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Applying Newton's first law of motion

Sal applies Newton's first law the answer some true/false statements about why objects move (or not).   Created by Sal Khan.

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  • blobby green style avatar for user Vinayak
    For statement #3, does LIGHT also apply?
    And what about radio waves, microwaves, etc. ?
    (182 votes)
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    • leafers ultimate style avatar for user Boris Stanchev
      Light is a bit tricky and although Newton did a lot to advance our understanding of light and optics on a basic level, you'll find out that its actual behavior isn't described by Newtonian physics. For example, Newton discovered that white light is composed of all the colors in the visible spectrum (he actually didn't know anything about invisible light, such as radio waves, microwaves, X-rays, etc) , but he had no good way of knowing the speed of light or what it is made up of. Today we know light is made up of things that have no mass called photons that act as both waves and particles depending on how they are observed. That alone can't be modeled by Newton's laws of motion and it's a pretty hard concept for anyone to really understand. Newton actually argued that light was made up of just particles. He wasn't aware of light's wave properties. Things like photons were first modeled in detail by quantum mechanics and the whole idea that photons are at once both waves and particles also came out of quantum mechanics.

      Another thing Newton didn't know about was that light also always moves at the same speed regardless of your frame of reference, which come to think of it doesn't make sense. You would think that if you were moving fast enough, light would appear to move slower next to you, but it never does! Also, as it turns out from Einstein's equations, Newton's laws of motion can't be used to describe anything at the speed of light or anything approaching such a speed. Therefore, light can't be modeled by Newton's 1st law of motion or any of the 3 laws of motion. These sorts of problems related to the speed of light as well as what happens when something approaches the speed of light weren't really dealt with until Einstein formulated his Theory of Relativity.

      So as you can see, the actual behavior of light is described by two branches of physics that didn't develop until the 20th century--- quantum mechanics, and Einstein's theory of relativity.

      One last thing, microwaves radiowaves, etc, are light except at much longer wavelengths (lower frequencies) than visible colors, while UV light, X-rays, and gamma rays are light with higher frequencies than visual light. In fact, the type of light we can see and that we think of as light, is really a rather small sliver in a much broader spectrum. Note that wavelengths and frequencies are properties that result from the wave aspect of light's behavior.
      (336 votes)
  • leafers sapling style avatar for user Matthew!!
    My teacher says that objects like the moon are in free fall, but why does it stay put and just rotate around the Earth? Is it like a vacuum or is it just the gravity is stronger than the free fall?
    (28 votes)
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  • spunky sam blue style avatar for user Shahabal Azhar
    What is the difference between speed and velocity? I thought both were the same thing.
    (9 votes)
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    • aqualine ultimate style avatar for user Sravanth
      Speed and Velocity are quite different. Speed is a scalar quantity, i.e it has only magnitude but no direction. Therefore, Speed = distance/time.

      But Velocity is a vector quantity, i.e. it has both magnitude and direction. Therefore, Velocity = displacement/time.

      I highlighted distance and displacement because, distance is scalar and displacement is a vector.
      (24 votes)
  • aqualine ultimate style avatar for user Pinakin Dutta
    What exactly do we mean by an unblanced force??.....
    It counteracts with Newton's third law of motion i.e. every action has an equal and opposite reaction

    Sorry...the question may be a little stupid, but not enough to defeat my cuiosity .
    (7 votes)
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    • male robot hal style avatar for user Charles LaCour
      The law about equal but opposite forces says that if you apply a force on something that it applies an equal but opposite force on you. The 2 forces are on different objects. When you are talking about unbalanced forces you are talking about the sum of forces on 1 object.

      For example if you are pushing a block of wood across a table to the left so that it accelerates you have a force from gravity on the block down, a force from the table pushing up and you pushing left and the force of friction to the right. If the sum of these 4 forces leaves an amount to the left they are unbalanced and the block is accelerating to the left.

      Newtons 3rd law would say that there is also a force up on the earth from the block, a force down on the table from the block, to the right on your hand from the block and to the left on the table from friction. All these forces are not on the same object so they can't be added.
      (16 votes)
  • blobby green style avatar for user Abheesht Sharma
    But doesn't friction decrease the magnitude of velocity i.e, speed? And friction is unbalanced. So how in the world doesn't an unbalanced force change the speed??
    (6 votes)
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  • blobby green style avatar for user Anjan Kumar Das
    What does 'velocity' mean?
    (0 votes)
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    • male robot donald style avatar for user Tomasz Bartkowiak
      Remember not to confuse "velocity" with speed!
      In physics, velocity is a vector which means that the value of velocity can be negative, whereas "speed" is a scalar (it only gives the value, without direction) and therefore, can be only positive.
      For example, if two cars are getting closer to each other (they are driving in opposite direction), we can say that the velocity of one of them is equal to 20m/s and the other one: -30m/s (minus sign because of the opposite direction), but their speed can be only positive so it is 20m/s and 30m/s respectively.
      (7 votes)
  • blobby green style avatar for user Trasan Apansson
    Statement2: What unbalanced force would be able to change a moving objects direction without also changing its speed via gravity?
    (3 votes)
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  • piceratops ultimate style avatar for user Mosaddek Islam
    at , sal says if it was velocity instead of speed,statment 02 would be right..i donot get.does anyone please explain?
    (5 votes)
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    • male robot hal style avatar for user muhammad khan
      When an unbalanced force acts upon an object, it impacts its speed and not its velocity because if anything is moving in a circular motion, and suddenly it stops moving, though the speed will be the same, but instead of stopping in a circular direction, its direction turns in a way it makes it stop moving and we know that if the direction changes the velocity turns to zero.
      (2 votes)
  • blobby green style avatar for user rachel
    A common situation to describe inertia:

    If a bus accelerates suddenly, the passengers will experience a backward push.

    If inertia didn't exist, what would happen?

    Pardon my english....
    (2 votes)
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  • duskpin ultimate style avatar for user HannyWils
    Why does he say unbalanced force? As opposed to a balanced force. Where is the unbalance coming from?
    (2 votes)
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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.