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Human beings have always observed that if you have

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an object that is moving,

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so this is a moving object,

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traveling to the right here,

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that it seems to stop on its own.

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That if you do nothing to this moving object,

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on its own, this object is going to come to a stop.

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It is going to come to rest.

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And on the other side of things,

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if you want to keep an object moving,

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you have to keep applying a force to it.

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We've never in our everyday experience

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seen an object that just keeps moving

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on and on forever without anyone acting on it.

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It seems like something will always stop.

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And this is why, for most of human history,

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probably pre-history, but we definitely know the ancient Greeks

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all the way until the early 1600s,

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so for at least 2000 years,

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the assumption was "objects have a natural tendency to stop."

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Objects ... have ... tendency ... to come to rest or to stop.

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And if you want to keep them moving,

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you have to apply some type of a net force to it.

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And once again, this is completly consistent with

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everyday human experience,

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this is what we've all experienced our entire lives.

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But then these gentlemen show up in the 1600s,

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and you might be surprised to see three gentlemen here,

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because this is about Newton's first law of motion.

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And, indeed, one of these gentlemen is

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Sir Isaac Newton.

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That's Newton right over there [middle].

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But these other two guys get at least as much credit for it

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because they actually described really what

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Newton's first Law describes,

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and they did it before Newton.

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This is Galileo.

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And this is Rene Descartes.

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And they describe it in different ways,

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and Newton frankly gets the credit for it

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because he really encapsulates into a broader framework

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with his other Laws, and the Laws of Gravitation,

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which was really the basics of classical mechanics,

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and seem to describe, at least until the 20th Century,

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most of how reality actually worked.

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And their big insight, and it was very unintuitive at the time,

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{so now we come to the 1600s}

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Is that these three gentlemen said,

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maybe it works the other way.

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Maybe objects have a tendency to maintain their velocity, their speed and their direction.

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And if their speed is zero, they'll maintain that restfulness.

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Unless they're acted upon by an unbalanced force.

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So the completly opposite way of thinking.

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For over 2000 years, objects tend to stop on their own,

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if you want to keep the movement, apply a force.

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These guys say,

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Objects have a tendency to maintain their motion forever

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and the only way that you're going to stop them is

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if you act on it, or accelerate them, or change their velocity,

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so either their speed or direction some way,

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is to act on them with an unbalanced force.

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But you might be saying,

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Hey, come on Sal, what's going on?

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You just went through this,

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you said for most of most of human history,

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including my own personal history,

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this is what I observed [top right].

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How can these guys say that this thing has a tendency to

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go on forever?

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This seems to break down.

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And their big insight was,

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well, maybe these things don't have, by themselves,

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a tendency to stop, but because of interactions

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with their environment, forces are being generated

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that are acting against their motion.

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So when you think you're leaving this thing alone,

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there is actualy a net force that is trying to stop it.

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And in this particular example over here,

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the net force is the force of friction.

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It's the interaction between the block and the ground.

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So, when you think you're leaving this thing alone,

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you actually have a net force going against its motion,

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which is the force of friction.

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And these guys realize that,

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because they said,

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look, if it was an innate property of the block,

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regardless of the environment,

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it should kind of always come to a stop

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in maybe a similar way.

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But they saw, if you made this surface a little bit smoother

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this thing would travel further and further.

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Maybe if you eliminated this friction,

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if you made this surface completely friction-less,

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completely smooth, this thing indeed would travel forever.

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And they didn't have the luxury of launching satellites,

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and doing things in deep space,

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so it was a very, very unintuitive thought experiment.

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And you might say, what about this other thing,

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what happens when I am applying the force?

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Becuase in my everyday life,

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If I want to drag my TV set across the room

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I apply a force to it.

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And what these guys would tell you

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is all you were doing,

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if you were keeping the velocity of that TV constant,

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all you were doing was counteracting this net negative force.

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So if this was a TV dragging across your carpet,

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this is the force of friction acting against the motion

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of the object,

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and so you are essentially just balancing it when you push it.

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If you balance it perfectly,

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you will be able to maintain it's velocity.

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If you want to accelerate it,

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you will have to apply even more force in the direction you are actually pushing it.

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Many thanks to Sal! :)