Physics: Newton's Second Law

Newton's Second Law states that force equals mass times acceleration. We hope our explanation of it doesn't go over like a, uh... lead parachute.

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Transcript

00:43

we've all taken a tumble once or twice like when you know you trip over your

00:47

dog or miss that last step coming down the stairs. or when you try to

00:52

use a rocket-powered motorcycle to jump over a canyon and things yeah don't go

00:58

well. well it turns out there's a lot of physics in this whole free-falling thing.

01:01

see we've fallen all these times for science. yeah for science .not narcissism.

01:09

now let's talk about free fallin. well in physics terms freefall occurs when the

01:15

only force acting on an object is gravity. meaning there's no normal force

01:20

or buoyancy or tension holding us up. it is just us headed down toward the ground. [Man and motorbike falling]

01:25

hoping our safety parachute there works. our old buddy Isaac Newton taught us

01:30

that if we have unbalanced forces we're gonna have acceleration. and if we only [man writes with quill pen]

01:36

have one force that's acting on us well that's pretty darn unbalanced. and here

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on earth the acceleration of gravity is 9.8 meters per second squared. and that

01:45

goes for everyone and everything it all falls at the same rate. you drop a [Man parachuting]

01:49

feather a congressman in a bowling ball all at the same time well and everyone

01:53

knows that they'll hit the ground yeah at the same time. okay so that's not

01:57

exactly true in real life because of a little thing called drag. which is the

02:02

force of friction created when a solid moves through a fluid. but if you drop a

02:07

bowling ball and a penny at the same time and from the same height they will [girl smiles from the top of the stairs]

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hit the ground at the same instant hmm. well for a lot of people that doesn't

02:14

make intuitive sense. after all we've all seen a sheet of paper gently swaying

02:18

back and forth as it falls or like that feather in the forrest gump member? and

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that piece of paper never lands with a thud like a bowling ball or a

02:27

congressman. same with the leaf falling from a tree or that strand of cat hair

02:32

that seems like it's defying all laws of physics by just floating there. well

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these are all lightweight things and as my toes will tell you something heavy

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like a bowling ball definitely doesn't seem

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to just float in the air. and I'm gonna be honest real actual freefall never

02:49

happens. at least not here on earth under normal circumstances. like I said

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freefall means only gravity is acting on an object. but we've got air all around [man smiles underneath parachute]

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us. don't get me wrong I'm very grateful for the atmosphere. I'm a big fan of

03:02

breathing. but all that air means we're always gonna be dealing with drag ,even

03:07

if it's much weaker than gravity .okay so technically speaking freefall really [Man standing with open parachute]

03:11

isn't impossible unless we're in a vacuum. but we can get close enough by

03:16

dropping objects that don't have a lot of drag, like that bowling ball in that

03:19

penny. so why exactly does the bowling ball fall at the same rate as the penny?

03:24

hmm? well after all the 16 pound bowling ball

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has about 7.3 kilograms of mass. and a penny ?well about 2.5 grams. which means a

03:32

falling bowling ball generates almost 3,000 times more force than a penny. why

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doesn't all that force mean more acceleration? we can figure this one out

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at the bowling alley. after we're done hiring all our trophies.. you know. then we

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can hit the lanes and start knocking down some pins. we'll take out purple

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penny and rack up our first strike of the evening. okay okay gutter ball. we can

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still pick up the spare .but this time we'll roll a penny down there- well at [bowling pins knocked down]

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least that one didn't end up in the gutter and knock over any pins either

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though. now before we get kicked out of the bowling alley for throwing around

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spare change, which one took more force to get moving? the bowling ball or the

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penny? yeah the bowling ball after all it has

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more inertia because it has more mass. and more inertia means harder to get

04:18

moving. and harder to stop moving too. just as it takes more effort from our bowling

04:23

arm to get that ball down Lane it takes more effort or force by gravity to move

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the ball as it falls. well Newton's second law boils down to this force

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equals mass times acceleration. so what are the forces being exerted on our

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bowling ball and on our penny? well the Penny's 2.5 grams but we want

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to convert that to kilograms to make sure we're using the same units that

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make up a Newton. so we've got point zero zero two five kilograms times 9.8 meters

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per second squared giving us a force of point

04:56

zero two five Newtons. as for that bowling ball when we multiply it seven point

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three kilograms times gravity we find gravity generates a force of about 72 [equation]

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Newton's which is a pretty big difference in force, and which is why the

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bowling ball is even knocked down any pins. while the penny just bounces off

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of them as they laugh at it angrily. but there's another aspect to this whole

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gravity thing that we need to understand. gravity is related to mass and distance.

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the closer two objects are to each other the more force gravity exerts. and the

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more mass an object has the more gravitational force it has. and we can't

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forget that every object has its own little pull of gravity. yep even a penny

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creates a teeny tiny gravitational field. and the bowling ball creates one that's

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a little less teeny-tiny. of course both of these things are miniscule compared

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to the planet Earth. so the effects of their gravity are too small to really

05:51

measure but they do exist. so it's like the penny and the planet pull towards

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each other like a virtual hug. now check out my trusty parachute here.

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a parachute only works because of drag which we can also call air resistance.

