Physics 7 (Winter 08) – Homework #6

Due:2/19

1. In a forest in Brazil a tree falls just as a guru in India levitates. The fallen tree blocks a stream that causes a small swamp to form while the guru comes down to earth and proceeds to drink a (very large) cup of tea. In what sequence would these events be seen by an observer moving at very high speed?

2. Imagine that we place a large number of negative charges in a straight line. An observer is moving at constant speed in a direction parallel to the lie of charges, then
1. The observer will measure neither electric nor magnetic effects
2. In the observer’s frame of reference the charges are moving and he will see a magnetic field since moving charges generate magnetic effects
3. In the observer’s reference frame the charges are moving but there will be no magnetic effects
4. The observer will see that the charges change sign or not depending on his direction of motion
5. The observer will see precisely the same electric and magnetic effects as when at rest with respect to the charges

3. John is in a spaceship and watches Mary in another spaceship moving at a very large speed with respect to him. Then
1. John sees Mary aging faster than he does and Mary sees herself aging faster than John does
2. John sees Mary aging slower than he does and Mary sees herself aging slower than John does
3. John sees Mary aging slower than he does and Mary sees herself aging faster than John does
4. John sees Mary aging faster than he does and Mary sees herself aging slower than John does
5. John and Mary see each other aging at the same rate

4. A plastic rocket is precisely 1 m long; in it there is a steel rod that just fits inside the rocket lengthwise. When the rocket travels at a constant speed close to c in outer space,
1. The metal rod suffers length contraction but to a lesser degree than the rocket because steel is more rigid than plastic, so that the rod will pierce the rocket
2. The metal rod does not suffer length contraction at all but the rocket does, so that the rod will pierce the rocket
3. Both the rocket at the rod suffer length contraction, but the effect is larger on the rod which will then float loosely inside the rocket
4. The rod will be “pushed” towards the back of the rocket since it is traveling at constant speed
5. Both rod and rocket suffer precisely the same degree of length contraction so that the rod will continue to just fit inside.

5. In the news today: a meteorite entered the atmosphere and hit Mr. A. Arnold breaking his leg. Mr. O, an observer moving very fast with respect to the Earth, will find that
1. Mr. A’s leg breaks before the meteorite enters the atmosphere
2. Mr. A’s leg breaks before or after the meteorite enters the atmosphere, depending on the direction of motion of Mr. O.
3. That Mr. A’s leg breaks after the meteorite enters the atmosphere, no matter how fast or in what direction Mr. O is traveling
4. Mr. A’s leg breaks just as the meteorite enters the atmosphere
5. Mr. A’s leg breaks before or at the same time as the meteorite enters the atmosphere, depending on the direction of motion of Mr. O.

6. If Fred weighs precisely 80kg when he is moving with respect to the scale then, when he is at rest with respect to the scales, he will
1. Still weigh precisely 80kg
2. Always weigh slightly less than 80 kg
3. Always weigh slightly more than 80kg
4. Weigh slightly less than 80 kg only if he was moving at speed c
5. Weigh slightly more or less than 80 kg depending on whether he was moving eastward or westward.

7. Nuclear weapons and reactors work by
1. Burning coal
2. Using reactions where a small amount of subatomic mass is converted into energy
3. Using reaction where a bit of energy is converted into mass
4. Accelerating atomic nuclei until they reach speeds close to c
5. Slowing atomic nuclei that naturally move at speeds close to c

8. Subatomic particles are often unstable and decay after a short time, which means they spontaneously break apart after that time. Imagine that Alfred in his laboratory here on Earth measures that, at rest, one type of subatomic particle takes 1 second to decay. Imagine also that Winifred (in her laboratory, also on Earth) finds that the decay of the same type of particle takes it 2 seconds when it is moving at a very fast speed. Then
1. One must discard Winifred’s results because time of decay cannot be measured accurately for moving particles
2. One must discard Alfred’s results because his particle was not really at rest since the Earth is rotating
3. Both can be right since the particle behaves as a clock and moving clocks slow down
4. If correct, this result would contradict the special theory of relativity which predicts that a moving particle would decay in a shorter time than when at rest
5. One of the measurements must be wrong, but one can only find out which by doing experiments in outer space.

9. A rocket which is precisely 100 m long goes through a spherical asteroid which has a diameter also of 100 m (and a hole for the rocket to go thorough). According to an observer on the asteroid the rocket is length-contracted and so will fit inside asteroid for a short time; according to the astronaut in the rocket the asteroid is length-contracted so the rocket will not fit inside the asteroid at any time. Who is right?

1. Both: whether the rocket fits or not inside is a statement that depend on the frame of reference
2. The observer on the asteroid
3. The astronaut
4. Neither

10. When Superman runs very fast with respect to us he looks
1. Shorter and stouter than when at rest
2. Shorter and thinner than when at rest
3. Taller and stouter than when at rest
4. The same height as when at rest but thinner
5. The same height as when at rest but stouter