15 Newton’s First Two Laws
The goal of this book is for us to learn to describe how objects move. This is a hard question in general, so let’s start with the simplest form of this question:
If an object is left alone, how will it move?
Well- if an object is at rest, and we leave it alone, the object just sits there. But what if the object is already moving?
Exercise 15.1: Stopping an Object
Suppose you and a little metal ball are the only two things in an otherwise completely empty Universe. You see the metal ball is moving away from you with some initial speed 
. If you wanted to stop the ball, what would you do?
Once you’ve answered that, think about this: suppose you fail to act, and leave the ball alone. What happens to the ball then?
The answer to the question above is the foundation on which physics is built. It is called Newton’s first law, though it was originally discovered by Galileo. It is also sometimes called the law of inertia. For completeness, here is my preferred wording of Newton’s first law:
If you don’t mess with an object, it keeps going.
More precisely, the object keeps moving with constant velocity. This sounds a little counter intuitive: when we roll a ball, we know it eventually comes to a stop. But- here is the thing: an object will keep going if left alone. When you roll a ball, the floor is clearly messing with the ball (if the floor weren’t there, the motion of the ball would be different!).
To really see what happens when an object is left mostly alone, we have to go to space. Fortunately, we can do that: check it out!
Original video can be found here.
So- if you don’t mess with an object, it keeps going with constant velocity. But what happens if we do mess with an object?
Let’s start by thinking what “mess with an object” means. Given our little metal ball, we can push it, or pull it. We can blow on it, but that is just another way of saying we push it. Any of these actions will make the velocity of the metal ball change. That statement is precisely Newton’s second law:
When you push or pull an object, its velocity changes.
We’ll have much more to say about the second law later in the book. For now, I want to be perfectly clear about one thing: I am simply asserting that these two laws are true. They are not derived from anything: they are simply Newton’s guess for how Nature works.
Saying that the laws of physics are just someone’s guess as to how things work may sound strange, but this is exactly how physics works, as discussed in Richard Feynman’s lecture below (Feynman was one of the titans of 20th century physics).
The rest of this book is about how we go from Newton’s guess to making all sorts of interesting predictions that we can compare to observations.
Exercise 15.2: Review
I don’t want you to forget about simple harmonic motion, so I have included this problem as a quick review.
A 
spring is attached to a weight, so that the weight is hanging from the spring. You then grab the weight, pull it down 
, and let go. The go box bobs up and down on the spring once a second.
Suppose you set 
at the equilibrium point of the box. Determine the amplitude and angular frequency of the oscillations, and write down the equation for 
that describes the motion of the box.
Key Takeaways
Fill in the words that complete Newton’s first two laws.