 Now we introduce a new concept called work. Now work is a word we're very well familiar with. You tend to think of it either as the job you go to work for your pay or the effort you put into getting a project completed. In physics, there's a very specific definition that we use for this term, work. And it's the transfer of energy to an object. Now in mechanics, we're going to specifically talk about the mechanical energy which is transferred when a force causes a displacement. So this is the key here, that when we have a force causes a displacement, then we're going to have mechanical energy which is transferred. So let's think about this here for a minute. If I've got an object, say this big box, and I've got a force pulling on it, well, if that force causes a displacement causing it to move, then that means I've got energy transferred into that object. The force caused the box to move, and that gave the box more energy than it had before. Similarly, I could have a force which resists the displacement. So for example, if I've got a box which is moving, but there's a force which causes it to slow down. Even if it doesn't cause it to slow down all the way to a stop, it still causes it to slow down. In that case, that force actually transferred energy out of the object, but there was still a transfer of energy that happened, so there's work in that case. Sometimes we have forces that don't cause any displacement at all. So if my box is just sitting there, well, I'd have a force of gravity downwards and a normal force upwards, but neither one of those forces is causing any kind of a displacement on the box. That means that no energy is transferred. If the force doesn't cause a displacement, there's no transfer of energy to that box. The forces are still there. The forces are still doing something, but they're not transferring energy. When we start talking about the language of work, we're gonna see situations where it talks about the work done by or the work done on. We wanna clarify what those things mean. If we're talking about the work done by, we're talking about what force is it that's providing that energy transfer. So if the work is done by a particular force or an outside agent which is providing that force, that's what the work is done by. When I talk about what the work is done on, that's the object. And so that's the object which is undergoing the displacement. Now we also see when we get into work that sometimes we have positives and sometimes we have negatives. Well, when the work done on an object is positive, that means the object gains energy. The positive means the gains it was transferred in. If I've got the work done on an object being negative, that means the object lost energy. I had a negative transfer of energy or the energy was transferred out of the object. So it all comes down to which way is the energy transferred into the object or out of the object. And then the object will gain energy or lose energy and will have a positive or negative associated with that work. This is just the beginning conceptual introduction to what work is. We still need to take a look at the equation for it though.