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Course: Physics library > Unit 3
Lesson 1: Newton's laws of motion- Newton's first law of motion introduction
- Newton's first law of motion
- Applying Newton's first law of motion
- What is Newton's first law?
- Newton's first lawNot started
- Newton's second law of motion
- More on Newton's second law
- What is Newton's second law?
- Newton's third law of motion
- More on Newton's third law
- What is Newton's third law?
- Newton's third law of motionNot started
- All of Newton's laws of motionNot started
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Applying Newton's first law of motion
Current time:3:04Total duration:7:13
Newton's first law quiz
+1. If the net force on an object is zero, its velocity won't change. (True)
+2. An unbalanced force on an object will always impact the object's speed. (False)
+3. Moving objects come to rest in everyday life because of unbalanced forces. (True)
+4. An unbalanced force on an object will always change the object's direction. (False). Created by Sal Khan.
+1. If the net force on an object is zero, its velocity won't change. (True)
+2. An unbalanced force on an object will always impact the object's speed. (False)
+3. Moving objects come to rest in everyday life because of unbalanced forces. (True)
+4. An unbalanced force on an object will always change the object's direction. (False). Created by Sal Khan.
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Video transcript
Now that we know a little
+bit about Newton's First Law, let's give ourselves
+a little quiz. And what I want you
+to do is figure out which of these statements
+are actually true. And our first statement is,
+"If the net force on a body is zero, its velocity
+will not change." Interesting. Statement number two, "An
+unbalanced force on a body will always impact
+the object's speed." Also an interesting statement. Statement number
+three, "The reason why initially
+moving objects tend to come to rest in
+our everyday life is because they are being
+acted on by unbalanced forces." And statement four, "An
+unbalanced force on an object will always change the
+object's direction." So I'll let you
+think about that. So let's think about these
+statement by statement. So our first statement
+right over here, "If the net force
+on a body is zero, its velocity will not change." This is absolutely true. This is actually
+even another way of rephrasing
+Newton's First Law. If I have some type
+of object that's just traveling through
+space with some velocity-- so it has some speed
+going in some direction, and maybe it's deep space. And we can just,
+for purity, assume that there's no
+gravitational interactions. There will always be
+some minuscule ones, but we'll assume no
+gravitational interactions. Absolutely no
+particles that it's bumping into, absolute
+vacuum of space. This thing will
+travel on forever. Its velocity will not change. Neither its speed nor its
+direction will change. So this one is absolutely true. Statement number two, "An
+unbalanced force on a body will always impact
+the object's speed." And the key word right
+over here is "speed." If I had written "impact
+the object's velocity," then this would be a true statement. An unbalanced force
+on a body will always impact the object's velocity. That would be true. But we wrote "speed" here. Speed is the
+magnitude of velocity. It does not take into
+account the direction. And to see why this
+second statement is false, you could think about
+a couple of things. And we'll do more
+videos on the intuition of centripetal acceleration
+and centripetal forces, inward forces,
+if this does not make complete intuitive sense
+to you just at this moment. But imagine we're looking at
+an ice skating rink from above. And you have an ice skater. This is the ice skater's head. And they are traveling
+in that direction. Now imagine right
+at that moment, they grab a rope that is
+nailed to a stake in the ice skating rink right over there. We're viewing all of this from
+above, and this right over here is the rope. Now what is going to happen? Well, the skater
+is going to travel. Their direction is
+actually going to change. And they could hold
+on to the rope, and as long as they
+hold on to the rope, they'll keep going in circles. And when they let
+go of the rope, they'll start going
+in whatever direction they were traveling
+in when they let go. They'll keep going
+on in that direction. And if we assume very,
+very, very small frictions from the ice skating
+rink, they'll actually have the same speed. So the force, the inward
+force, the tension from the rope pulling on the
+skater in this situation, would have only changed
+the skater's direction. So and unbalanced force
+doesn't necessarily have to impact the
+object's speed. It often does. But in that situation, it
+would have only impacted the skater's direction. Another situation like
+this-- and once again, this involves centripetal
+acceleration, inward forces, inward acceleration--
+is a satellite in orbit, or any type of thing in orbit. So if that is some
+type of planet, and this is one of the
+planet's moons right over here, the reason why it stays in orbit
+is because the pull of gravity keeps making the object
+change its direction, but not its speed. Its speed is the
+exact right speed. So this was its
+speed right here. If the planet wasn't
+there, it would just keep going on in that
+direction forever and forever. But the planet right
+over here, there's an inward force of gravity. And we'll talk more about the
+force of gravity in the future. But this inward
+force of gravity is going to accelerate this object
+inwards while it travels. And so after some
+period of time, this object's velocity
+vector-- if you add the previous velocity
+with how much it's changed its new velocity vector. Now this is after its traveled
+a little bit-- its new velocity vector might look
+something like this. And it's traveling at
+the exact right speed so that the force
+of gravity is always at a right angle to
+its actual trajectory. It's the exact right speed so it
+doesn't go off into deep space and so it doesn't
+plummet into the earth. And we'll cover that
+in much more detail. But the simple answer is,
+unbalanced force on a body will always impact its velocity. It could be its speed,
+its direction, or both, but it doesn't have to be both. It could be just the speed
+or just the direction. So this is an
+incorrect statement. Now the third
+statement, "The reason why initially
+moving objects tend to come to rest in
+our everyday life is because they are being
+acted on by unbalanced forces." This is absolutely true. And this is the example we gave. If I take an object,
+if I take my book and I try to slide
+it across the desk, the reason why it
+eventually comes to stop is because we have the
+unbalanced force of friction-- the grinding of the
+surface of the book with the grinding of the table. If I'm inside of a
+pool or even if there's absolutely no
+current in the pool, and if I were to try to
+push some type of object inside the water,
+it eventually comes to stop because of all of the
+resistance of the water itself. It's providing an unbalanced
+force in a direction opposite it's motion. That is what's slowing it down. So in our everyday
+life, the reason why we don't see these
+things go on and on forever is that we have
+these frictions, these air resistants, or the friction
+with actual surfaces. And then the last statement, "An
+unbalanced force on an object will always change the
+object's direction." Well, this one actually is
+maybe the most intuitive. We always have this situation. Let's say I have a
+block right over here, and it's traveling with some
+velocity in that direction-- five meters per second. If I apply an unbalanced
+force in that same direction-- so that's my force
+right over there. If I apply it in
+that same direction, I'm just going to accelerate
+it in that same direction. So I won't
+necessarily change it. Even if I were to act against
+it, I might decelerate it, but I won't necessarily
+change its direction. I could change its direction
+by doing something like this, but I don't necessarily. I'm not always
+necessarily changing the object's direction. So this is not true. An unbalanced force on
+an object will not always change the object's direction. It can, like these
+circumstances, but not always. So "always" is what makes
+this very, very, very wrong.