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Physics Videos 34 videos

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Physics: Impulse and Newton's Second Law 12 Views


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Description:

What is Newton's Second Law, and how long do you go to prison if you break it? Long enough to study friction, buoyancy, closed systems, and more.

Language:
English Language
Subjects:

Transcript

00:00

All right impulse and newton's second law the best pairing

00:04

since pb met jay did you fire him over when

00:10

and where A sound for buoyancy girl and seeing their

00:15

kids all right you're in second launching for men and

00:19

a funny thing emoji rico's graphic impulsive way her pg

00:27

thirteen site way legal we do ok moving on every

00:33

sport involves physics just think of the parappa ulla of

00:36

a basketball when steph curry shoots a halfcourt buzzer beater

00:41

for how when ah hockey player is checked into the

00:44

boards the boards push back on the player just as

00:47

hard as he slams into them or how the acceleration

00:51

of gravity can really ruin a gymnast day in their

00:55

most sports fans don't think about any of that stuff

00:57

when they're watching the game but if you're a physicist

01:00

while sometimes it feels like all you can see is

01:03

physics not ghosts but when you're actually playing a sport

01:06

you're going to be in big trouble if you've got

01:08

physics on your brain like when you're standing in the

01:11

batter's box looking to hit a walk off home run

01:14

well if you're thinking about how your bat's going to

01:16

change the balls Momentum Chances are pretty good that the

01:18

only thing you'll be hitting our some molecules in the

01:21

air But once you start thinking about this stuff well

01:24

it's hard to stop After all the system between the

01:26

batter the baseball bat and the ball itself is about

01:30

as close is will come to having a closed system

01:33

here on earth In physics terms a closed system is

01:37

where no matter enters or leaves the system and there

01:41

are no effects from forces outside of the system Got

01:45

that All right Well if i'm trying to catch a

01:47

foul ball and some jerk in the stands feels a

01:50

soda on me He's just introduced some matter into the

01:53

system made up of me and the ball Some very

01:57

cold sticky matter And i'm not happy about it All

02:00

right Well even in the system when i'm up to

02:02

bat there is still the force of gravity that's pulling

02:05

on everything And everyone on the ball also has to

02:09

deal with drag In fact it's pretty much impossible to

02:12

find a true closed system Of course if you could

02:15

consider the whole universe to be one big fat close

02:18

System Well that'd be a different thing but you hate

02:21

to see the equation for conservation of momentum in that

02:24

system Pretty big Well here on planet earth many things

02:27

can enter into a closed system Like the force of

02:31

friction We can all use sorts of goop to reduce

02:34

friction Yeah but we could never make it go away

02:37

completely And if you go bowling underwater while you're gonna

02:41

be dealing with drag and buoyancy so if you're hoping

02:44

to get a strike in the bikini bottom bowling alley

02:47

when well it'll take a lot more room than on

02:48

dry land So how do we figure out mo mentum

02:51

when there's an outside force messing with it Well luckily

02:54

for us some guy named isaac newton came up with

02:56

a way that can help Newton's second law of motion

02:59

tells us that if an unbalanced force is applied to

03:02

an object that object will experience acceleration Will this law

03:07

boils down one simple equation That equation says that the

03:10

force equals mass times acceleration We can tinker around with

03:14

this equation to show just how force and mo mentum

03:17

get along Alright now buckle up This might be a

03:20

bumpy ride Like we said we've got one of newton's

03:23

greatest hits Force equals mass times acceleration All right well

03:28

what is acceleration Acceleration is the change in velocity over

03:32

the change in time And get that little triangle there

03:34

Delta means change in whatever So let's get rid of

03:38

a and put this thing into our force equation instead

03:41

Okay now how do we calculate change in velocity It's

03:45

Not too hard We just subtract the initial velocity from

03:48

the final velocity so we can add that into our

03:50

equation too So now our equation for force looks like

03:54

this force equals mass times final velocity minus initial velocity

03:58

over the change in time Kick our algebra skills into

04:01

another gear and use the distributive property for that parent

04:04

medical Just in case you're having a bad math day

04:07

which hey happens to the best of us The distributive

04:09

property means that when we have a parenthetical where addition

04:12

or subtraction is being done and we're multiplying the result

04:15

by something outside the parentheses were gonna multiply that outside

04:19

term by each of the terms inside and then ditch

04:21

the parentheses altogether So instead of mass times the result

04:28

of final velocity minus initial velocity we can change it

04:31

to mass times final velocity minus mass times initial velocity

04:36

got all that so we're distributing the mass within the

04:38

parent medical Okay great fun little math trick but what's

04:41

the point Well we have mass multiplied by velocity In

04:45

fact we have that twice And what is mass times

04:49

velocity It's momenta More mojo more technically speaking So now

04:53

we can swap out the m v stuff and put

04:55

in momentum And instead of just saying final momenta minus

04:59

initial moment and we can call it the change in

