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In the field of physics, circular motion is a term that defines an object’s movement in a rotating manner. In this motion, an object navigates around a circle, such as the planets moving around the Sun in each of their orbits. The object also moves around its center at an equal distance, but accelerates as it goes around in order to maintain a circular motion.
One important element that makes circular motion possible is an inward force; this is also called the centripetal force. According to Newton’s Law of Motion, an object generally requires a certain force acted upon it for the object to move, accelerate, and react. In circular motion, the absence of the centripetal force will make the object move only in a straight line because there is nothing that pulls the object inward. For example, a pebble tied to a string can be swung continuously because the string pulls the pebble toward the center. The pebble falls straight to the ground, however, when the string is cut.
A circular motion has two kinds: uniform and non-uniform. In uniform motion, the object travels along at a steady speed. This does not take into account the object’s velocity, which is the change of speed or direction, because a rotating object is always changing its direction.
A non-uniform circular motion, on the other hand, makes an object move in shifting speeds. The distance between the object and the center of the sphere also changes. The velocity of the object will increase if the distance is smaller, while a greater distance slows down the rotation of the object. For example, the pebble attached to the string will spin faster if the string is shorter, but will spin at a slower rate if the string is longer. This is because the force acting upon the object becomes either weaker or stronger when the distance is longer or shorter, respectively.
Everyday situations where uniform and non-uniform circular motions apply can be found in a carnival rides. A Ferris wheel travels in a uniform motion because the wheel moves at a constant speed and maintains the same distance from the center. The roller coaster, on the other hand, is a good example of a non-uniform motion, where the carts go up slower than when they are falling down. The size of the circles the carts go along also accounts for the change of velocity.
@Iluviaporos - Gravity does more than contribute to circular motion equations.
It is also the force that pulls the whole earth together just enough. If gravity was a stronger force, the universe might not be expanding the way it is, because it would have stuck together for longer after the initial Big Bang.
Kind of weird that the same circular motion might one day enable us to create gravity on space ships by setting them into a spin.
Like water stays in a bucket when you twirl it around, because it too wants to fly off in that direction, but can't because of the bucket, which itself is acting like the string in your original comparison.
It's funny to think that we still don't fully understand what gravity is, or whether or not it is a universal force, but if it suddenly disappeared, our planet would just shoot into space.
Because in the example given above, our planet is the pebble and gravity is the string that swings us around the sun. If gravity didn't exist, or was slightly off in either direction (stronger or weaker) we wouldn't be here.