Physics 7. Homework # 7

Due: 5/17

1)      What was the one peculiarity in Newton’s universal law of gravitation that led Einstein to seek a replacement for it?

a)      That it was valid only for non-inertial observers

b)      That it assumed instantaneous propagation of interactions

c)      That it required light to travel at infinite speeds

d)     That was inconsistent with Newton’s second law

2)      Becky and Tom are in outer space, each in her/his own spaceship and have identical clocks. Both their reference frames are inertial and they are moving with respect to one another at a large speed. When Becky compares Tom’s clock to hers, she finds it is slow. When Tom compares his clock to Becky’s, will he find it is

a)      Fast, because Becky find his clock slow

b)      Slow, because in his reference frame Becky’s clock is moving and it will slow down because of relativistic effects

c)      Synchronized, because time flows in the same way for all inertial observers

d)     Stopped

3)      The distance from Earth to Alpha Centauri (a near-by star) is about 5.4 light years (one light year is the distance light covers in one year, or about 160 billion km). Yuri the astronaut decides to take a trip to this star and he will travel very fast. When he arrives will he find that

a)      In his reference frame he covered a distance smaller than 5.4 light years because of relativistic length contraction

b)      In his reference frame the distance he traveled was also 5.4 light years

c)      In his reference frame he covered more than 5.4 light years

d)     That the trip takes a very long time, much more than 5.4 years, even if moving close to the speed of light with respect to us.

4)      What physical principle makes atomic bombs possible?

a)      The absolute nature of the speed of light

b)      The equivalence of mass and energy

c)      The absence of the ether

d)     Maxwell’s theory of electromagnetism

5)      A laser is placed on top of a hill (here on Earth) and turned on. Assume that the beam encounters no obstacles and that there are no losses when it travels through the air, and that the beam does not spread. Then,

a)      The gravitational force pulls the beam towards the center of the Earth

b)      The gravitational force pushes the beam away from Earth

c)      The gravitational force has no effect, for otherwise the beam would move around the Earth in an ellipse

d)     The gravitational force affects the beam as long as it is inside the atmosphere, once the beam leaves the atmosphere behind the gravitational effects stop abruptly.