One problem for humans living in outer space is that they are apparently weightless. One way around this problem is to design a space station that spins about its center at a constant rate. This creates 'artificial gravity' at the outside rim of the station. (a) If the diameter of the space station is 800 m, how many revolutions per minute are needed for the 'artificial gravity' acceleration to be 9.80 m/s2?

A science-fiction tale describes an artificial “planet” in the form of a band completely encircling a sun (Fig. 6–38). The inhabitants live on the inside surface (where it is always noon). Imagine that this sun is exactly like our own, that the distance to the band is the same as the Earth–Sun distance (to make the climate livable), and that the ring rotates quickly enough to produce an apparent gravity of g as on Earth. What will be the period of revolution (this planet’s year) in Earth days? <IMAGE>

A 3kg rock spins horizontally at the end of a 2m string at 90 RPM. Calculate its centripetal acceleration.