PHYS 221 Recitation Kevin Ralphs Week 12

Overview HW Questions Chapter 27: Relativity – History of Special Relativity (SR) – Postulates of SR – Time Dilation – Simultaneity – Length Contraction – Velocity Addition – Mass, Energy and Momentum

History of SR Historically, the nature of light was a source of much debate – Did it travel instantaneously? – Is it a wave or a particle? If a wave, how do we explain reflection and aberration of starlight? If a particle, how do we explain refraction? Humans tend to explain new experiences through old ones and multiple theories and interpretations were given centering around the idea of an aether that mediated light’s transmission

History of SR In trying to understand this aether, most of the results of SR and its mathematics were already developed prior to Einstein’s proposal Einstein’s contribution was to explain all these results from two postulates with no reference to an aether

Postulates of SR 1.All the laws of physics are the same in all inertial frames 2.The speed of light in a vacuum is a constant, independent of the motion of the light source and all observers An inertial frame is one that is not undergoing any kind of acceleration

Time Dilation

Simultaneity In general, observers in different inertial frames will not agree on the order of events However, all observers will agree on the order of events that occur at the same position or are causally connected

Length Contraction

Velocity Addition

Relativistic Momentum

Mass and Energy

Quiz Question Consider the motion of a muon as it moves through he Earth’s atmosphere. A particular muon is created 15,000 m above the Earth’s surface and just reaches the ground before it decays (2.197e-6 s). What is the speed of the muon? a)6.818e9 m/s b)2.995e8 m/s c)2.997e8 m/s d)6.792e9 m/s e)2.986e8 m/s

Quiz Question In studies of elementary particles such as electrons, physicists use machines called “accelerators” in which the particles move at very high speeds. Suppose an electron in one of these machines has a speed of c and it travels along a straight line for a distance of 500 m as measured by physicists doing an experiment. How long is the accelerator as measured by a person riding along with the electron? a)22.3 m b)11.1 km c)35.3 km d)7.07 m

Quiz Question

Quiz Questions