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What is 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 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.

Scientist with beakers
Scientist with beakers

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.

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Discussion Comments


@Mammmood - As a teenager I loved physics. But one of the most interesting lessons was when my professor got to talking about perpetual motion machines.

These were devices that, once set in motion, would never stop and need no additional energy source besides the initial force that set them in motion. These devices defy the laws of physics, but I remember letting my mind run wild with ideas of how to build such a machine, using a flywheel and magnets.

I sketched it out on paper and it looked like a Rube Goldberg invention. However, at some point, I could see the system would wind down and lose energy because of friction.

Perpetual motion is indeed impossible, but if you do a search for people who have vainly tried to build such contraptions, you’ll all find that many of them try to create uniform perpetual motion.


It’s funny that the article cites a carnival ride as an example. It’s that time of the year for us now, when we are having our annual state fair.

I don’t go on any of the roller coaster rides (I’m too old) but I do get into this G-force space ride. We step inside this circular room on a platform and get strapped against the wall.

Then the thing just spins around and the G-forces pin us to the wall. It’s a wild feeling as the ride moves in a set rotational motion at a constant speed. I can feel the flesh on my face begin to peel back against these forces.

At some point we are no longer standing on the floor, but are virtually airborne as the centripetal forces push us against the wall. This must be a small taste of what Air Force flight training is like.

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      Scientist with beakers