in this lesson we are going to introduce the idea of work i'm going to explain exactly what it is i'm going to do a few examples and so by the end of this lesson you will have a much better idea of what work actually means so the mathematical or scientific formula for work is the following f is the force acting on the object change in x is the displacement that the object undergoes and then cos theta is something we're going to explore later on but pretty much what it's going to do is it's going to help us to decide whether energy is being added to the object or if energy is being taken away from the object oh by the way work is the same as energy so if i say that a certain force is doing positive work it means energy is being added to the object and if i say any work is being done or work is being removed from the object or energy is being removed from the object then the energy of the object would obviously decrease but we're going to explore that a little later so for now i'm just going to say that the cos theta part is going to help us to decide whether work is being added to the object or taken away and so we can say that cos theta helps us decide if energy is added to the object or taken away so let's get started with some examples here we have a person who exerts a force of 5 newtons on an object and it causes the object to move a distance of 20 meters determine the work done on the object so we know the formula for work is the following and so they told us that the force is 5 newtons the distance or the displacement is going to be 20 meters and now with the cos theta part this is where it becomes interesting so let's say we've got a box that someone is pushing with 5 newtons and the box is moving a distance of 20 meters so what you've got to do is the following you've got to look at the direction of motion which is like this then you have to look at the direction of the force which is also like that now you need to look at the angle between those two well because they're going in the same direction that is zero degrees so what we will say is cos of zero now cos of zero is equal to one if the for the object was moving to the right but the force was going to the left that would be something like friction then the angle would be 180 degrees and so the cos of 180 is -1 and so that is how this part here helps us to work out whether the work is positive meaning it's being added to the object or whether it is negative meaning it's going to be taken away from the object and so cos of zero is just one and so five times 20 is a hundred now work is the same as energy so if you look at any food item in your fridge that is measured in joules of course we could say calories but when we are busy with physics we use the s i unit the standard international unit which is joules here's another example but wait just one thing we never choose a direction with energy you could say added to the object added to the object but you won't say left or right because remember energy is a scalar now just to get your brains starting to get used to a certain concept that's going to come up in a few in in one of the later lessons would this object speed up or slow down over the 20 meters well well done if you said speed up because if you add energy to an object it will speed up if you take energy away from an object it will slow down that's going to be quite important later on in number two we have a lady called mandy who's trying to move a fridge and she exerts a force of 75 newtons the fridge does not move determine the work done by mandy so remember work is a scalar so we do not have to choose direction so we don't do any of that we just go plug in the force which is 75 the distance that it moves is zero and we don't even have to look at this part because the object's not moving so there is no angle really so we can ignore that part because straight away we can see here that when you multiply something with zero you're gonna get zero so mandy is exerting a force okay so she is she is using some of her own energy of course she's getting red in the face she's trying to push this thing but she's not adding any energy to the fridge and i've just realized i spelt fridge wrong i wonder if any of you noticed sorry about that guys so she doesn't add any energy to the fridge she is using some of her own energy as i've said but she's not adding any energy to the fridge if she was the fridge would start moving it would start speeding up it would have kinetic energy so the energy that she is busy exerting it's probably just going to it's being wasted in the sound maybe she's making a noise trying to push this thing maybe her arms are starting to heat up a little bit so all of the energy is going into heating up her arms making her face red she's making a bit of a noise trying to push this thing but none of that energy is transferring to the fridge and so zero work is being done by mandy on the fridge and here's the last example for this lesson so here we have a person called felix who's holding a box in his hands and then walks 10 meters to the right he has to apply a vertical force of 20 newton to keep the object in his hands because obviously if he lets it go it's going to drop due to gravity determine the work done by felix to the object okay so here's felix i actually won an award when i was in school for my artistic skills and if you're paying attention you'll know that that's complete sarcasm so here we have a person called felix who is busy applying a force of 20 newtons upwards obviously because he has to keep the box from falling he then walks 10 meters to the right okay 10 meters we need to determine the work done by felix so let's see so we know that work is equal to the force so the force he's applying is 20 newtons the distance he's going to walk is going to be 10 meters now we need to look at the angle so think about this he's going in this direction but the force is in that direction and so that's a 90 degree angle and cos of 90 is zero and so the work done by felix is zero now that makes sense guys think about this he's not adding any energy to this box because you can imagine that felix is already walking okay and he's going to cover a distance of 10 meters so he's the speed he's not changing his speed okay we can assume he's walking at the same direction and so he's not adding any more energy to the box than what it already had and the main thing is is that the direction of his force is upwards whereas he's moving to the right so if you are simply holding something and then moving to the right or moving to the left or to the right you're not adding any work or energy to that object because your force is in the vertical direction whereas the object is moving in the horizontal direction and so that has a 90 degree angle and so zero work is actually being done and so guys that's a small little introduction to work and in the next couple of lessons we're going to go more into detail about the different types of questions and applications that you would need to know so thank you for watching