galilean Transforms Tsokos, 2005, p553 y x velocity, v with respect to ground origins coincide when clocks at both origins show zero vt y x the train has moved away; when the clocks show 3 s, lightning strikes at x = 60 m Calculate and if v = 15ms -1

IB Physics – Relativity Calculating relative velocity A ball rolls on the floor of the train at 2ms -1 (with respect to the floor). The train moves with respect to the ground at (a) 12 ms -1 to the right and (b) 12 ms -1 to the left. Find the velocity of the ball relative to the ground. x An object rolling on the floor of the ‘moving’ frame appears to move faster as far as the ground observer is concerned y vt v v Tsokos, 2005, p553

IB Physics – Relativity Nature of Light Oscillating magnetic and electric fields at right angles to each other. Maxwell’s equations predict that the speed of light is independent of the velocity of the light source.

IB Physics – Relativity Conflict with Galilean relativity. Observer in the train measures the speed of light = C Observer on the ground measures the speed of light as C + V C V

IB Physics – Relativity The Michelson-Morley Experiment Read in Kirk p149 Explain how the null result from this experiment shows that the speed of light is unaffected by the motion of the Earth. Link to Michelson-Morley powerpoint Link to Michelson-Morley explanation

IB Physics – Relativity Modified equations

IB Physics – Relativity Postulates of Special Relativity The laws of physics are the same for all inertial observers. The speed of light in a vacuum is the same constant for all inertial observers Animations showing time dilation and length contraction.

IB Physics – Relativity Postulates of special relativity video

IB Physics – Relativity The laws of Physics are the same

IB Physics – Relativity Electromagnetism and relativity

IB Physics – Relativity Speed of light is constant video

IB Physics – Relativity Maxwells equations lead to constant c

IB Physics – Relativity Simultaneity Two events which happen together are simultaneous. If two events are simultaneous in one frame of reference they may not be in another. See train example in Kirk p173 If the two simultaneous events occur at the same point in space then they are simultaneous for all observers e.g. two flies landing at the same point on a table in a train. This is simultaneous for all observers. Compare with the two files landing simultaneously at each end of the table (as seen by the observer on the train)

Example IB Physics – Relativity

Answer (learn) IB Physics – Relativity

Simultaneity video

IB Physics – Relativity B A Light clocks B A L B A B A S x = 0 x = v  t v  t

IB Physics – Relativity Time dilation video