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well there are a lot of factors that play into how much drag effects an

06:09

object .those factors include the objects density which is why lead parachutes

06:14

never really caught on. and velocity also makes a difference less velocity equals

06:22

less drag. also the size of an object's tongue seems to have an effect .although [drag demonstrated with girl riding on a bike]

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that usually only comes into play when the object is a dog .but it does also

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happen on our motorcycle from time to time. so you told you. now there is at least one

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place on earth where we can get rid of drag. NASA has a huge vacuum chamber in

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Sandusky Ohio. they use it to test equipment that's going to be used in

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outer space. and if you want actual proof that a bowling ball and a feather will

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fall at the same, time well check out this video right. here you can go.

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so acceleration is the result of unbalanced forces. but what does an

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unbalanced force you look ?like it can be hard to actually see a force like drag.

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no one ever says look up in the air it's a bird it's a

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plane! no it's drag. yeah they don't say that. but we do have

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one tool that can help us visualize forces the good ol FBD or Free body

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Diagram. well a Freebody diagram is a way for us to sketch out all the forces that

07:27

are acting on an object. so let's do a diagram for an object in freefall. what [man rides motorcycle]

07:32

about when a certain bird chasing canine falls off a cliff. well the first step

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for our FBD is to draw something basic to represent our object. we'll make our

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hungry friend into a square like this. and we put a dot in the middle there to

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represent his center of mass. now cartoon coyotes might be able to ignore the law

07:51

of physics but our diagram cannot .so we need to draw out our forces there. at the

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very beginning of the fall the moment gravity kicks in well there wouldn't be

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any drag. but as soon as oh let's call him Willie

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to avoid any lawsuits .as soon as Willie not Wiley but Willie .starts to gain

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velocity drag will kick in acting in opposition to his movement. since he's

08:16

still picking up speed though the vector arrow for gravity should be longer than

08:20

the one for drag. that lets us know that the forces are unbalanced and that

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acceleration is occurring. eventually as the velocity increases. the drag will

08:31

reach a point where equals the force of gravity. that state is called terminal

08:37

velocity .by the way terminal velocity is the maximum velocity a falling object [terminal velocity explained]

08:41

can reach. its terminal, like it's terminaly ended its acceleration. when terminal

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velocity happens there won't be any more of it.

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so both arrows should be the same length there. no it looks like Willie had a rough

08:56

landing. listen in an ACME gift card to a cheer him up. if you don't think all this

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is really clever from the answer there. how about an FB D for us on our super

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bad motorcycle picking up speed before we hit the ramp. Oh what would that force

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diagram look ?like so we can draw the ground first a plane a horizontal line

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will work just fine and then we can simplify ourselves. looks like well

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really capture my best side there yeah. now let's give ourselves a center of

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mass. and we'll start adding in our vectors and we like to kick things off

09:28

with the one for gravity since that's never going away. at least none so we

09:32

finalize our plans to jump over that flag on the moon. so we'll draw the [the moon and earth pictured]

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vector. and we've got the normal force from the ground pushing us up then we've

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got our engine working in the forward direction we'll call that F sub a for

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applied force. and any time you've got motion in any direction

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you've got friction working the other way, so that's F sub F. like we said we're

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picking up speed here so that F sub a vector should have more magnitude than

10:00

the friction vector. that way we might not know how much the acceleration is

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but we know that we are accelerating. so here we go.

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all right well now as we're flying over these buses let's take a minute to

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do the force diagram for the other part of this journey we're on when we hit the

10:20

landing ramp and hit the brakes we can draw the ramp. as an incline and we've

10:25

got another square us there nothing tricky so far how about forces well

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we've always got the big G to deal with. straight down. for that vector but the

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normal force vector isn't straight up. the normal force is always perpendicular [lever used to illustrate forces]

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to the surface the object is on. so it'll be going up at an angle this away. and

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here's where things might get a little tricky. our motion is going down the ramp

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in this direction. any time we have motion we've got friction acting in the

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opposite direction. so let's draw our F sub F vector. but we don't actually have

11:00

any force that's pushing us in the direction of motion. well be hitting the

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brakes and holding on for dear life. you definitely won't be given any gas. but we

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do have acceleration it's just negative acceleration .so if the positive

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direction is the direction of motion this negative acceleration will be in

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the opposite direction. like the friction. well here the brakes are providing that

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negative acceleration so we can consider that our applied force. we don't want it [diagram explaining forces]

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to overlap with the friction vector so we'll draw it okay. about that one. all

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right and we've only got the best brakes on our bike so we'll assume that the

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braking force is stronger than friction. remember diagrams were meant to help us

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think about what forces are acting on an object. if we have to improvise a little

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bit well that's just fine we just need to make sure our diagram is clear and

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understandable. okay coming in for landing now we're sure

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everything will go just fine. Wow .yeah. our multiple fractures can prove we

11:56

always have forces acting upon us. there's no getting around gravity and as

12:00

long as we're on firm ground we've got the normal force doing its thing too.

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Newton told us a long time ago that force equals mass times acceleration and

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that's never gonna change .a law is a law after all .so even as I'm lying in my

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hospital bed drinking all of my meals through a straw there's physics going on [man in hospital bed]

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all over the place. but don't worry I'll be back on my feet soon you can't keep a

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daredevil down. I wonder if you could attach some rocket

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boosters to this bed and clear the grand canyon.