Wednesday, April 16, 2008

Handed back time dilation quiz. Asked students who did not get it correct to redo it on back and hand in.

Introduced Length Contraction with Thought Experiments. Came up with equation for length contraction. Showed by proof by contradiction that the length is contracted only in the direction parallel to the direction of motion, NOT in a direction perpendicular to the motion.

Redid the quiz first using time dilation and then using length contraction to calculate what the astronaut's "odometer" would read. The results were consistent as they must be.

Introduced idea of increase in mass with speed along with the mass equation.
Used examples of bouncer with roll of quarters, Andy and Bill trying to punch each other, and Bonnie and her sister.

Like other waves, light has energy, but unlike other waves, light also has momentum (comet tails always point away from the Sun). If light has energy, it also has mass. It has mass because it has energy.

The total energy of an object is, from Einstein's equation, E = mc^2 = mo c^2 + KE
where mo is the rest mass. You cannot use KE = 1/2 m v^2 when dealing with relativistic speeds.

Assigned problems 8-13 for homework.

Test next Tues/Wed