[Music] hi welcome back to ap physics 1 unit 4 um the work energy power unit so in this video we're going to look at that first concept known as work okay so work is defined as when you apply a force um over on an object and causing that object to move all right so um really so if you look at our gentleman here with the briefcase right and so we've got this force he's applying a force on the object notice that it is a vertical force but when you look at the next picture it shows that he's actually doing no work on the object and so that's because the force that he's applying is not in the direction of the force so that force has to be parallel to the motion okay now for work really to be done you have to look at it in three ways right so you have to have three conditions met the first one there has to be a force applied okay no force equals no work all right and why because it means there's no change in motion of the object the second thing there has to be motion right no motion [Music] equals no work oh it's about the word work put now so if an object doesn't move or does not move in the direction of the force there's no work being done and now here's the last condition there has to be cause and so what we say is the force applied has to cause the motion and so that's the that's this scenario with the gentleman the briefcase right if he's not he's applying a force yes but the force that he's applying does not cause the change in motion of the of the briefcase okay so sometimes what they like to do is they like to ask you those questions hey is there work being done or identified situation work is being done and they give you these different scenarios and you're asking yourself is there a force applied is there a change in emotion and then does that force cause the change in motion okay now to calculate work you can see the equation up there with the um up here with the roll with the lawnmower so work equals force times distance times cosine theta so this cosine theta only appears because remember we want parallel forces right or parallel directions okay and i put fours as in four for golf so we want parallel forces okay and that's why we have to include that cosine now each force that you apply to an object has the ability to determine to do work and so sometimes we you're asked is to find the network okay and so because work is dependent on that cosine so if you think about the relationships work is directly proportional to the force so more work or more force equals more work it's also dependent on the displacement or the distance traveled so the more you apply the force the more work there's done but it's also dependent on the cosine of the angle right or the angle so and and we're really looking at where does a lie between cosine of 0 and cosine of 90 okay and because cosine is 0 1 cosine of 90 is 0. now if you if your theta equals 0 degrees right excuse me i got that it should be 90 here's what i want to write there right so that means your work equals zero if your angle is between 0 and 90 your work is positive and you if your work is between 90 and 180 your work is negative and the big force that does negative work is friction uh okay so let's look at a couple examples so here we have a pumpkin we're lifting it up into up you know a 0.8180 meters to check it out how much work do we do on the pumpkin so we start with the equation work equals f delta x cosine theta well the good thing is we don't have to worry about the cosine part because we're lifting it straight up so our force is applied in the same direction as our motion so we can just drop it down to f delta x now that f can take on a lot of looks in this case the f that we have to apply to the object is the weight of the pumpkin so what i'm going to do is i'm going to substitute in the weight right to mg delta x because that is at minimum to lift the pumpkin right to get the move at a constant velocity we have to apply a minimum the weight of the object and so now i can plug in my numbers i have 3.2 right times um sorry i gotta grab my notes because i didn't bring my calculator with me so times 9.8 or you can use 10 depending on where this question lies and we're going to move it 0.80 meters so the work done is going to be 25 joules okay and this is the si unit for work it's joules named after jh prescott joule and the reason why we use joules is because later we're going to talk about how work is a transfer of energy so work and energy are essentially the same thing and so that they have to have the same unit sometimes you will see this unit as a newton times meter because you're applying a force over a distance but we just simply just call it joules okay here in the next um here in the next example we're kind of putting it all together right so we have a drum set we're going to push with 24 newtons it's going to experience some friction uh and then we're going to slide it for 1.5 meters and the weight of the drum tells us is 54 newtons okay so way i like to do this is start with a free body diagram because the free body diagram helps you find what forces are actually doing work on the object right so if we're sliding this you know drum set to the right okay then that means the only forces that are doing work are going to be friction and the applied force so the normal force and the weight aren't going to do any work but they do have a factor here because normal force is in the friction formula right so mu fn which most of the time mu is or fn is mg in this case so what i can do is find the net force so the net work is equal to the net force times delta x and we don't have to worry about the angle because again we're pushing in that same direction and so that's going to be 24 errors let's put it let's do it variable first so 24 so it's gonna be our apply force minus our friction times our delta x well as our apply force minus friction which is mu mg right delta x and now i can plug in some numbers so i got 24 minus um 0.41 times 54. don't multiply this by 10 because they already tell us the weight if it was kilograms then you multiply by 10. and we're going to move it 1.5 meters and what you find is that the friction is 22 or roughly 22 it's a little bit greater than 22. and so the work done is going to be 3 joules right so the net work on the on this drum set is going to be uh three joules okay so in the next video we're going to talk about kinetic energy and then we're going to start talking about how does this all kind of fit together in something called the work energy theorem