Uniform circular motion is when an object moves at a constant speed around a circle, with centripetal force keeping it in motion. Nonuniform circular motion is when the velocity changes. Planetary orbits are often not uniform circular motion.
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 satisfy neither the requirement that the velocity always remain the same nor that the motion must be circular.
The key aspect of uniform circular motion is that the direction of the object changes while the speed at which the object moves relative to its environment remains the same. In physics terminology, the speed with which an object moves through space is known as “velocity”, while the term “velocity” describes both the speed of movement and the direction of movement simultaneously. For uniform circular motion, therefore, the speed changes while the speed is constant.
In nonuniform circular motion, by contrast, while the object is still moving in a circular path, the velocity is not constant. For example, a car that slows down as it enters a curve and then speeds up as it exits is moving in part of a circular path, but as the car’s speed changes, it’s not a smooth circular motion.
The uniform circular motion is caused by something called a centripetal force. A force is a push or pull; centripetal force is the particular force that causes an object to keep moving in a circle. It prevents 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 speed. However, centripetal force does not push or pull in the direction the object is moving, so the velocity remains the same. In uniform circular motion, this centripetal force is always of the same magnitude, which is what keeps the object moving in a circle versus a different shape.
To examine a sample case, there is a carousel where the riders are strapped inside a large rotating cylinder. When the cylinder is moving at a set speed, those riders are in a uniform circular motion. The centripetal force pushing them inward can be felt in the pressure exerted by the cylinder walls against them. If the cylinder suddenly disappeared, the pilots would take off in a straight line. Fortunately, the thrust of the cylinder forces their movement to be circular instead.
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