Circular motion

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Circular motion is where a particle moves in a circle. It occurs when the acceleration of the particle is constant in magnitude and always perpendicular to the velocity of the particle. As such, the speed of the particle remains constant as no work is done on the particle (the force acting on the particle is perpendicular to its displacement). The acceleration, of a particle undergoing circular motion may be describe as


is the angular frequency, defined as over the period of oscillation,
is the radius of the orbit

Since , this may also be written as


Kepler's third law of planetary motion states that the square of the time period of a planet is proportional to the cube of the orbital distance. This can be seen as the earth undergoes circular motion around the sun. The force, , acting on the earth is just gravity so


is Newton's gravitational constant
is the mass of the sun
is the mass of the earth
is the distance between the earth and the sun

From Newton's second law, this is can be related to the acceleration as:

Since the angular speed, , is equal to where is the time period, we can rewrite the previous equation as

This is Kepler's third law.

In reality, the orbits of the planets are not circular, instead they are elliptical. This derivation also assumes the mass of the earth is small compared to that of the sun. This is reasonable in this case but not for two stars that orbit as part of a binary system. When this more general case is accounted for, Kepler's third law becomes

Where is the semi-major axis of the ellipse.