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smell impulse problems maybe we can't control impulses but when we can solve [writing on chalk board]

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them you know it's a graphing them that's one way to solve I mean that was

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our kind impulsive formula clever I mean for some impact that's Steven Seagal

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America where like when Tiger Woods hits the golf ball there it's impact you all

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right well you know jugglers don't get a lot of respect these days juggling used [juggler on busy street]

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to be a pretty good career few hundred years ago you could travel the

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countryside as part of some sort of merry band and if you were lucky well [juggler as part of a band]

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maybe you'd get to perform for a king or a queen or at least maybe some lousy [juggler as a mime in front of royalty]

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Duke or something well these days it's a tough gig but I've got a PhD in physics

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and I've got to put my degree to use somehow okay so this is an

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unconventional career choice for someone with an advanced physics degree and sure [man talking from couch]

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all that time I spent studying the best ways to detect neutrino interactions

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doesn't really help me keep five burning torches from setting me on fire but as

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you probably realize by now there's a lot of physics going on in juggling for

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example my hands have to provide a certain amount of force to throw a ball

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up in the air and they also have to provide a force to stop the ball from

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falling on its way down and when I'm doing all of this while riding a

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unicycle yeah there are a whole bunch of forces going on all right let's focus on

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the moneymakers the hands these puppies are what bring in the crowds and they're [hands on screen]

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what creates the impulse that changes the ball's momentum [writing on chalk board]

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well impulse is the measure of a change in momentum another way to describe

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impulses as a measure of force applied over a span of time so when I throw a

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ball the air my hands and arms apply force to the ball to give it enough [juggler with two balls]

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acceleration to move upwards and that acceleration also affects the balls

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momentum well same thing when I catch the ball my hand applies force to create

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negative acceleration slowing the ball down and reducing its momentum and all

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this impulse stuff is graphical woohoo we can whip up a force versus time graph

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and just start drawing all over the place well not all over the place the [graph on screen]

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graph should actually you know make sense all right well here's a graph of

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me chasing after some key who stole one of my juggling balls the

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amount of force being applied is reduced over time as I lost my breath and you

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know slowed down all right we can calculate the impulse by finding the

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area of this triangle that our graphed line creates everything under this line

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represents the change in momentum well how do we find the area of a right [writing on chalk board]

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triangle yeah well the good old Pythagorean theorem it tells us that a

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squared over B squared equals C squared well C is the hypotenuse of the triangle

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and a and B are the other sides yeah it doesn't matter which side is a which [graph on screen]

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side is B just so long as neither of them should actually be C you see all

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right well now we could have all sorts of other shapes on this graph rectangles

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trapezoids ocean waves okay the last one would mean there's some pretty crazy

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impulse going on but well you know still if you need to refresh your geometry

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skills well you can find the handy dandy little shmoop learning guide here all [Shmoop website]

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right well the equation for impulse is pretty straight forward impulse which is

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that J signe there that equals force times a span of time remember just

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because something happens really fast doesn't mean that it happens

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instantaneously at least not in physics terms like when a 6-iron hits a golf

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ball well the ball in the club are only in contact for less than half a

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millisecond which is about a thousand times faster than it takes you to blink

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your eye admit it right now you're trying to blink your eye really fast [woman blinking on couch]

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aren't you yeah try as hard as you can but your blink will still take about a

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hundred milliseconds but less than half a millisecond is enough time for us to [woman using calculator]

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calculate an impulse we just saw the equation for impulses force times time

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but another way to find the impulse is to figure out the change in momentum [writing on chalk board]

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over time so we can find a starting momentum in a final momentum subtract

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that starting figure from the ending one and get our impulse now I don't know how

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you play golf but when I play the ball starts out and not moving at all meaning

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its initial velocity is zero and let's say that after impact the ball moves it [man hits golf ball]

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in 85 miles an hour sure the pros hit the ball with a bit more speed but well

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that's why they're pros I'd like to see them juggle four chainsaws

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at a time 85 miles an hour converts to about 38 meters a second so that's our

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final velocity to calculate the final momentum though we also need to know the

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ball's mass like any good juggler I know the mass of every single spherical [man measuring balls]

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object I own so this ball has mass of 45 grams or point 0 4 or 5 kilograms

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momentum is mass times velocity in point zero four five kilograms times thirty [writing on chalk board]

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eight meters a second gives us a final momentum of 1.7 kilogram meters a second

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and since the starting momentum is Zil to our final momentum is also our

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impulse well those units are important enough

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for us to take a quick look at them the units for impulse and momentum are

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kilogram meters a second which makes sense since momentum is mass measured in

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kilos times velocity which is measured in meters per second you following here

