Newton’s Third Law

One might assume that a discussion of Newton’s three laws should start with the first. When I started writing these posts I tried that. I started with half of Newton’s first law and in the discussion I found that I could only really explain Newton’s First Law if you knew the other two laws already. Then thinking about it, I realized that really I should start with Newton’s Third Law.

You’re going to have to trust me on this and just roll with it. Ok?

So Newton’s Third Law is most commonly stated as:

“For every action there is an equal and opposite reaction.”

More clearly worded, but less catchy, this clarifies to:

“The forces of two bodies on each other are always equal and are directed in opposite directions.”

It’s important to remember the second clarifying sentence, because otherwise, things might get confused when we’re discussing the other laws later on. But I’ll help you through those when they come up. For now, what you need to know is the first catchy one.

Sometimes this law is a little hard to grasp. After all, when I push against a wall, is it pushing back on me? Is that what we’re saying? If that’s true, why don’t I fall over when I push against the wall?

But let’s not talk about walls and people. Let’s talk about two kids. Essentially this means that if two kids sitting in chairs with wheels, push against each other (directly, in a line, let’s not add weird angles to this), they will both be moved by the force of the push. Yes, one kid might go further than the other, but that’s related to the fact that the two kids don’t weigh exactly the same (and that’s related to the conservation of momentum, which we’re not discussing yet). So if two kids, weighing exactly the same amount, are sitting in chairs with wheels–like the kind you use for your computer–and they press their hands against each other and both push forward, against the other, both kids will be pushed back the same distance.

Let’s clarify this yet further. Even if only ONE of the kids had pushed (meaning the other kid sat there completely stiff), BOTH kids would still have moved an equal distance back.

This is because even though the kid not moving doesn’t actively use his muscles to push on the other kid, the fact that the other kid is pushing on him, means he’s exerting an equal and opposite force.

But let’s go back to a harder to grasp example, the example of something very small (like a human) and something very big (like the earth).

You, as a small human, are exerting your weight on the surface of the earth. Gravity is pulling down on you, trying to pull you into the very earth itself, trying to pull you closer and closer to its center. (This force of gravity on you, is your weight. Aka, the more you weigh, the greater the force of gravity is on you).

So if gravity is acting on you so consistently why don’t you get pulled through the surface of the earth?

Well, because of Newton’s third law!

You are exerting a force on the ground–the force of the earth wanting to suck you in. But the ground is exerting an EQUAL and OPPOSITE force, counteracting gravity.

The equal and opposite is very important. It has to be opposite in order to directly counter act gravity. Otherwise, if the ground was exerting a force on you at an angle, you’d feel it and sort of slide sideways, because gravity isn’t counteracting that business. Gravity acts straight down, not at an angle. The equal is important, because if the force of the ground was less than gravity, you would be pulled through the ground. If the force was greater than gravity, you would actually be pushed up and away from the earth.

Because remember, a force is a push or pull. So if two people are pulling on something equally, it’s not going to move. Like a tug of war rope, when the sides are evenly matched.

So the earth and ground are both pushing on you with the same force, so you don’t go anywhere.

And by the way, we call that upwards force the ground exerts on you a Normal Force.

I hope you’re not confused, and that this adequately explained the third law.

Let me know if you have questions!

Defining Some Basics

So I was going to start today with explaining Newton’s First Law. I started writing up my basic explanation and then I realized that within my “basic” explanation, I had assumed you knew some things. Like what a force is.

Granted, I don’t actually think anyone reading this doesn’t have an intuitive feel of what a force is. And I hope this doesn’t come across as patronizing. But there is sometimes a difference between our intuition and what something actually is in a scientific sense.

For example, the word “power” causes confusion. I used to TA an Introduction to Aerospace Engineering class and this was one word that consistently gave my freshmen problems. The intuitive definition of power brings up thoughts of something a person can hold and have. Like the President of the US has power. Or the CEO of a company. People also think a “powerful car” is one with a big engine, one that can go from zero to sixty in a few seconds, or one that can pull tons of weight. This then leads people to confuse power and force.

Don’t be confused.

Force, for purposes of understanding the First Law, is a push or pull. Like hitting a baseball with a bat. When the ball hits the bat, I start pushing the ball forward using the bat. This is a force.

Mechanical power is then a force times a velocity—so how much force I’m putting on the baseball times the velocity the baseball is going at that moment. So these aren’t the same thing.

Now please put power from your mind entirely for now. I just wanted to use it as an example of why I’m explaining something so simple and obvious as force. Because sometimes the simple and obvious answer isn’t the right one.

So yes, for now, a force is a push or pull.

And this definition of it, this understanding, is directly related to Newton’s First Law.

And you can look forward to a more detailed explanation of what a force is in our discussion of Newton’s Second Law.

Two more definitions we should get out of our way here are velocity and acceleration.

Many people use velocity interchangeable with speed, but this isn’t quite true. There is an important difference. Velocity is speed and direction. So your speed is 50 mph. Your velocity is 50 mph North. Speed is merely the magnitude of the velocity (that is, the number without the direction). This directional aspect is extremely important, as you will see more when we discuss Newton’s laws.

Acceleration is like velocity, the direction matters. What is acceleration? Well, it’s your change in velocity. But there is something important to point out here. If you’re going at a constant speed of 50 mph but you change direction then you are accelerating. Why? Because changing your direction changes your velocity, and acceleration is a change in velocity. So if you’re traveling at a constant speed in a circle, then you are accelerating the entire time, because your direction and therefore your velocity are constantly changing.

So there are a few of the topics we need to understand to begin discussing Newton’s laws. Force, acceleration, and velocity. Let me know if you have questions!

Why Reveal My Secrets?

Today I start a new blog, one that deals solely with science and engineering. Every since the days of Werner von Braun, people have been viewing rocket science as a difficult field that only geniuses can deal with. But this is simply not true. I am not a genius. I’m just a girl with a passion for rockets, satellites, and space.

As someone who has had this passion all my life, I’ve accumulated a lot of knowledge about science and engineering, even from a young age. Because of this, I often don’t realize that other people don’t know the facts I view as “common knowledge.”

For instance, when I was in college, I once did a church event that involved leading a group of high school girls. They were mostly juniors in high school. These girls who were almost done with their basic education, girls who had gone to the best schools in Atlanta, girls from affluent families destined to go to excellent colleges, asked me with all seriousness and sincerity, “What is bigger: the galaxy or the universe?”

I was baffled. How could they not know the universe is bigger. I mean its right there in the name. UNI-verse! There can only be one! This was something I had known since at least fifth grade, if not earlier. How had girls with such good educations and bright futures failed to learn this fact?

This sort of event happened to me over and over again. Somewhere, somehow, the average person has gotten the idea that science is insurmountable. It’s something too difficult to know. Something beyond the grasps of the average man. Only an Isaac Newton or Albert Einstein can dare to grasp it.

So here I am. Prepared to start at the basics and explain the in’s and out’s of science and engineering as I know them.

Therefore, look forward to the basics of my world: an explanation of Newton’s laws. We’ll build up from there.

And if you have any burning questions, anything you’d really like to know about, just leave a comment. I promise, I will not allow a question to go unanswered.