Uniform circular motion is when an object moves at a constant speed around a perfect circle. Perfect examples are very rare in the physical world, but approximations include a rider on a carousel, or a pendulum moving in a circle parallel to the ground. While planets and moons are often cited as examples of uniform circular motion, most planetary orbits are elliptical and fulfill neither the requirement that the speed always stay the same nor that motion must be circular.
The key aspect of uniform circular motion is that the direction of the object changes while the rate at which the object is moving relative to the environment remains the same. In physics terminology, the rate at which an object moves through space is known as the “speed,” while the term “velocity” describes both the rate of movement and the direction of movement at the same time. For uniform circular motion, then, velocity is changing while speed is constant.
In non-uniform circular motion, by contrast, while the object still moves in a circular path, the speed is not constant. For example, a car that slows down going into a turn and then speeds up coming out of it is moving in part of a circular path, but because the speed of the car changes, it is not uniform circular motion.
Uniform circular motion is caused by something called centripetal force. A force is a push or pull; centripetal force is the particular force that makes an object continue to move around in a circle. It keeps the object from going in a straight line by pulling it towards the center of the circle, changing the direction of the object and therefore its velocity. Centripetal force doesn’t push or pull along in the direction that the object is moving, however, so the speed remains unchanged. In uniform circular motion, this centripetal force always has the same magnitude, which is what keeps the object moving in a circle as opposed to a different shape.
To examine a sample case, there is a carnival ride in which riders are strapped to the inside of a large spinning cylinder. When the cylinder is moving at a set speed, those riders are in uniform circular motion. The centripetal force pushing them inwards can be felt in the pressure from the walls of the cylinder against them. If the cylinder suddenly disappeared, the riders would fly off in straight lines. Luckily, the push of the cylinder forces their motion to instead be circular.