Physics Education Department Sebelas Maret University Surakarta

1. Fundamental Physics 3(Modern Physics) Physics Education Department Sebelas Maret University Surakarta Physics Education Department - UNS

2. Which reference frame? Suppose you are on the bus to tawangmangu driving at 80 km/hour, and throw a ball forwards at 40 km/hour. From your seat on the bus, the speed of ball is the same as in this classroom. To someone on the side of the road, your 40 km/hour throw has become a 120 km/hour fastball. Who is correct? But what exactly is the absolute velocity of the ball? Physics Education Department - UNS

3. Earth spins (rotates) on its axis • One rotation in (24 hrs)(60 min/hr)(60 sec/min)=86400 sec • Point on surface moves 2πRE in one rotation. • Surface velocity = 2πRE /T=2π(6.4x106m)/86400 sec = 465 m/s • Earth revolves around sun • One revolution in (365 days)(86400 sec/day)=3.15x107 sec • Earth velocity = 2π(1.5x1011 m)/ 3.15x107 sec=3x104m/s Physics Education Department - UNS

4. Quick Quiz 39.1 Which observer in the right Figure 39.1 sees the ball’s correct path? the observer in the truck (b) the observer on the ground (c) both observers. Physics Education Department - UNS

5. Galilean Relativity – Example • A passenger in an airplane throws a ball straight up • It appears to move in a vertical path • The law of gravity and equations of motion under uniform acceleration are obeyed Physics Education Department - UNS

6. Galilean Relativity – Example, cont • There is a stationary observer on the ground • Views the path of the ball thrown to be a parabola • The ball has a velocity to the right equal to the velocity of the plane Physics Education Department - UNS

7. Galilean Relativity – Example, conclusion • The two observers disagree on the shape of the ball’s path • Both agree that the motion obeys the law of gravity and Newton’s laws of motion • Both agree on how long the ball was in the air • Conclusion: There is no preferred frame of reference for describing the laws of mechanics Physics Education Department - UNS

8. Ayem Tentrem What about electromagnetism? • Maxwell equations say that • Light moves at constant speed c=3x108 m/sec in vacuum • Seems at odds with Galilean relativity: • Tentrem would expect to see light pulse propagate at c+v • But Maxwell says it should propagate at c, if physics is same in all inertial reference frames. • If it is different for Ayem and Tentrem, then in which frame is it c? Physics Education Department - UNS

9. The Ether • To resolve this, 19th century researchers postulated existence of medium in which light propagates (called ether), rather than vacuum. • i.e. similar to gas in which sound waves propagate or water in which water waves propagate. • Then Maxwell’s equations hold in the ether Physics Education Department - UNS

10. The Michelson-Morley experiment • If the earth moves through a medium in which light moves at speed c, along the direction of the earth’s motion, light should appear from earth to move more slowly. Physics Education Department - UNS

11. M1 Angin eter v Lengan 1 M0 Lengan 2 Sumbercahaya M2 teleskop • Ether wind would change average speed of light on the different paths. • Waves will interfere when they recombine. • Both can not be observed! Physics Education Department - UNS