Ch 26 Chapter 26 Relativity © 2006, B.J. Lieb Some figures electronically repr

Ch 26  Chapter 26 Relativity © 2006, B.J. Lieb Some figures electronically repr www.phwiki.com

Ch 26 Chapter 26 Relativity © 2006, B.J. Lieb Some figures electronically repr

Williams Tostrup, Gretchen, Food Editor has reference to this Academic Journal, PHwiki organized this Journal Ch 26 Chapter 26 Relativity © 2006, B.J. Lieb Some figures electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Giancoli, PHYSICS,6/E © 2004. Ch 26 Galilian-Newtonian Relativity Relativity deals with experiments observed from different reference frames. Example: Person drops coin from moving car In reference frame of car: coin is at rest in addition to falls straight down In “earth” reference frame, coin is moving with initial velocity in addition to follows projectile path. Ch 26 Inertial Reference Frames Inertial Reference Frame: one in which Newton’s First Law (Law of Inertia is valid.) A reference frame moving with constant velocity with respect to an inertial reference frame is an inertial frame. If the car is moving with constant velocity relative to the earth, it is an inertial reference frame. Accelerated or rotating reference frames are noninertial reference frames. The earth is approximately inertial.

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Ch 26 Relativity Principle “the basic laws of physics are the same in all inertial reference frames” This principle was understood by Galileo in addition to Newton. Space in addition to time are absolute– different inertial reference frames measure the same length, time etc. All inertial reference frames are equivalent– there is no preferred frame. Ch 26 Special Theory of Relativity Einstein asked the question “What would happen if I rode a light beam” Would see static electric in addition to magnetic fields with no underst in addition to able source. Underst in addition to ing electromagnetic radiation requires changing E in addition to B fields. Concluded that: no one could travel at speed of light. No one could be in frame where speed of light was anything other than c. No absolute reference frame Ch 26 Postulates of Special Theory of Relativity First: The laws of physics have the same as long as m in all inertial reference frames Second: Light propagates through empty space with a definite speed c independent of the source in addition to observer This means that an observer trying to catch a light beam in addition to moving at 0.9c will measure the speed of that light as c in addition to an observer on earth will also measure c. In order as long as this to be true, observers must differ on measurements of distance in addition to time The special theory of relativity deals with reference frames that move at constant velocity with respect to an inertial reference frame. The General Theory deals with accelerated reference frames in addition to is primarily a theory of gravity.

Ch 26 Relativistic Clock In the above clock, light is reflected back in addition to as long as th between two mirrors in addition to timer counts “ticks” This is an ideal clock because of special properties of light An observer at rest with respect to the clock concludes that the time as long as a tick is In order to study time dilation, we will places this clock in a spaceship moving past earth. t0 is called proper or rest time because clock is at rest in spaceship (note, we don’t call it “correct” time) Ch 26 Time dilation Spaceship moves by earth at speed v. (both observers agree that speed is v.) Observer on earth sees light move distance per tick. Observer on earth writes this equation as long as c Observer on earth sees spaceship moving Ch 26 Time dilation The as long as mulas on the previous slides can be combined to give Clocks moving relative to an observer are measured by that observer to run more slowly (as compared to clocks at rest). Clock is in spaceship so this measures the proper or rest time t0. It is often convenient to write v as fraction of c, thus v = 3.0×107 m/s is written v = 0.10 c. We call this effect time dilation because the time in the moving reference frame is always longer than the time in the proper reference frame

Ch 26 Time dilation factor Consider how this depends on v Ch 26 Length Contraction A spaceship passes Earth in addition to continues on to Neptune Earth in addition to spaceship observers disagree on time, in addition to they also disagree as to length L (distance to Neptune) Since earth is at rest, it measures the “proper” length L0. Both observers agree on the relative velocity v, so we can use the time dilation to derive length contraction equation Ch 26 Length Contraction The length of an object is measured to be shorter when it is moving relative to the observer than when it is at rest. This contraction is only in the direction of the velocity. The drawing shows the changed shape of a picture when a person moves by horizontally.

