# A few quick questions

1. Oct 8, 2004

### Purduenub03

First off, i'm new to the board and want to say hi to everyone! I hope i can give and take from this site, i should be helpful in the chem forums, but physics is not my best. I have 2 quick questions if you could help me that'd be great.

A space station is shaped like a ring and rotates to simulate gravity. If the radius of the space station is 120 m, at what frequency must it rotate so that it simulates Earth's gravity? [Hint: The apparent weight of the astronauts must be the same as their weight on Earth.] - in rev/s

I did the following and got it wrong
9.8 = V^2/120, found v = 34.29m/s
34.29 / 2*pi*120 = .045 rev / s

Where have i gone wrong?/

A coin is placed on a record that is rotating at 33.3 rpm. If the coefficient of static friction between the coin and the record is 0.3, how far from the center of the record can the coin be placed without having it slip off?

I'm plain lost on this one.

2. Oct 8, 2004

### HallsofIvy

Unless it's a roundoff problem, I don't see anything wrong. I did it a different way and got 0.0455 rev/ s.

[/quote]A coin is placed on a record that is rotating at 33.3 rpm. If the coefficient of static friction between the coin and the record is 0.3, how far from the center of the record can the coin be placed without having it slip off?

I'm plain lost on this one.[/QUOTE]

Let r be the distance from the center of the record to the coin. Then the acceleration of the coin is r(66.6&pi;)2 so the force necessary to keep it rotating without slipping is rm(66.6&pi;)2 (m is the mass of the coin). The maximum force the record can exert is 0.3 times the weight of the coin: 0.3(9.81)m= 2.943m. Set those equal and solve for r.