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### Course: High school physics (DEPRECATED)>Unit 2

Lesson 2: Newton's first law: Mass and inertia

# Newton's first law of motion introduction

Newton's first law states that objects move with constant velocity unless acted upon by an unbalanced force. If the net force on an object is zero, it will remain at rest (if already at rest) or continue moving with constant speed and direction. A force is not required to keep an object in motion unless an opposing force is present. Created by Sal Khan.

## Want to join the conversation?

• In science my teacher said that theres a lot of weight pushing us down because theres a lot of air above us and he said our body is pushing back but how can other things push back to like a blade of grass and how can they push back too
• There is pressure outside, which pushes us, but there is high pressure inside us which doesn't let us crush because of the pressure outside. And measure your height when your lying down or sleeping on the floor.
• what is the definition of newton's first law of motion
• The definition of Newton's first law is: The velocity of an object will not change unless the object is acted on by an outside force.

The definition of Newton's second law is: When an object is acted on by an outside force, the strength of that force is equal to the mass of the object times the resulting acceleration.

A Newton= kg.m/sec2.
Force=(mass).(acceleration).

Here's an example of Force=(mass).(acceleration):
F=(m).(a)
F=(300 kg).(0.12 meters/second2
F=36 Newtons.
• what other forces exist?
• The universe consists of 4 major forces: Gravity, Electromagnetism (Light), Weak Nuclear, and Strong Nuclear. Some scientists say there is a fifth, but these 4 are the proven forces that basically "run" our universe.
• This may be a really simple question but I want to know, what force is keeping earth moving or orbiting the sun. You might say that in Vacuum, with one force, the object will keep moving. Okay but where did the first force come from if that is true. Also, Why is it that the earth is moving in circles (orbiting) when when you push something, it goes straight, not in curves?
• It's not such a simple question, at all! In fact until Newton came along and figured it out, nobody had a really good explanation for your really important observation, that the Earth and other objects in space going around the Sun follow a precise and predictable path, or orbit. You know what he called the force that keeps planets (and anything else!) from flying off in a straight line once they start moving-- gravity. I think it makes more sense to think of gravity as a pull, not a push, because gravity is a force of attraction that exists between any objects with mass. And the force is actually a pretty small one... you need a lot of mass involved before we start to notice it. Andrew M explained pretty well to another question how the solar system got moving at its formation and its been going ever since. But why in elliptical orbits? Well most of the mass in our solar system is found in the Sun and so the Sun has the greatest gravitational pull in our neighborhood in space. When you get an object pulling you toward it, but you're already moving, you start moving in a curved path. That's what happened to the planets going around the sun. I've heard the Sun's gravity described like a rope or tether pulling on the Earth, with not so much force that we spiral in toward it (which would be bad), but enough force to keep us from spiraling away (which would be pretty bad, too.) There's just enough gravity to keep us going around and around and around...
• Why is it that when a car accelerates the person on the passenger seat feels like they are being pressed into their seat?
• All objects want to stay at rest is they are at rest. This is also called inertia. When the car accelerates, your body's inertia kept your body from moving until it was pushed by the car seat.
• So if Galileo and Descartes had the same general ideas before Newton why does Newton get ALL the credit
• Does inertia have any relation with gravity?
• Yes, very good question!
In fact there are two different concepts of "mass":
1.) the mass which is attracted by gravity (let's call it mass(g))
2.) the mass which resists any accelleration, i.e. the mass which is responsible for inertia (let's call it mass(i))
It is not obvious at all that mass(g)=mass(i) This was the discovery by Galileo and indeed it is one of the most important physical relationships: it is called the "equivalence principle"
• So, thus, if the block was "pushed" in space it could float forever, right? Well, there is no friction and putting aside any astronomical objects like stars, planets, black holes aside; then it would just go on forever, right?
• Yes, if nothing applied a force to the block after the initial push the block would continue to move with the same velocity forever.
• If the objects at rest and costant velocity have 0 net force,does it mean that to give an object speed you have to exert some force on it.And once you stop exerting force,an object will just infinitely move forth?
• I believe that this would be true, as long as the is no counter-acting force, such as friction, gravity, air resistance ( Which, come to think about it, could be the name of a band... ), etc.
Good Question!
(1 vote)
• At - , Sal mentioned a net force... would that be the same thing a gravity? Or would gravity full under one of the net forces?
• The term "net force' is another way of saying "overall force".
For example, if you pushed a block to the left, but your friend pushed with a greater force to the right, then the net, or total, force would be to the right.