Ch 26 Example 26-1 (7) Suppose you decide to travel to a star that is 85 light-years away at a speed that tells you the distance is only 25 light-years. How many years would it take you to make the trip We determine the speed from the length contraction. The light-year is a unit of length. The rest or proper length is L0 = 85 ly in addition to the contracted length is L = 25 ly. which gives We then find the time from the speed in addition to distance: Ch 26 Example 26-2: A muon is an elementary particle with an average lifetime of 2.2 s. A muon is produced in the earth’s atmosphere with a velocity of 2.98 x108 m/s. What is the distance traveled by the muon be as long as e it decays as measured by observers in the muon’s rest frame We note that the muon’s reference frame is the proper frame as long as time in addition to the earth is the proper frame as long as distance. What is the lifetime of the muon as measured by observers in the earth reference frame Ch 26 Example 26-2 (continued): A muon is an elementary particle with an average lifetime of 2.20 s. A muon is produced in the earth’s atmosphere with a velocity of 2.98 x108 m/s. What is the distance traveled by the muon as measured in the earth reference frame Use the above results to calculate the velocity of the muon in each reference frame. Notice that observers in the two reference frames disagree as to distance in addition to time but agree as to velocity.

Ch 26 Four-Dimensional Space-Time Since observers in different reference frames often do not agree on time measurements as well as length measurements, time is treated as a coordinate in our coordinate system along with the three spatial coordinates X,Y in addition to Z. Thus we speak of a four-dimensional space time. Space Time The red line represents a light ray in addition to the blue line represents the “world-line” of an object through space-time. Ch 26 Mass Increase The mass of an object increases as the speed of the object increases: Mass increase is observed in particle accelerators m0 is the “rest mass”, the mass as measured in a reference frame at rest with respect to the object. This mass increase can be seen as the reason that objects can’t travel at the speed of light. Ch 26 Relativistic Kinetic Energy Einstein used the work-energy theorem to derive a new, relativistic equation as long as kinetic energy When v c, this equation can be approximated by KE = (1/2) m v2, so this equation can still be used as long as non-relativistic speeds. The equation as long as kinetic energy is the difference between two quantities which have units of energy

Ch 26 Total Energy This is the total energy of the object. It implies that there is energy in mass in addition to mass can be converted into energy in addition to energy can be converted into mass This is the rest energy of the object. We can thus write the total energy as Ch 26 Units of Mass-Energy Earlier in the course we defined the electronvolt (eV) as a unit of energy where 1 eV = 1.6 x 10-19J. We will also use keV (103) in addition to MeV (106) From E=mc2, we can solve as long as m = E / c2 in addition to thus MeV / c2 is a unit of mass. Example: electron mass is me = 0.511 MeV/c2 It is useful to use c2 = 931.5 MeV / u. Ch 26 Additional Relativistic Equations The Special Theory of Relativity is a revision of the laws of mechanics as long as objects with velocity close to the speed of light. The relativistic equations all can be approximated by the non-relativistic equations when v c. For problems where v 0.10 c, the relativistic correction is less than 1 percent in addition to the non-relativistic equations are used. Momentum: Energy-Momentum:

Ch 26 Example 26-3. An electron is accelerated through an electrical potential difference of 2.00×106 V. Calculate the mass energy, kinetic energy, total energy in addition to veloctiy of the electron. ( One eV is the energy gained by electron accelerated through 1.0 V. Ch 26 Example 28-4. Neutrons outside of the nucleus are unstable in addition to decay with a half-life of 10.4 mins into a proton, electron in addition to a neutrino. If a neutron decays at rest, calculate the energy released by this decay. This energy is shared by the proton, electron in addition to neutrino. Note: The neutrino mass is only a few eV/c2 so we will assume it is zero. Table 30-1 on page 838 contains the masses we need. Mass of electron = me = 0.00054858 u = 0.511 MeV/c2 Mass of proton = mp = 1.007276 u = 938.27 MeV/c2 Mass of neutron = mn = 1.008665 u = 939.57 MeV/c2 Ch 26 Alternative Solution Example 28-4. Neutrons outside of the nucleus are unstable in addition to decay with a half-life of 10.4 mins into a proton, electron in addition to a neutrino. If a neutron decays at rest, calculate the energy released by this decay. This energy is shared by the proton, electron in addition to neutrino. Solution in MeV/c2: me = 0.511 MeV/c2 mp = 938.27 MeV/c2 mn = 939.57 MeV/c2 (slight difference due to round-off)

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