okay so the first one we have here is um what hap emma we have an external torque okay this external torque in this case i'm going to talk about hands on a pottery wheel right so here's kind of the side view of the wheel right and we've got one hand here we've got one hand here pushing on it and there's friction here right and we would have some l initial and as the hands interacted with the wheel it would turn into a final angular momentum now this torque is external and that's key is this a conservation of angular momentum problem no because you're not worried about the angular momentum of the hands and the angular momentum of the wheel you just said hey let's talk about the angular momentum of the wheel right so this actually right we recognize would be better approached with torque equals delta l over delta t okay can i draw this looking down from above is that okay all right if i look down from above i have a hand here and a hand here let's make it go around this way if it goes around this way there's a frictional force here there's a frictional force here and how did i know to draw them that way it goes against the motion right and so now i have those two forces could we find the torques yes and there would be two of them they would sum up i'm going to ask you do we want up to be positive or negative okay so up is positive okay and uh ryan you know how to find these torques this isn't even a hard one because it's 90 degrees right so i would have one torque here let's think about the left hand one okay the left hand one the force is upward the the force is trying to twist it this way yes but but emma said that up is plus but this would be down right so it must be a negative torque okay so we have a minus force of friction times r for that one you didn't like that no i didn't do my left i didn't do it with my left hand the left-handed one right now i have to do the right-hand one okay we got to be good at this as long as you're good everybody's good now the right hand one right yes the force is down it's trying to spin the wheel i don't care which way the wheel is spinning right i'm asking about which way would that force like to spin the wheel yeah that force is down it's trying to spin the wheel that way still in with the picture which would be down so i get another one minus force of friction r equals l final minus l initial over however long it works and if you knew if you knew more information if you knew the coefficients of friction or you knew how hard they were pushing on it where we could put numbers into this and do it but this looks remarkably like our previous f equals delta p over delta t yeah and it works exactly the same way once again we know to use that because we haven't asked about the angular momentum of the whole system right you know i i suspect you better be sitting on a good seat because if you go like this on a 200 kilogram pottery wheel right you're going to have a tendency to go okay if you grab on to it right but we don't we're not going to worry about that all we've said is hey we're dis focused on the wheel with these external forces rather than the system of you and the wheel yes no it's not no wait a minute sorry maybe you're asking if this was a real pottery wheel a real pottery wheel has one of two things either it has a plate on the bottom that the potter kicks to keep it moving okay or it has an electric motor attached to it okay but this is just a big stone spinning around but what you're or what you're saying is well wait a minute isn't there inertia isn't there some force that keeps objects moving but no no ah stab me in the heart right when you shoot the ball through space you don't need a force to keep it moving you shot it no there is no other force yeah it just has inertia so it can would continue to spin unless we act on it with torques very you're you you've gone very aristotalian on me we got i got to keep you newtonian right things in motion stay in motion unless acted on by a force things spinning stay spinning unless acted on by a no use the other word the fancy word the torque yes all right okay now one where we have no external torque shall be no external uh now we have no external torques we have the problem that that brook want wanted to see okay uh so i've got i've got a bird feeder and a squirrel okay you know squirrels love to raid your bird feeders right uh this it's a three kilogram feeder it's hanging on a 1.5 meter string it's a two kilogram squirrel and it it jumps horizontally i know you don't jump horizon but we're going to keep it easy right it jumps with one meter per second and it's it's straight okay so the question here is what's the what's the angular velocity of the bird feeder after the squirrel collides with it did you hear the magic word what was the magic word collides that's a big clue right or just from reading the idea here is this an external set of forces or are they torques are they internal they're internal so this is a conservation of angular momentum problem okay we have our b4 right hanging bird feeder squirrel one day come on don't laugh at my squirrel okay did you laugh at my squirrel she did all right so that's the before and then we have the after right and this is just before and just after where our hapless squirrel right is clinging for dear life to the bird feeder so it doesn't get thrown off all right justin ah addison okay so the total angular momentum before equals the total angular momentum afterwards right so you want the easy question the hard question to come next okay what's the angular momentum of the bird feeder before yes why careful because it's not rotating okay so yes l of the squirrel initial plus l of the feeder initial equals l total afterwards and yep that one is zero ian pick another one ain't no i'm gonna scroll now it's a funny one right because it's going in a straight line okay so i'll put my origin up here yep i'll look and say hey here's the dashed line of the linear momentum vector what's the straight uh perpendicular distance that would be up i'm going to look at that there's my l equals 1.5 so i have what is it a two kilogram squirrel one meter per second kilogram times 1.5 meters plus zero yes