Time for another physics lesson.
I like teaching about Newton’s 3 laws of motion. Most people know the first and third. They get quoted all the time in different places. The thing is that most people don’t understand what they mean. I think a lot of people think they know what they mean, but in reality there is a lot of room for misconceptions and misunderstandings and when tested many people will fail in full understanding of them.
So I want to write today about the first law. When asked, “What is Newton’s first law of motion?” people will say something along the lines of, “An object in motion will remain in motion.” Or if they have had some physics, they may add to that and say, “An object in motion will remain in motion, until acted on by an outside force.” The second one is definitely better than the first, but it still has issues.
So what are my issues you may ask. Well, I am glad you asked. The first is with the word motion. What does that mean? Most people will say that motion means an object is moving, and I get that. But what does that mean? How do you describe moving? I would say that any kind of motion would be defined as moving. So, any of the following would be motion:
- An object going straight along the road at a constant 45mph.
- An object slowing down.
- An object speeding up.
- An object moving in a curve or circle (like a planet around the sun) possibly at a constant speed.
- An object being hit by a bat.
All of these objects are in motion, but only #1 fits with Newton’s 1st law. The object needs to have a constant (not changing) motion to be classified under Newton’s 1st law. People tend to leave that out of their description, and as a result misconcpetions about motion can occur. Will object #2 keep slowing down forever? What happens when it reaches a stop? What about #3, will that object just keep speeding up forever? That doesn’t just happen. You need something to speed an object up or slow it down (that is another one of Newton’s Laws). And #4 is even more interesting. It isn’t speeding up or slowing down (possibly), but because it is always changing directions it doesn’t have a constant motion. Its motion is always changing, and therefore it isn’t constant, and doesn’t not fall under Newton’s 1st law.
So what do we need to add? We need to add the word constant to the definition. We need to clarify that the motion must not be changing. As a result this brings up another kind of motion that one may not think of: rest. Rest is a form of motion. It just happens to be a motion of zero. This is my water bottle sitting next to me right now on the table. It is at rest and will stay that way forever, unless something else acts on it to change that motion. So, rest is also a state of motion. I used to call out rest in the definition of Newton’s 1st law, but now I include it in the description of motion because that allows one to think about rest in a new way that is important in understand motion of an object (like at the top of a throw when a ball comes to “rest” it is still in a state of motion).
The other part I see causing issues if with the “outside force” part at the end. What is an outside force? It is any force that is external to the object and acting on that object. Well, if the object is on earth then it will always have the force of gravity acting on it (unless you fall into the group of people that think gravity isn’t a force, I am not going there, go see the veritasium video on gravity not being a force if you want to get a mind-trip https://www.youtube.com/watch?v=XRr1kaXKBsU). So, you have at least one outside force acting on you all the time, but that doesn’t mean you won’t stay in constant motion. I am sitting in a chair at rest right now, and you may be too. You are in a constant state of motion, but you have that force of gravity acting on you. So, why are you in a constant state of motion? Well, it is because you have another force acting on you: the normal force. I am not going into what the normal force is in detail, but in the case of you sitting in a chair it is the force on your butt right now pushing you up. It is also an outside force. Wait, now you have 2 outside forces acting on you while you are sitting in a chair reading this blog, and you are staying in a constant state of motion, rest.
What is happening, and most of us realize this, is that the normal force is pointing up and gravity is pointing down. They happen to have an equal strength and as a result they cancel each other out, and therefore your state of motion remains the same. So, what change do I add to the law? I always say end Newton’s 1st law with “an outside, net force.” That word net is very important because now the implication is that there is an unbalanced set of forces acting on the object and as a result the state of motion can change.
By just adding a couple words to the “standard” definition of Newton’s 1st Law I have seen many students over the years gain a better understanding of what is going on. They are less likely to think that an asteroid floating through the void of space has any forces acting on it at all because it is moving at a constant velocity. So its state of motion is constant and therefore there is no net, outside force acting on it.
Here is my wording of Newton’s 1st Law:
An object in constant motion, will remain in constant motion, unless it is acted upon by a NET outside force.
Oh and I just read an article about how someone just wrote an article about how for 300 years people have been misinterpreting Newton’s 1st law. The authors basically said that Newton’s 1st law should be explained as… the exact way I have been stating it for the last 17+ years. I guess I could have written a science article about it and had it published. Oh well. Here is the link to it:
I hope you enjoyed the read today, and maybe you learned something. Or maybe you disagree and need me to make a change. Well, let me know.