Physics 7. Homework # 8

Due: 5/24

1)      What is the equivalence principle? What does it imply when considering the motion of objects under the influence of gravity?

a)      The equivalence principle states that all inertial observers are equivalent, and it implies that gravity is absent in all inertial frames

b)      The equivalence principle states that inertial and gravitational masses are identical, it implies that all objects experience the same acceleration under gravity

c)      The equivalence principle states that gravitational and inertial masses are identical  only in inertial reference frames; and it implies that gravitational acceleration is proportional to the square of the mass

d)     The equivalence principle states that speed of light is absolute, and it implies that mass and energy are equivalent

2)      Alma and Omar are twins born on the surface of a very massive planet. When they were 25 years old Alma went to live to a space station in outer space far from any heavy celestial body. When Alma turned 50 years old Omar came to visit her, when he arrived

a)      He was also 50 years old

b)      He was younger than Alma because he lived in a region where gravity was strong and his biological clock slowed down accordingly

c)      He was older than Alma because of time dilation

d)     He is shortened permanently because of relativistic length contraction

3)      Why a black hole is called that way because,

a)      Its escape velocity is larger than c,  so light cannot escape its vicinity

b)      It converts all its energy into matter

c)      It emits only X-rays, so it looks black to our eyes

d)     It emits only microwave radiation, so it looks black to our eyes

4)      When we see astronauts inside the space shuttle they appear weightless. Is there a force acting on the?

a)      No, because there is no gravity in space

b)      Yes their weightlessness is an optical illusion

c)      Yes, but it is countered by the sun’s gravitational attraction

d)     Yes, the Earth’s gravity acts on them (that is why they keep in orbit and not move at constant speed in a straight line), however, astronauts and shuttle are accelerated in precisely the same way so with respect to the shuttle the astronauts do not accelerate.