okay sarah jane's dad yeah okay phil phil did everybody say hi to phil okay now i have to do the after right okay well first of all if the squirrel is hanging onto the bird feeder are they rotating together yes they are so i could expand this to say i of the feeder omega plus i of the squirrel omega but those two omegas would be the same wouldn't they because it's they're rotating together so we could make this i of the feeder plus i of the squirrel times omega and hopefully we have enough information in there to find the eyes so that we can find the omega so far so good now emily that that both of those now they kind of look like point masses don't they right we don't go to the table and find an eye for bird feeders we don't find an eye for squirrels but you know we did just get done looking and saying oh if we have blobs of masses following circular thing we know how to do that right all right so this would be i the feeder would be m three kilograms times 1.5 meters squared i'm going to take that one away plus the squirrel the squirrel is two kilograms the squirrel is also located at 1.5 meters and we could take our handy dandy calculator and pull the numbers in here and pull out the final omega yeah now after we have the omega we might have to do something how high does it swing i don't know we'd turn to conservation of energy or something like that but basically this is a conservation of angular momentum problem and they come in all sorts of varieties like a dog runs across the playground drums on the merry-go-round the little kids are on the merry-go-round and somebody pushes their big brother off the the child is sitting at the edge of the merry-go-round and doesn't feel safe so they crawl to the middle right there are all sorts of these variations that but all of these conservational momentum angular momentum problems hey what's the initial what's the final okay it's just a question of parsing the words uh you could see a variety of these the classic one for in physics land charles yes is the ice skater right because you've always you've watched the olympics right right the ice skater is out there and they get to the dramatic end of the performance and they're there and they're spinning and suddenly they pull their arms in really really tight and they spin really really fast and i don't know how they do that without throwing up but i guess it's a learned skill yes but here i'm not going to work this one out necessarily but there's no collision here right this one they just changed their moment of inertia so you end up with i initial omega initial equals i final omega final because angular momentum is conserved as long as there are no external torques and that's the whole key to that spinner that that skater spinning around frantically they start with their arms out here and they're spinning and suddenly they do this and you can watch them and they make themself as skinny as possible so that their moment of inertia goes down and when their moment of inertia goes down their omega has to go up because they're on ice and it's really really slippery and there are no external torques and they do they do this frequently dancers do this acrobats just divers any divers no divers this year divers do this right they will tuck in really tight and spin spin spin spin spin and then they'll fling their arms out and their feet out so that their moment of inertia gets big so that they stop spinning so that they can go straight down into the water right but these are again they're all just applications of the fact that momentum is conserved but you can change the moment of inertia and change the spinniness yes the force that well actually the force comes from their arms okay because their arms would rather be out here than in here so their arms are doing this and exerting the torque that makes them spin okay it's a little hard to see in this it's easy to see in the bird right here you have to believe in conservation um oh we're doing good okay so like i said um robin yes um a dog on a merry-go-round runs from an edge to the center of the merry-go-round okay it's just changing the moment of inertia so you know what the system was before or after and two different eyes before and after okay doesn't always have to be a collision okay now it's unfortunate that you i need you because you have the longest arms i'm sure yes it's demo time well you look like you had the longest you have longer arms than robin is your health insurance up to date let's see here you go have a seat put your feet on the bar yeah the little the little bar right there now we're going to start with them in because i believe in safety if you start with them out and you pull them in you're going to go really really fast and fall off if i start with them in and you push them out what are you going to do you're going to slow down you won't stop okay you ready okay push them out oh my goodness pull him in yeah push him out right you could be an olympic skater okay now oh he's still oh he did give me a string okay this is why you need long arms i'll take these now you're going to hold this like this one hand on top one hand on the bottom i'm going to pull this and it's going to spin this way okay if it spins this way which way is the angular momentum it will be down and i'm going to ask you to turn it over okay that's that's what we're going to do but now if if this system starts with down angular momentum and he turns it over what's going to happen he's going to spin because we have to keep the whole system having down angular momentum and so in fact i predict you're going to spin around this way well i know i got to get it you ready i'm just going to turn it when you start well i'm going no you i'm going to pull the string all the way off and then you can turn it whenever you want okay hang on now turn it over oh i got my vectors wrong turn it back yeah because the system has to keep the same angular momentum you like that yeah what are we going to do to stop we're going to exert a torque on the side that's external see i get all the pieces in here in one day right thank you yeah give him a hand give him a hand all right end the show keep put it down yeah wait for it