Solving for Diameter and Mass of a New Kuiper Object with a Satellite

[astronomystudent]

PROBLEM
If the size of a new Kuiper object is 0.0150 arc sec in angular size as seen from a distance of 42.00 AU, what is the true diameter? If it has a satellite with a period of 5.50 hours at a semimajor axis of 12500.0 km, what is the mass? What is the resulting density? What do you it is composed of?

What I know:
I know that I use V=4/3(pi)r^3 to get the density and previous equations to solve for mass. I don't know how I am supposed to apply the 0.0150 arc seconds or distance to solve for the diameter. What is the equation for that?

 

[Astronuc]

What is the relationship of the some length of segment on the circumference of a circle and the angle subtended by that segment?

 

[astronomystudent]

I have no idea what you mean when you say "angle subtended..."

 

[BerryBoy]

Is the concept of angular size the trouble here. Here is a diagram, with this you should know what to do...
http://www.berrys.plus.com/hh5.gif

So the furthur away the object is, the smaller the angular size will become.

Hope this helps...
Sam