# What are Newton’s laws of motion?

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space.

There’s a reason — actually, several — why Sir Isaac Newton is often considered the No. 1 scientist of all time. And while we’re all forced to learn about his laws of motion and concepts of gravity in high school, we rarely get a glimpse into why his seminal work, “Philosophiae Naturalis Principia Mathematica” (or, in English, “Mathematical Principles of Natural Philosophy”), is so dang important. So let’s dig in a little bit into the mind of a genius:

Stay put

Philosophers throughout time have been searching for fundamental laws, simple rules of the universe that could explain the wide and wild variety of phenomena that we see in the world around us. They had been working, and largely failing, at that task for a few millennia until Newton showed up in the late 1600s and showed them how to do it.

In “Principia,” Newton laid out three simple rules of the universe. At first glance, over three hundred years later, they seem simple, intuitive, and obvious, but that’s only because we’ve had over three hundred years to let them sink in. At the time, they were total revolutions in thought.

His first law stated that objects at rest tend to stay at rest, and objects in motion tend to stay in motion. In other words, there’s this thing called “inertia,” which is a measure of an object’s resistance to motion.

This idea was … new. Previously, most thinkers thought that individual objects had a natural inclination to either move or not move (e.g., to explain why the wind tended to blow but rocks preferred to stay put). Likewise, some objects preferred to float (like clouds) while others didn’t (like people). But Newton flipped this on its head: all objects had an innate resistance to new motion, and it took a force to get them to change.

A little push

Speaking of forces, that was Newton’s second law: forces applied to an object give them acceleration, with the amount of acceleration dependent on the object’s mass. This, too, went against the prevailing wisdom, which thought that forces applied to an object gave them velocity. This is partly true, because acceleration is a change of velocity, but it misses the bigger picture that Newton was going after. Once accelerated to a certain velocity, an object will maintain that velocity unless and until a new force is applied to speed it up or slow it down.

Newton’s second law is really the law of conservation of momentum written in another way. Objects will maintain their momentum until a force is applied, and that force will change their momentum. All interactions between objects (e.g., collisions, bumps, knocks, smashes and so on) will preserve the total amount of momentum between them.

If you’ve never met momentum conservation before, you should know that this concept is a cornerstone of every single branch of physics. Seriously, all of it: general and special relativity, quantum mechanics, thermodynamics, particle physics and so on. They all rest and rely on conservation of momentum to guide them. All of modern physics boils down, at the very deepest levels, to expressing conservation of momentum in different scenarios.

From electrons in an atom to the expansion of the universe, it’s all tied to the same concept, which can trace its roots to Newton’s second law.

Equal and opposite

Newton’s last law, that each force has an equal and opposite force, seems like a minor addition. But it too was a major revolution in thought.

When you push on something, you’re applying a force to it and causing it to accelerate. Easy peasy, right? But did you know that the object is simultaneously pushing back on you?

How could that be, if you’re not moving and the object is?

The key is that while the forces are equal, the accelerations aren’t. If you’re more massive than a football, then when you kick it your acceleration will be small, while the football will go flying. But that force back on you is what gives you the sensation of resistance. Another example: when you sit on a chair, you’re applying a force to it, but the chair is also applying a force to you — it’s what you feel pushing up on you.

This last insight is how Newton unlocked the entire cosmos. As he watched an apple fall from a tree, he realized that since the Earth is applying a force to the apple, then the apple must also be applying a force to the Earth. But we don’t see the Earth move because it’s so massive.

With this line of reasoning, Newton was able to argue that the gravitational force wasn’t just something felt near the surface of the Earth, but that it was truly universal: all objects in the cosmos were tied to all other objects through invisible chains of gravity. Armed with that insight and his newfound laws, Newton was able to explain everything from the orbits of the planets to the cycles of the tides.

That’s the power you get from correctly understanding the fundamental laws of nature, laws that were the only paradigm for over 200 years (until the developments of relativity and quantum mechanics), and continue to play a central role in our everyday lives.

Learn more by listening to the episode “What was Newton’s big deal?” on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Chris C. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

Source: space.com

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