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No hello mo mentum an introduction that might knock you

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on your feet that momentum anyway Double espresso physics Who

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wrote this thing All right The math A momentum of

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their p equals We got you to come every right

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Conservation of momentum All right here we go Well a

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lot of people think that football is all about violence

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and those people Well they're not wrong Look the whole

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sport is about guys crashing into each other And you

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know what Sometimes it feels really good to land a

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nice clean tackle as long as you're the tackler not

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the tackle but football players aren't just big dumb brutes

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A lot of us are really smart Well some of

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us till we get hit in the head enough smart

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enough to know that this game is all about real

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world physics All right like when i sack a quarterback

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Yeah well what's really happening is that i'm building up

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momentum as i run And then i transfer some of

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that moment um to be but what is all this

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moment Um anyway well you probably have an intuitive understanding

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of momentum after all You know if a friend tosses

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you a golf ball it's not going to be too

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hard to catch if he tosses you a bowling ball

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in a different story But physicists know better than to

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just rely on intuition And relying on intuition is for

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english majors Not that there's Anything wrong with being an

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english major Of course So what is momentum in physics

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terms Momentum is the amount of movement and object has

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which feels weird We don't usually say boy look at

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that cargo It sure has a lot of moment Um

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momentum is related to inertia Remember inertia It's quite a

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newton's first law of motion one that says an object

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at rest tends to stay at rest and an object

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in motion tends to stay in motion so what's an

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object is emotional Keep on keeping on and awesome type

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of force changes that motion well that force khun b

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anything friction gravity my shoulder pads whatever And the more

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momentum and object has the more forces required to change

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that objects motion So let's go back the golf ball

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versus the bowling ball Like we said everybody generally toss

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you a golf ball Easy catch but there's no such

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thing as a gently toss the ball because the bowling

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ball has more momentum it's harder to stop its motion

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What if your friend really whips that golf ball Actually

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well it's going to be pretty hard to catch then

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and painful to hit you because a golf ball that's

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thrown faster has more momenta mme than one that's moving

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slowly man she's got a good arm maybe she could

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be our backup q b so mo mentum involves both

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velocity and mass In fact that's the whole equation for

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mo mentum mo mentum equals mass times velocity go inside

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now don't get confused with that lower case p there

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in the equation Yes momentum doesn't have the letter p

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anywhere in it but scientists used to use the latin

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for everything And the letter p was used in one

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of the original roots of the word mp tests which

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relates to mo mentum So you know just go with

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b o and don't get it confused with uppercase pete

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that stands for power and then there's row which looks

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an awful lot like the letter p but it isn't

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rowe is the symbol used for density Yes sometimes all

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these abbreviations just make things more complicated But let's get

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back the equation Since momentum is made up of mass

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and velocity it's based units are kilograms times meters over

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seconds which looks a lot like the base units for

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newton's remember and newton is it derived unit made up

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of kilograms times meters over second squared Pretty similar huh

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Well just one little superscript two that sets them apart

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But that itty bitty two is pretty important Newton's our

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unit of force and force equals mass times Acceleration and

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acceleration is measured in meters per second squared Well momenta

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muses velocity not acceleration So make sure you don't get

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confused and mix up your mo mentum with your newton's

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bad things will happen not bad is in the end

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of the world That is in getting the test question

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wrong which can sometimes feel like the end of the

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world have been there Well one of the components of

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momentum is velocity which is a vector quantity meaning it

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has both a magnitude and direction because one of its

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parts is a vector That means mo mentum is a

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defector too And it's really important that it is back

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when smarty pants people like rene day card were working

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to understand mo mentum Well they had a hard time

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with something See in a closed system there can't be

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any change in the total momentum within the system which

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means that if you have two objects each with a

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momentum of five kilogram meters per second The total momentum

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is ten kilogram meters per second If those objects interact

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with each other well then the total mo mentum after

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the interaction is still ten kilogram meters per second But

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if you have two bowling balls each with the same

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mass and velocity going in opposite directions and they crash

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into each other well you're going to end up with

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total momentum of zero and a really bad score in

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bowling momentum has to be conserved It can't just disappear

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The refs will definitely throw a flag if it does

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But this dutch dude christian huygens pointed out that if

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you set one direction is positive and the other is

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negative than everything works out just fine mathematically if you

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have one bowling ball with the momentum of five kilogram

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meters per second and the other with mo mentum of

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negative five kilogram meters per second Well known your total

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initial mo mentum is zero after they crash they'll still

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zero No momentum has gained and none is lost and

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loss of physics prevail As always Of course in the

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real world we're not in a lot of closed systems

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If i get the interception and head for the end

