Orbit

Why don’t planets fall into the Sun?

They are falling. They are also moving sideways. By the time a planet has fallen, the Sun is no longer where it was falling toward. Falling and missing, without end, is an orbit. There is no outward force holding them up.

A straight dashed path is what the planet would do with no gravity. A pull toward the Sun bends that path. The closed curve is the miss.

Both halves of the question are fair

The Sun really is pulling every planet toward itself, constantly. Nothing is holding them up, so falling in is a fair expectation. And a planet really is moving fast enough that, with no gravity, it would sail off in a straight line. Flying away is a fair expectation too.

Both are happening. They do not cancel like two people pulling a rope. Gravity only changes the direction of the motion. The planet falls, and the ground it was falling toward has moved.

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There is no outward force

Textbooks sometimes draw a centrifugal arrow. That arrow is a description in a spinning frame, not a second force out in space. In the inertial picture there is one force, inward, and a velocity, sideways. Newton’s first law does the rest.

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