Isaac Newton is a household name for his invention of calculus, laws of motion, and realization that gravity exists in the heavens (contrary to Aristotle’s beliefs!). College students that have studied introductory physics or math may have been exposed to Newton’s three laws of motion, but how exactly are they relevant in the context of astronomy? Let’s restate them and provide some astronomical context:
Newton’s First Law: an object moves at a constant velocity (or stays at rest) unless acted upon by an outside force. This law explains why the spaceships we launch do not need fuel after they are in space; without air resistance or friction in space, ships can simply move at a constant velocity without a need for thrusters (hint: a net force!).
Newton’s Second Law: mass multiplied by acceleration equals force. Large planets can exert a greater gravitational force on, say a comet, than a smaller planet would—mass is a direct factor in the determination of force (stay tuned for a future post on the Universal Law of Gravitation).
Newton’s Third Law: each force has an equal and opposite reaction force. Celestial objects, or any objects for that matter, exert forces on each other. You heard that right—that same gravitational force that keeps you grounded to Earth is also being exerted on Earth by you!
While this brief blog post is not an exhaustive astronomical explanation of Newton’s laws, I do hope it provided a little more context than simply F = m • a.