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Introduction to work and energy

Introduction to work and energy. Created by Sal Khan.

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  • leaf green style avatar for user Sukanya Sudhiram
    I don't quite understand the difference between negative work and no work?
    (5 votes)
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    • spunky sam red style avatar for user V_Keyd
      Negative work can be understood this way:
      Car breaks down while going uphill and it starts to roll downward. You, as a good samaritan, try to stop the car from rolling down the slope by pushing on it with all your might. The force you apply on the car is directed uphill but alas, it's not enough to stop the car. The car continues to move downhill, that is, it's displacement is in a direction opposite to the force you are applying. This means you are doing negative work on the car. If you draw the free body diagram for this car, rolling down the incline, you'll see that the component of gravity parallel to the incline is doing the positive work because both this component and the displacement are in the same direction.

      Now suppose, more bystanders come along to help stop the car. Everyone pushes on the car uphill so that the car slows down and eventually stops. This happened because the negative work that each person was doing eventually became big enough to cancel out the work that gravity was doing. Once the car stops and the people continue to push it uphill, the car will start rolling upward, in the direction of the applied force and now you'll get positive work.
      During this scenario, note that the normal force on the car from the incline is perpendicular to the surface and the car and it does zero work because there is no motion along its direction.
      I hope this helps to assuage your doubts.
      (46 votes)
  • piceratops ultimate style avatar for user Hemanth
    Sal says that work = force . distance... how can a vector be multiplied with a scalar to yield a scalar?
    (13 votes)
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    • leafers seed style avatar for user Ordo Ab Chao (Quizzaciously Sesquipedalianized Eleemosynary)
      The actual formula for work is

      W = F · s,
      i.e. Work is the scalar product of the vectors Force and displacement, not necessarily distance.

      Scalar product is so called because it returns a scalar, which in this case is Work. (It is also called "dot product" because of the "·" symbol between the vectors in the formula.) So in fact, Work does not equal Force times distance; Mr. Khan did a white lie there.

      Why do I say he did a white lie? Well, in the case of moving in a straight line (with force in same direction as displacement),

      the scalar product has the same effect as multiplying the magnitudes of the 2 vectors, and the magnitude of displacement vector is equal to distance:
      so W=F · s=||F||*||s||=F*d.

      So in this case, Mr. Khan's definition of Work is correct. But this obviously won't work for other cases (e.g. NOT moving in a straight line); Mr. Khan gave this definition for introduction's sake.
      (11 votes)
  • male robot hal style avatar for user han zengxiang
    what is the difference between work,energy and power?
    (5 votes)
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  • hopper cool style avatar for user Himanshu Rana
    the law said that the energy can neither be created nor be destroyed if its true and we need energy to move our body then where did the energy went for example before moving our hand the energy was stored there but after moving it the energy is used the question is where is the used energy after moving the hand ?
    (4 votes)
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    • duskpin ultimate style avatar for user Amanda Haley
      So the energy associated with moving out hands is dissipated in several ways. One way is release as thermal energy within our actual body - so when we use our muscles, they actually heat up, because our bodies have to do a lot of exothermic (heat releasing) chemical reactions to actually create muscle contraction. We also interact with air molecules, kind of "swatting" them, which will vaguely increase their speed.

      When we do small motions, like waving our hand once or twice at a friend, this isn't going to release a noticeable amount of heat... but if you run a marathon, I guarantee your legs will feel warm, and your body will heat up at the massive amount of work your body does over the course of the marathon!

      Hope this helps.
      (5 votes)
  • blobby green style avatar for user Harshini Nadella
    what is negative force?
    what angle must me there between displacement and force in order to do negative work?
    (2 votes)
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  • aqualine tree style avatar for user Phoebe Dillon
    So when it comes to a problem and we need to find velocity...and it isn't a projectile or impulse, should we use this instead of Newtons Laws? (i.e. vf=vi+at, etc. etc.)
    (2 votes)
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  • starky seed style avatar for user Rashi Manvar
    I had a doubt about this example.

    If I am carrying a book in my hand ( till chest ) and then walk till 10 m then is it called work done? or not,
    as 2 forces are acting to each other
    - gravitational
    - my hand's force to keep it upright
    (1 vote)
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  • hopper happy style avatar for user Khurram Solangi
    At , He says work is energy transferred by force, Using this definition shouldn't the formula be Work = Energy times Force ?
    (2 votes)
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  • leaf red style avatar for user RandallAlex2014
    In the equation work equals force times distance, but cant distance be replaced by displacement?
    (2 votes)
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  • piceratops seedling style avatar for user Ayushi Jain
    Work is a scalar quantity and therefore negative work is only used to denote the direction.Right?
    (2 votes)
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    • spunky sam blue style avatar for user CHRISTIAN MOMAH
      Firstly, since it is scalar, the sign has nothing to do with the direction. Negative work is an indication that work is done against the object. For example, to stop a fast moving car (Vo), you need to apply work against it motion (V). The same goes for work done by friction which acts against the direction of motion.
      (1 vote)

