Defining Some Basics

21 09 2011

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!