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zone all have a lot of momentum And if someone

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on the sidelines enters into this closed system and trips

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meanwhile all that mo mentum will be transferred into the

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ground two yards short of scoring out Well that's a

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case where something or someone enters into a closed system

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changing its internal workings But for now at least we're

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not gonna worry about some jerk tripping us up We're

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going to work with situations where mo mentum will be

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conserved within the system We've looked formula for mo mentum

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already Now let's look at the formula for total mo

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mentum When we have two or more objects interacting with

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each other it might look a little frightening at first

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But it's not as rough as it looks with this

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equation means in english is that the sum of mo

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mentum for all objects is the same before and after

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any interaction between the objects We can rewrite it toe

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look a little better like this Yeah while this equation

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says the some of the initial momentum is equal to

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the sum of the final moment um how does that

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work in the real world Well let's say that the

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quarterback standing still looking for an open receiver then i

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come along from the blind side and sack the heck

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out of them I've got a massive one hundred kilograms

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and i'm moving in about ten meters a second Well

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the initial mo mentum will equal the cubes mo mentum

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plus mine since quarterback is standing still his momentum's zero

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regardless of how much mass he has But we'll say

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he has a mass of eighty kilograms Those puny quarterback

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as for me my mo mentum will equal one hundred

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kilograms times ten meters per second or a thousand kilogram

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meters per second and the initial total momentum is the

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same So now once i hit him will basically act

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as one object and our combined mass will be one

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hundred eighty kilograms But our total mo mentum would still

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have to be a thousand kilogram meters a second which

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means the velocity would have to slow way way down

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In fact it would slow down to almost half five

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point six meters per seconds to be exact Yeah when

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i collide with hubie i transfer some of my initial

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mo mentum to him so the total momentum remains the

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same Now while this game of football's going on that

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doesn't mean i'm not taking care of things at home

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In fact thanks to modern technology i'm vacuuming my floors

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as well Speak thanks to my last contract extension i

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was able to buy a top of the line Roomba

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well first my cat princess flutter bottom wasn't down with

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this machine but now she hops on for a ride

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Princess has a mass of four kilograms and the roomba

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has a mass of three point five if they're traveling

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at zero point two meters a second to the southwest

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what's the total mo mentum ah cats and room bozz

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They share a connection as all this time and i

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can waste hours of time watching videos of them together

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Well we have separate objects here is much as it

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hurts to call princess and on the way but we

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can think of them as one combined mass after all

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since the cat is riding on the robotic vacuum cleaner

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And while they have the same velocity vector our equation

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is p e Equals m times v the combined mass

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is seven point five kilograms When we multiply that by

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point two meters per second we find a mo mentum

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of one point five kilogram meters per second But this

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is a vector quantity so we need to include that

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direction The official answer One point five kilometers per second

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to the southwest Good kitty Now on my days off

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from football i liked to sit in the backyard and

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relax with family because sitting is about all i'm capable

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of after a big game my nephew is planning on

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being the next tony hawk So he's always on a

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skateboard Of course he's Only three So he's not going

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to be doing a three Sixty anytime soon We hope

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he skates from left right and covers one hundred twenty

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meters in nine seconds If he and the skateboard of

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a combined momentum of two hundred twelve kilogram meters per

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second skateboard has a mass of one kilogram what's the

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mass of my nephew Okay nothing we can't handle here

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after all way Have our handy dandy mo mentum equation

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p equals m times v We've got our p we're

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trying to find em we don't have our vey well

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but we can figure it out real quick velocity is

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displacement over time and our displacements one hundred twenty meters

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and it took nine seconds to roll that far That

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comes to about thirteen point three meters per second Now

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it can plug those numbers into our equations First let's

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re arrange it so we're solving for mass If momentum

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equals mass times velocity than mass equals momentum divided by

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velocity So we divide two hundred twelve kilogram meters per

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second by thirteen point three meters per second and find

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a mass of fifteen point Nine kilograms See how nicely

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those units all cancel out leaving us with kilograms real

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nice but is that it Well not quite We're trying

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to find the kid's mass not the total mass way

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need to subtract the skateboard which has a mass of

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one kilogram leaving us with fourteen point nine kilograms which

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rounds the fifteen used the right amount of sick figs

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there Well hopefully you've built up a lot of momentum

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learning all this physics stuff Once you get started it

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can be hard to stop Sometimes Just remember that this

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starting mo mentum Within a closed system always equals the

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final moment Um and if we're dealing with more than

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one object when we have to add up all the

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mo mentum from the objects it's Time to cut this

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lesson short Turns out coach isn't too happy that i've 00:11:04.782 --> [endTime] spent most of this quarter talking about momentum

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