Spacecraft i, containing students taking a physics exam, approaches the earth with a speed of
0.660
c
(relative to the earth), while spacecraft ii, containing professors proctoring the exam, moves at
0.220
c
(relative to the earth) directly towards the students. the professors stop the exam after 40.0 min have passed on their clock.

a. for what time interval does the exam last as measured by the students?

b. for what time interval does the exam last as measured by an observer on earth?

In an old-fashioned amusement park ride, passengers stand inside a 3.0-m-tall, 5.0-m-diameter hollow steel cylinder with their backs against the wall. the cylinder begins to rotate about a vertical axis. then the floor on which the passengers are standing suddenly drops away! if all goes well, the passengers will “stick” to the wall and not slide. clothing has a static coefficient of friction against steel in the range 0.60 to 1.0 and a kinetic coefficient in the range 0.40 to 0.70. part a what is the minimum rotational frequency, in rpm, for which the ride is safe? express your answer using two significant figures. f f = rpm