It’s said that Sir Isaac Newton was sitting under an apple tree when an apple fell on his head, and that’s when all his discoveries began.
Personally, I doubt that story—just as I doubt that Galileo Galilei ever dropped iron and wooden balls off the Leaning Tower of Pisa. His goal would have been to show that both objects hit the ground at the same time. Unfortunately, wind resistance would have gotten in the way.
Regardless of how Newton discovered gravity, his scientific achievements are monumental. In fact, we recognize him today as one of the greatest scientists to ever live, second only to the famous Albert Einstein.
Newton’s revelation that gravity draws objects toward Earth changed the course of modern science. But what exactly did he find out?
Through his experiments, Newton came up with three laws of motion that still accurately describe all motion in the universe today.
The first, Galileo first noticed—but Newton proved it and elaborated upon it.
Law I: A body continues at rest or in uniform motion in a straight line unless acted on by some force.
I’ll make it simpler for you. Newton’s first law can be summed up with one single word: inertia.
Inertia is, as Bill Nye’s opening theme song states, a property of matter. And it just means that objects don’t like to change how they’re moving. As my physics professor is fond of stating, matter is stubborn.
Matter, by the way, is just “stuff.” That’s all it is. A skyscraper is matter, air is matter, the tiniest bacteria is matter, you and I are matter, and—guess what—the Earth is matter.
In fact, every object in the known universe is matter. Anything that has mass—is made of some kind of stuff—and takes up space is matter.
And inertia just means that matter likes to stay exactly how it is. If it’s still, it will resist being moved. If it’s moving, no matter how fast it’s moving, it will resist changing speed or direction.
This is why driving is so dangerous. Cars don’t want to slow down once they’re at a certain speed. And because they’re big and have momentum, it takes a lot of work to force them to slow down—or to change their direction.
Wait a second. What’s momentum?
You can think of it as how much motion an object has. Basically, if an object has more momentum, it will resist changing its motion more than if it has less momentum.
You can have two objects moving at the same speed that have different momentums, if they’re different sizes.
Why? Think of it this way.
Imagine you’re in space. Now venture a guess for me and imagine how easy it would be for you to reach out and give the Earth a push. How far can you get it to move?
Not very far. In fact, chances are its motion will be so little it’s not even worth measuring.
But what if you’re in space, and you accidentally let go of your walkie-talkie?
Now imagine reaching out to that walkie-talkie before it drifts away, and grabbing it. Do you think you can move it back to you?
Now, Earth and the walkie-talkie are both moving at the same speed. They’re both traveling at about 30 km/s (that’s 18.5 miles/second) around the sun. But one’s easier to move than the other, because one is much smaller than the other.
They have different momentum.
Momentum, by the way, is an object’s velocity times its mass. Meaning, how fast it’s going times how much stuff it’s got inside it. Does that make sense? I mean, obviously mass plays a part, because of the Earth/walkie talkie example, but what about speed?
Well, think of cars again. How hard is it to stop a moving car? And if you have to get out and push, how hard is it to get the car to move?
Really hard. Because a car has momentum, depending on both its velocity (its speed and its direction) and its mass.
Newton’s second law of motion is that the acceleration of a body is inversely proportional to its mass, directly proportional to the force, and in the same direction as the force.
Okay…let’s tackle this bit by bit.
First let’s define acceleration. In physics, it doesn’t just mean speeding up. It also means slowing down or even changing direction.
Well, remember how it was easier to move the walkie talkie than the Earth? Doesn’t it make sense that how much we can accelerate an object would depend on how big it is, and big objects resist changes in motion more than smaller objects?
That’s the first part of the law.
Now…directly proportional to the force. Imagine pushing the same box a few times, but each time, you push a little harder than before. Don’t you think that when you push harder, you’ll get the box to move farther?
See, that’s the second part of the law. Force is directly proportional to the motion it causes.
And then there’s the third part, that the motion will be in the same direction as the force. What do you think will happen when you push that box away from you? There’s no one else pushing on it, and the ground is flat. Will it move to your left instead?
Nope. It’s as simple as that. The motion will be in the same direction as the force that causes it.
As for the third law….
To every action, there is an equal and opposite reaction.
Okay, that sounds too complicated for my liking. Let’s say it a bit simpler.
If you make an object move, it’s going to make you move too.
Yup, it’s true. Think of it this way. When you push on that box, if you don’t push hard enough, it pushes you back a little, doesn’t it?
Think of a balloon. What if you blow up a balloon but forget to tie it, and it flies out of your hand? This happens because air is being pushed out of the ballon. But that also makes the balloon move through the air.
Air is being pushed out of the balloon with the same force that’s pushing the ballon forward. And that’s the magic of it. Whatever force you exert on an object, the same force is going to be exerted on you.
So…how to we explain bugs splattering on car windshields? Obviously the car is exerting a force on the bug. Can the bug possibly be exerting a force on the car?
Here’s where momentum comes in, not to mention Newton’s famous equation:
Get this: the bug’s force on the car is the same as the car’s force on the bug!
But because the car is bigger, faster, and has more momentum, the car keeps moving fast while the bug gets absolutely splattered. If something bigger hit the car, like, say, another car, then you can probably imagine what would happen.
The lesson here is that the force is an object’s mass times its acceleration.
There’s Newton’s laws of motion for you. Next up, let’s take a peek at how Newton took those laws and used them to explain planets’ gravity…