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## Physics Tutorials

### Why does not the moon fall?

This discussion is an attempt to help someone who asks “Why does not the moon fall?"

Well, it does. It has an acceleration vector that points toward us. As we look up at the moon, it has a velocity vector that is perpendicular to our line of sight. Newton's First Law says that an object in motion continues in motion, in a straight line, unless acted on by an unbalanced force. If the moon were not falling towards us, it would continue in a straight line along that path perpendicular to our line of sight and leave the earth behind. A stone in the tread of a tire goes in a straight line tangential to the circle it was following when it slips out of the tread. The moon’s gravitational attraction toward the earth is the force on the moon that Newton's 1st Law requires to explain why it does not continue going straight. So, because the moon is also accelerating towards us, its path is bent. But not so much that it crashes into the earth. The path is bent into a circle by the fact that it is accelerating towards us.

Another way of describing what is going on is to say that it does fall toward the earth, but because the earth is a ball, it misses. And continues missing. From the moon, the earth is a ball in the sky. True, the Earth from the moon is a bigger ball than our view of the moon but still a ball. If the earth were a flat surface, yes, it would continue bending its path so that it would crash on the earth. But the earth is not flat, it's a ball. And the moon keeps missing the earth.

copyright: Steve Johnson, sojphysics@gmail.com

Steve Johnson is an electrical engineer. He earned a B. Sc. in Physics, and MS Electrical Engineering. Steve welcomes your questions.

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