## Video transcript

Human beings have always observed that if you have an object that is moving, so this is a moving object, traveling to the right here, that it seems to stop on its own. That if you do nothing to this moving object, on its own, this object is going to come to a stop. It is going to come to rest. And on the other side of things, if you want to keep an object moving, you have to keep applying a force to it. We've never in our everyday experience seen an object that just keeps moving on and on forever without anyone acting on it. It seems like something will always stop. And this is why, for most of human history, probably pre-history, but we definitely know the ancient Greeks all the way until the early 1600s, so for at least 2000 years, the assumption was "objects have a natural tendency to stop." Objects ... have ... tendency ... to come to rest or to stop. And if you want to keep them moving, you have to apply some type of a net force to it. And once again, this is completly consistent with everyday human experience, this is what we've all experienced our entire lives. But then these gentlemen show up in the 1600s, and you might be surprised to see three gentlemen here, because this is about Newton's first law of motion. And, indeed, one of these gentlemen is Sir Isaac Newton. That's Newton right over there [middle]. But these other two guys get at least as much credit for it because they actually described really what Newton's first Law describes, and they did it before Newton. This is Galileo. And this is Rene Descartes. And they describe it in different ways, and Newton frankly gets the credit for it because he really encapsulates into a broader framework with his other Laws, and the Laws of Gravitation, which was really the basics of classical mechanics, and seem to describe, at least until the 20th Century, most of how reality actually worked. And their big insight, and it was very unintuitive at the time, {so now we come to the 1600s} Is that these three gentlemen said, maybe it works the other way. Maybe objects have a tendency to maintain their velocity, their speed and their direction. And if their speed is zero, they'll maintain that restfulness. Unless they're acted upon by an unbalanced force. So the completly opposite way of thinking. For over 2000 years, objects tend to stop on their own, if you want to keep the movement, apply a force. These guys say, Objects have a tendency to maintain their motion forever and the only way that you're going to stop them is if you act on it, or accelerate them, or change their velocity, so either their speed or direction some way, is to act on them with an unbalanced force. But you might be saying, Hey, come on Sal, what's going on? You just went through this, you said for most of most of human history, including my own personal history, this is what I observed [top right]. How can these guys say that this thing has a tendency to go on forever? This seems to break down. And their big insight was, well, maybe these things don't have, by themselves, a tendency to stop, but because of interactions with their environment, forces are being generated that are acting against their motion. So when you think you're leaving this thing alone, there is actualy a net force that is trying to stop it. And in this particular example over here, the net force is the force of friction. It's the interaction between the block and the ground. So, when you think you're leaving this thing alone, you actually have a net force going against its motion, which is the force of friction. And these guys realize that, because they said, look, if it was an innate property of the block, regardless of the environment, it should kind of always come to a stop in maybe a similar way. But they saw, if you made this surface a little bit smoother this thing would travel further and further. Maybe if you eliminated this friction, if you made this surface completely friction-less, completely smooth, this thing indeed would travel forever. And they didn't have the luxury of launching satellites, and doing things in deep space, so it was a very, very unintuitive thought experiment. And you might say, what about this other thing, what happens when I am applying the force? Becuase in my everyday life, If I want to drag my TV set across the room I apply a force to it. And what these guys would tell you is all you were doing, if you were keeping the velocity of that TV constant, all you were doing was counteracting this net negative force. So if this was a TV dragging across your carpet, this is the force of friction acting against the motion of the object, and so you are essentially just balancing it when you push it. If you balance it perfectly, you will be able to maintain it's velocity. If you want to accelerate it, you will have to apply even more force in the direction you are actually pushing it. Many thanks to Sal! :)