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people let's take a look at our first equation force times the time span

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well forces measured in Newtons otherwise known as kilogram meters per

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second squared and time is measured in well seconds what happens when we

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multiplied Newtons by seconds well one of the instances of seconds is cancelled

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out on top and the bottom leaving us with just a plain old s on the bottom

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there now 1.7 kilogram meters a second looks

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like a pretty small number which is because we're using a pretty small ball

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but how much force is required to generate that momentum well to find out

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we can rearrange our impulse equation if impulse equals Force Times time then

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force equals impulse divided by time and our time span is just an itsy-bitsy

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little thing about 41 10 thousandths of a millisecond four point zero zero zero

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four one seconds yeah when we divide our impulse by that number we find a force

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of about forty 100 Newton's which is not an itsy-bitsy number at all 4100 Newtons

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is about 920 pounds of force it's enough to make you feel sorry for that poor

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golf ball well another important thing to [Isaac Newton's portrait]

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recognize is that Newton's laws of motion x' aren't going anywhere so when

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I'm juggling bowling balls and I apply a certain amount of force upwards on the

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ball to get it airborne the ball applies the same amount of force down on me [man juggling balls]

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luckily I've got more math than a bowling ball so I'm not pushed through

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the stage here although my arms sure do get tired

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and the life of a juggler isn't as glamorous as it seems people in fact the

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fast-paced lifestyle of juggler makes me fantasize about how to get away what if

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aliens were out there just looking for someone to take back to their home [flying saucer in space]

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planet of course they'd probably have little probes first so what yeah moving

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on alright say there's a probe out there with a mass of four kilograms yeah not

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moving just chilling in space analyzing data to find the perfect specimen to

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teach the beings on its world all about it no I don't know you know the finer

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points of the Boston shuffle well it senses an incoming asteroid so it fires

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its thrusters to get out of the way and here's a graph of the force versus time

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there what's the impulse on the probe and what's its final velocity like I [graph on screen]

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said before when we have a graph like this we can find the impulse by

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calculating the area under the line well the last time we saw a graph like this

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we had a triangle to deal with because the force changed over time but now

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we've got a rectangle one of geometries greatest-hits really and to find the

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area of a rectangle we just multiply the base by the height with a height of 800 [writing on chalk board]

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Newtons and a base of three seconds we find an impulse of 2,400 kilogram meters

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a second now we need to find the final velocity it'll be a little more

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complicated but nothing we can't handle we know impulse is the measure of a

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change in momentum here's the equation when we break momentum down into its

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components we're trying to solve for V sub F and the probes mass is four

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kilograms since the alien probe started off with a velocity of zero well that

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initial momentum will also be zero so that leaves us with 2,400 kilogram

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meters equaling four kilograms times the final velocity but when we divide both

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sides over the equation by four kilograms we find a final velocity of

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600 meters per second Wow alien tech is no joke at least not the alien tech I

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daydream about but sadly no aliens are gonna take me away anytime soon so I

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gotta keep practicing days like this make me think I could have found a [man juggling]

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better use for that doctorate in physics now what job brings you more respect in

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juggling it does get frustrating sometimes though and sometimes I have to

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lash out like when I'm working on a new trick and the balls fall all over the

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place my passion might lead me to kick one of the

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stupid stinking falls across the room all right deep breaths remember the

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jugglers code if the ball has a mass of 0.1 kilogram said my foot sends it

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flying at a velocity of 12 meters a second what's the impulse on the ball

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and if the collision between foot and juggling apparatus last point oh three

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seconds what's the force of impact okay so impulse equals Force Times time

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right now we don't know the impulse we don't know the force and all we know is

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the time two unknown variables equals in the oh solution but just like last time

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we can use the other equation for impulse that's the one using the change

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in momentum well the ball was at rest when my toe connected so the initial

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momentum is zero the final velocity was 12 meters a second and when we multiply

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that by the mass point zero one kilograms we find an impulse of 1.2

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kilogram meters a second now we can go back to our first equation rearrange it

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to solve for force and plug in our numbers well 1.2 kilogram meters a

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second divided by 0.03 seconds equals a force of 40 Newtons I'm okay with that

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maybe I need to incorporate more kicking into my act so sure I could probably be [man kicking ball with foot]

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making more money if I used my physics degree to work for a saw or if I used it [man juggling inside NASA control room]

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in some sort of quantum computing job or even if I just flipped burgers but

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juggling isn't about the money it's about love of art and love of merely

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burning my face off with lit torches and it's about physics acceleration impulse

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forces all that stuff and more is involved in this ancient tradition which

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leaves me with just one more question specifically for my mom and I brought

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some money

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