In uniform circular motion, which quantity stays constant?

In uniform circular motion, the net work done by the centripetal force over one full revolution is zero.

A stone tied to a 0.5 m string is whirled in a horizontal circle at 10 rev/s. Its centripetal acceleration is:

Rain falls vertically at 10 m/s. A person walks north at 10 m/s. The umbrella should be tilted at what angle with the vertical?

Two trains approach each other on parallel tracks at 40 km/h and 60 km/h. Their relative speed is:

Two cars travel in the same direction at 60 km/h and 40 km/h. The velocity of the first car relative to the second is: