Students know the path of a planet around the Sun is due to the gravitational attraction between the Sun and the planet.

Gravitational attraction is obviously a major force in keeping the planets and other solar system bodies orbiting the sun. However, if it were the only physical force in the interaction, then the massive sun would simply attract these bodies so much that they would crash into it.

We’re still here, and we know that Earth has not crashed into the sun. A different physical principle works in conjunction with gravity to keep planets and other solar system objects in their orbits. This is the principle of inertia, that a body moving in a particular direction will keep moving in a straight line in that direction unless it encounters another force. Without gravity, all solar system bodies would keep moving in straight lines away from the sun. Gravity and inertia combine so planets neither escape in a straight line nor crash into the sun. Instead, they remain in stable orbits around the sun.