Video transcript

Welcome back. I'm now going to introduce you to the concepts of work and energy. And these are two words that are-- I'm sure you use in your everyday life already and you have some notion of what they mean. But maybe just not in the physics context, although they're not completely unrelated. So work, you know what work is. Work is when you do something. You go to work, you make a living. In physics, work is-- and I'm going to use a lot of words and they actually end up being kind of circular in their definitions. But I think when we start doing the math, you'll start to get at least a slightly more intuitive notion of what they all are. So work is energy transferred by a force. So I'll write that down, energy transferred-- and I got this from Wikipedia because I wanted a good, I guess, relatively intuitive definition. Energy transferred by a force. And that makes reasonable sense to me. But then you're wondering, well, I know what a force is, you know, force is mass times acceleration. But what is energy? And then I looked up energy on Wikipedia and I found this, well, entertaining. But it also I think tells you something that these are just concepts that we use to, I guess, work with what we perceive as motion and force and work and all of these types of things. But they really aren't independent notions. They're related. So Wikipedia defines energy as the ability to do work. So they kind of use each other to define each other. Ability to do work. Which is frankly, as good of a definition as I could find. And so, with just the words, these kind of don't give you much information. So what I'm going to do is move onto the equations, and this'll give you a more quantitative feel of what these words mean. So the definition of work in mechanics, work is equal to force times distance. So let's say that I have a block and-- let me do it in a different color just because this yellow might be getting tedious. And I apply a force of-- let's say I apply a force of 10 Newtons. And I move that block by applying a force of 10 Newtons. I move that block, let's say I move it-- I don't know-- 7 meters. So the work that I applied to that block, or the energy that I've transferred to that block, the work is equal to the force, which is 10 Newtons, times the distance, times 7 meters. And that would equal 70-- 10 times 7-- Newton meters. So Newton meters is one, I guess, way of describing work. And this is also defined as one joule. And I'll do another presentation on all of the things that soon. Joule did. But joule is the unit of work and it's also the unit of energy. And they're kind of transferrable. Because if you look at the definitions that Wikipedia gave us, work is energy transferred by a force and energy is the ability to work. So I'll leave this relatively circular definition alone now. But we'll use this definition, which I think helps us a little bit more to understand the types of work we can do. And then, what kind of energy we actually are transferring to an object when we do that type of work. So let me do some examples. Let's say I have a block. I have a block of mass m. I have a block of mass m and it starts at rest. And then I apply force. Let's say I apply a force, F, for a distance of, I think, you can guess what the distance I'm going to apply it is, for a distance of d. So I'm pushing on this block with a force of F for a distance of d. And what I want to figure out is-- well, we know what the work is. I mean, by definition, work is equal to this force times this distance that I'm applying the block-- that I'm pushing the block. But what is the velocity going to be of this block over here? Right? It's going to be something somewhat faster. Because force isn't-- and I'm assuming that this is frictionless on here. So force isn't just moving the block with a constant velocity, force is equal to mass times acceleration. So I'm actually going to be accelerating the block. So even though it's stationary here, by the time we get to this point over here, that block is going to have some velocity. We don't know what it is because we're using all variables, we're not using numbers. But let's figure out what it is in terms of v. So if you remember your kinematics equations, and if you don't, you might want to go back. Or if you've never seen the videos, there's a whole set of videos on projectile motion and kinematics. But we figured out that when we're accelerating an object over a distance, that the final velocity-- let me change colors just for variety-- the final velocity squared is equal to the initial velocity squared plus 2 times the acceleration times the distance. And we proved this back then, so I won't redo it now. But in this situation, what's the initial velocity? Well the initial velocity was 0. Right? So the equation becomes vf squared is equal to 2 times the acceleration times the distance. And then, we could rewrite the acceleration in terms of, what? The force and the mass, right? So what is the acceleration? Well F equals ma. Or, acceleration is equal to force divided by you mass. So we get vf squared is equal to 2 times the force divided by the mass times the distance. And then we could take the square root of both sides if we want, and we get the final velocity of this block, at this point, is going to be equal to the square root of 2 times force times distance divided by mass. And so that's how we could figure it out. And there's something interesting going on here. There's something interesting in what we did just now. Do you see something that looks a little bit like work? Well sure. You have this force times distance expression right here. Force times distance right here. So let's write another equation. If we know the given amount of velocity something has, if we can figure out how much work needed to be put into the system to get to that velocity. Well we can just replace force times distance with work. Right? Because work is equal to force times distance. So let's go straight from this equation because we don't have to re-square it. So we get vf squared is equal to 2 times force times distance. That's work. Took that definition right here. 2 times work divided by the mass. Let's multiply both sides of this equation times the mass. So you get mass times the velocity. And we don't have to write-- I'm going to get rid of this f because we know that we started at rest and that the velocity is going to be-- let's just call it v. So m times V squared is equal to 2 times the work. Divide both sides by 2. Or that the work is equal to mv squared over 2. Just divided both sides by 2. And of course, the unit here is joules. So this is interesting. Now if I know the velocity of an object, I can figure out, using this formula, which hopefully wasn't too complicated to derive. I can figure out how much work was imputed into that object to get it to that velocity. And this, by definition, is called kinetic energy. This is kinetic energy. And once again, the definition that Wikipedia gives us is the energy due to motion, or the work needed to accelerate from an object from being stationary to its current velocity. And I'm actually almost out of time, but what I will do is I will leave you with this formula, that kinetic energy is mass times velocity squared divided by 2, or 1/2 mv squared. It's a very common formula. And I'll leave you with that and that is one form of energy. And I'll leave you with that idea. And in the next video, I will show you another form of energy. And then, I will introduce you to the law of conservation of energy. And that's where things become useful, because you can see how one form of energy can be converted to another to figure out what happens to an object. I'll see