If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:8:37

Video transcript

in our daily life this term called work is very loosely defined for example me recording this video right now I could say I am working but my brother could look at me and say hey no he's just playing on his computer he's not working at all see what I mean loosely defined hard to agree whether somebody's working or not but in physics we want precise definitions so in this video we'll see how do we define work in physics so according to physics work done on an object equals the force on an object multiplied by the displacement of that object so this immediately means that if you want to do work then one you need to push on it push or pull on it because you need to provide a force and while pushing or pulling on it you have to make it move that's the condition to do work in physics okay so you have to push on an object because if you don't push our pull the force will be zero work will be zero and while pushing you have to make it move because if you don't make it move there will be no displacement and again work would be zero so let's quickly look at some examples to make more sense of this imagine somebody is pushing a grocery cart on grass apparently do you think that person is doing work on that cart well let's see since he's pushing on that cart she's putting a force on it and notice while pushing it she is making that cart move and so she's putting a force and she's displacing the cart therefore she's doing work on that cart on the other hand what if there's a person who's trying to push on a wall do you think he's doing work well let's see again since he's pushing on that wall he's putting a force on it but that wall it's not moving anywhere the wall is just staying there so he's not making that wall move so he's putting a force but he's not making it move so he does not do any work over here now he might say hey no I got so tired by pushing that wall for three hours doesn't matter he didn't make it move he didn't displace it and therefore no work was being done all right let's take another one let's go to outer space where there is no gravity no friction so that we can keep things simple imagine we have an astronaut with a ball in his hand say he throws that ball do you think he did work think about this for a while alright let's look at it when the astronaut throws the ball he's clearly pushing on that ball right to make that throw and while he's pushing on that ball notice he is displacing that ball he's making it move while pushing and therefore he put force and indeed displacement and therefore he did work on that ball but what happens once the ball gets thrown do you think now he's doing work on that ball think about this what do you think well if I get that back once that ball is released notice he's no longer pushing it so yeah he's the ball is moving if the ball is moving the ball is being displaced but now nobody's pushing on it right and so now no longer work is being done on that ball because there is no force acting on that ball remember we have to push it and while pushing make it move so the astronaut only did work when that ball was in his hand the moment the ball started moving away from his hand no longer work is being done by that astronaut so this is fun right let's do more examples this time imagine a man who's just holding a barbell like this and just standing there do you think he's doing work well let's see is he putting a force on that barbell yes he is he must be pushing up on it right otherwise due to its own weight the body would have fallen down so he is balancing the weight of that barbell pushing up on it so he's a force on it but he's just holding it over there so do you think he's doing any work I'm pretty sure you can now answer this no he's not doing any work because he's not making that barbell move now of course he might say that no just standing like that he's getting very tired he starts sweating profusely and he's getting super tired but just because you're getting tired doesn't mean you're doing work in physics and that's important okay so yeah he's not doing any work because he's not displacing that barbell on the other hand now imagine if he starts moving that barbell up mmm now he did work because now he pushed it and while pushing he displaced it okay one last example now let's say we take that same barbell and we just drop it now do you think there was work being done on that barbell when it was falling down think about this all right at first we might say no there's nobody pushing on it so clearly nobody's doing work but remember to do work you just need a force people don't have to push on it any force will do and you might know there is force of gravity pulling down on it and that's why it's falling in the first place right so gravity is pulling down on it and as it's being pulled down notice it is displaced it's falling so it's moving and therefore work was done on that barbell it was not done by people in this example but it was done by gravity any force can do work as long as it can displace that object it doesn't have to be living things okay now that we know when work is being done in physics let's go ahead and calculate working for a particular example so let's say this lady pushes the grocery cart with a force of 30 Newtons and displaces it by two meters as is the letter that we use for displacement what is the work that she does on that grocery cart can you try and answer this question okay work done is force times the displacement right so in our case the work done will equal to force which is 30 Newton's times the displacement which is two meters so that gives us 30 times two is 60 and now let's look at the units of this the units become Newton into metre so the units of work done becomes Newton meters now because work is something that we'll be using in physics over and over again we decided not to write Newton meter over and over again so we gave Newton meters a name we call Newton meter a Jew given by capital J so capital J stands for a Jew it's named after a scientist and so the unit of work done is also joules so over here we can say she did 60 joules of work on that shopping cart now before I wrap up one question you might have is what is the meaning of Newton meter what does it say or why do we even define work as force into displacement where does it all come from well we can only answer this in the future videos once we start introducing energy and we start connecting work and energy and and they will all start to make sense okay but as of now in this video what did we learn we learned that in physics work is Force Times displacement which means that to do work you need to push on something or pull on it and while pushing or pulling you have to make it move okay we also saw its units turns out to be newton meter and newton meter is often called Joule so newton meter or Joule is the standard unit of work