Physics 7. Homework # 3

Due: 4/19

1)      According to Galileo, would a Martian astronomer looking at the Earth find that our planet

a)      Is stationary

b)      Moves around Mars

c)      Exhibits phases like Venus

d)     Does not exhibit phases like Venus

2)      Two spaceships, labeled L and R, pass each other. An astronaut in L claims that R moves to the right at 1,000 km/s,

a)      Then R will claim that L moves left at 1,000 km/s, and they are both right since only relative motion is significant

b)      Then R will find that L is stationary

c)      Then R will claim that L moves left at 1,000 km/s, and they are both wrong since one can only measure velocity with respect to a stationary object like the Earth

d)     The whole statement is inconsistent since in space there is no notion of velocity

3)      What is an inertial reference frame?

a)      A reference frame glued to the Earth

b)      A reference frame glued to the center of the universe, which is in absolute rest

c)      A reference frame where free bodies move at constant speed in a straight line

d)     A reference frame where free bodies are uniformly accelerated

4)      Imagine that two crates are on the surface of an icy lake; one crate is twice as heavy as the other. Imagine now that you kick both crates, one after the other, with the same force, then

a)      The heavier crate will move slower, this is due to Newton’s second law, F=ma,  for the same F the larger m  is the smaller the acceleration

b)      They will move at the same speed since they are acted on by the same force

c)      The heavier crate does not move at all, but you will move backwards

d)     The crates will move in opposite directions.