Stuff keeps doing what it is doing unless something pushes it.
Here’s a good question: When I throw a ball, why does it keep going? We know why it *started* going - I pushed it into the air - but why does it *keep* going once it’s left my hand? Students often give one possible solution: “It keeps going because of the push you gave it when you threw it.”
I often ask if I’m still pushing it while it flies through the air. They agree that, no I’m not, so *why* does keep moving? This is a fundamental question, and one that didn’t escape Isaac Newton. He observed that objects tend to keep moving how they were (or weren’t) unless we interfere with them. That is, they’ll stay at rest or moving at a constant speed in a straight line unless we push them in a certain way.
In physics-speak, we say The object will not accelerate (turn, speed up, or slow down) as long as there is no net (unbalanced) force acting on it. No force is required to maintain that motion. The physics word for the tendency of objects to resist changes to their motion is **inertia**.
There is evidence for this observation everywhere: Interstellar spacecraft like the Voyager probes haven’t had any engines on for decades, yet they still move through space at more than 35,000 mph. Air hockey pucks move seemingly forever, even with a very light starting tap. Finally, stationary rocks never spontaneously start moving. In these examples, motion is maintained without any force at all.
There are myriad counterexamples to consider as well. When sliding an object across the table, does it not slow to a stop? Why do I need to constantly use the engine in a car to maintain speed? When I toss a ball, why doesn’t it keep moving in a straight line upward to space? How to explain these events which are seemingly at odds with Newton’s 1st Law? There clearly is something missing. Check back next week for Newton’s 2nd Law where we’ll look at how something hiding in plain sight explains all these counterexamples and more.
Physics and Math are hard! Ask your teacher for help if you need it. Ask me if you need more.