The radius is 5000 meters.
The circumference is 2i*r = 10,000 pi = 2pi radians; so
1 radian = 5000 meters.
We are circling at 100 m/s so we'll travel 5000 meters = 1 radian in 50 seconds.
If we travel 1 radian in 50 seconds then we'll go 1/50 = .02 radians in 1 second.
So the answer is .02 radians per second.
In general, here is how to stop your headaches and learn to know and love radians.. The unit circle has radius 2pi and everything flows from that. 1/4 of the way around the circle is pi/2; halfway round is pi; 3/4 of the way around is 3pi/2; all the way round is 2pi, which is the same as zero, where we started.
Forget you ever heard of degrees. They're a fading memory from your past. From now on, think only in radians. When you get confused remember that the circumference of the unit circle is 2pi radians and all other circles are proportional to that. And if t is a fraction between zero and one, going t of the way around the circle is 2pi*t radians.
If t = 1/2 then you went halfway around the circle, to pi. If t = 1/3 then you went 1/3 of the way around the circle to 2pi/3. After a while you won't even care about how many degrees that is.
That's the mindset you want to strive for. Think only in radians from now on.