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PHYSICS
Asked by Sarah

What forces keep a planet in orbit and how does each one work on the planet?

There is only one force which keeps a planet in orbit, and that is gravity. Gravity pulls planets towards the sun, and also the sun towards the planets. However, since the sun has much more mass than any individual planet, the planets are each pulled (accelerated) towards the sun to a greater degree than the sun is to a planet. The equation which describes the acceleration of a planet towards the sun is: a=GM/r^2. a is the acceleration of the planet towards the sun, G is a gravitational constant, M is the mass of the sun, and r is the distance between the planet and the sun. Now the reason why every single planet doesn't hurtle straight into the sun is that they're also moving very fast. Specifically they all have a sufficiently high tangential velocity, this is velocity in the direction perpendicular to the line which connects a planet to its sun. This speed means that while the planet is being pulled towards its sun, the surface of the sun also curves away from the planet. If the tangential speed is equal to a quantity called the orbital speed then you get a perfectly circular orbit, and if its higher than that you get an elliptical one, as most planets have. I would look Newton's Cannonball on wikipedia for some fantastic gifs representing this effect. Now, you might have been taught about something called the centrifugal force. This isn't a proper physical force, rather an 'inertial' or pseudo force. This is a force which appears in the equations when you do them from the perspective of the planet itself.

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