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I have four statements here and I want you to think about which of

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of these statements are true and the first statement is an object in motion will slow down

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unless it is acted on by an unbalanced force in the direction of motion - interesting

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second statment, an object in motion will maintain it's speed and direction

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forever, unless acted on by an unbalanced force- also very interesting.

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Third statement, an object at rest will stay at rest, unless acted on by an unbalanced

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force. And then the fourth statement, an object acted on an by a unbalanced force will always

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acclerate in the direction of the unbalanced force

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So, i'll leave you to think about those statements, and figure out

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which of those statements are actually true.

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It might be none of them, it might be all of them, or some combination of them.

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Now that's try to think about each of these statements.

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So first of all an object in motion will slow down, unless it is acted on

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by an unbalanced force in the direction of motion. Well this, at least, on a first cut

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kind of seems consistant with our everyday experience. If i'm pushing some furniture

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across the room, so let's say that this is, that this is the carpet of my house,

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and i'm pushing a television set -let's say it's one of those old school heavy television set-

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(so i'm pushing a television set), the only way that I can keep it in motion is that I keep

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pushing on it. So let me draw my force of pushing (my force of pushing). So let's call

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that right there the force of Sal pushing the television set. So the only way that

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I can keep (the only way that I can keep) my television set moving seems like if I keep applying

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this pushing force on it. And I were to stop doing that, it seems like it will slow down.

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So on a first cut everday experience, this seems like it would actually maybe be true.

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It only seems that way, because i'm actually not accounting all of the forces in this situation.

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There is not just the force of me pushing. There is also the force of friction.

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(There is also the force of friction.)

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So i'll call that, there is also the force of friction. And if I want to keep this

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TV moving at a constant veloctiy, my force of pushing (that make that the vectors look

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about the same magnitude, becuase they need to be the same magnitude)

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If I want to keep my television set at a constant velocity in the direction that i'm

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pushing, I have to exactly offset the force of (the force of) friction.

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If my force of pushing is less than the force of friction, then the TV set will to decelerate.

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If my force of pushing is larger than the force of friction, then my TV set will accelerate.

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But in the scenario right now, I have an object in motion, but there's actually a

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balanced force. My pushing is completely balanced by the force of friction.

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Another scenario, if we were to go to deep space (i'm not even thinking about air resistance

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right here), but if we were to go into space, where, especailly deep space, no planets around, complete

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vacuum, if an object is in motion (if an object is in motion), and there is absolutely

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no forces balanced or otherwise, there are absolutely no forces acting on it,

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then that object is going to stay in motion. The only way it might change its direction

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or decelerate or accelerate is if there is a net force on it.

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And just by going back to this example, the only reason why it will slow down, if I

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were to stop pushing, is becuase there is a net force on it, the force of friction.

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And if you want to think about it, there would probably be very minor amount of

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air resistance. So this actually isn't true. An object in motion does not have to slow down

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,unless it is acted on by an unbalanced force.

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An object in motion can stay in motion, as long it is being acted on either by a balanced force, or (i guess)

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a set of forces that balance each other, or no forces at all.

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So this is not (that is not) true. An object in motion will maintain it's speed and direction forever

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unless acted on by an unbalanced force.

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This is true! This is exactly what's happening right over here in the case of a (I guess) a block

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moving through space in a vacuum. If there are no forces acting on it (no net forces acting on it)

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it is not going to change its velocity. Similarly, you could have this situation,

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where you actually have forces acting on it, but they balance each other out.

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In this situation, as long as I can keep pushing forever, and the floor doesn't change, and so the

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friction doesn't change, this thing will keep moving in that direction, theoretically forever.

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The only way to change its direction or accellerate in one direction or another direction, would

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be to have an unbalanced force. Maybe if my force of pushing were larger than the force

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of friction, or the force of friction were larger than my force of pushing.

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So this right over here is absoluely true.

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Third statement, an object at rest will stay at rest unless acted on by an unbalanced

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force. And this is actually pretty intuative, I think, for people. If you just leave

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something alone (if you just leave something alone) it's just going to stay alone.

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It's not going to start spontaneously moving.

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And so, this is true. It's not just the situation where there is no forces; it could also be

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the situation that's it's going to stay at rest, if there are balanced forces acting on it.

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And so the only way you can actually get this thing to come out of that rest position

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is if you have a net force, an unbalanced force.

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If, let's say, this force right here is (is) any amount larger than this force

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right over here, if they don't completely balance out. So this, once again, is true.

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Last statement, an object acted on by an unbalanced force will always accelerate in the direction

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of the unbalanced force. So if I have an object, and there's an unbalanced force, and if we

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net out all the forces, let's say, the direction is in the force's net out and we get a force

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acting in this direction. This is actually true! The object will always accelerate

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if it has an unbalanced force acting on it in that direction, it will accelerate if there's

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a net force in that direction. And there's another way of thinking about it.

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If you see an object that is not accelerating, like my TV that's moving at a constant velocity or this thing

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that's just traveling through space, if you see something that is not accelerating, that

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means that there must not be an unbalanced force acting on it.

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So could have balanced forces acting on it, or you could have no forces on it at all,

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but you can't have an unbalanced force. So if you have an unbalanced force, the thing will

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be accelerating.