05:02

mo mentum so now we have a whole new equation

05:05

for force to go along with the old school f

05:07

equals m a force equals the change in mo mentum

05:10

over the change in time and this new equation will

05:13

definitely come in handy But we can also rearrange the

05:16

new equation assault for the change in momentum Change of

05:19

momentum equals force times change in time I don't let

05:23

anyone tell you that math and physics aren't creative it's

05:27

like people who don't think baseball's creative sometimes you gotta

05:30

find a new way to avoid getting tagged out like

05:33

that right Well it's the same in science Creativity is

05:35

huge thing way have to figure out ways to get

05:37

from point a to point Q You can't just sling

05:40

the alphabet song under our breath to figure out how

05:42

to get there either But the other important thing is

05:45

this change in momentum right here Well changing mo mentum

05:49

is important because it happens all the time Momentum is

05:52

changing everywhere You look okay If you're not looking at

05:55

your cat sleeping in the sun where it maybe not

05:57

But you might look out your window and see car

06:00

slowing down at a stop sign or birds picking up

06:03

momentum as they fly off three brands or might see

06:06

the left fielder's momentous change very suddenly when he runs

06:09

into the shortstop there Changing mo mentum is so important

06:13

that it has its own names called impulse has its

06:16

own symbol to jay Well you know impulse starts with

06:20

a j jj impulse like that How you remember it

06:24

Look these symbols are always gonna make sense Just roll

06:26

with impulse is the measure of force applied over time

06:30

so it makes sense of units for impulse are newton

06:32

seconds or to break it down to base units Units

06:35

for impulse are kilogram meters per second just like mo

06:39

mentum Yeah because you know it is mo mentum or

06:42

at least the change in mo mentum And because impulses

06:45

force applied over time we can slap it onto a

06:47

graph The y axis is the force in the x

06:50

axis is the time like you might have seen on

06:53

other graphs Such a velocity versus time Here's graph for

06:56

a constant force flight over specific amount of time Let's

06:59

look at all this green area right here under the

07:02

line Don't get too excited This crap isn't going to

07:04

reveal the meaning of life Where is money python when

07:07

you need him it's just a rectangle And like any

07:09

other rectangle we can find its area with a simple

07:12

formula Yeah Base times height Remember that from like third

07:15

grade Well what is the height represent in this green

07:17

shape That would be force And the base is time

07:22

We'll force times Time is impulse So the area under

07:26

the line on our graph equals the total impulse we

07:29

knew Geometry class Would come in handy someday All right

07:32

Well what if we don't have constant force That's Harder

07:36

Well here's a graph for force that decreases at a

07:38

constant rate Same deal We find the area of everything

07:43

under the line and here we just use the formula

07:46

for the area of a right triangle Eight times be

07:48

divided by two Well anytime we apply a net force

07:51

to an object over any span of time the objects

07:53

acceleration will change anytime acceleration changes Velocity will change in

07:58

anytime velocity changes Mo mentum will change All of these

08:02

things are our separate but related to each other And

08:05

in the real world forces pretty much always applied over

08:08

a span of time That span might be tiny like

08:11

when a baseball hits a fast the ball in the

08:14

bat might only be in contact for a millisecond but

08:17

that still counts is a time span in physics but

08:20

most time spans air a lot longer than that Like

08:22

if we floor the gas pedal for five seconds in

08:25

our car or even when my feet hit the ground

08:28

is i round second base and it applies to negative

08:31

acceleration to like When our center fielder runs into a

08:33

wall trying to make it a catch In fact in

08:36

our impulse equation the force has a specific name The

08:40

force of impact and the relationship between changing mo mentum

08:44

in time determines just how hard that impact will be

08:48

In fact what makes something soft is completely linked to

08:51

changing momentum Let's look back at our center fielder here

08:55

Well you know he's getting to know the wall there

08:58

likes a great defender but he's going to get hurt

09:00

if he keeps on doing that or have to move

09:02

to game of thrones and try something with white walker

09:04

is doing that thing Yeah all right Well the wall

09:07

has a little padding to soften the blow but trust

09:09

me it doesn't feel too good Let's Say my kind

09:12

of law Save nine meters a second right before he

09:14

hits the wall and he has a mass of ninety

09:16

kilograms So right before he makes impact his momentum is

09:19

eight hundred ten kilogram meters per second What's his momentum

09:23

after he hits the wall Really kind of bounces off

09:26

of it So there's negative momentum here Walk it off

09:30

mike But there Will be a point in this crash

09:33

where his velocity reaches zero meters A second and that's

09:37

what we're looking for Shmoop so the change in momentum

09:46

will be negative Eight hundred ten kilogram meters per second

09:50

and let's say it takes a tenth of a second

09:52

for this change to be complete So what's the force

09:55

of the impact well we just rearrange our impulse equation

09:58

to solve for force before sequels impulse divided by time

10:02

Oh and we're not i'm gonna worry about having a

10:04

negative momentum We'll just use the absolute value Eight nine

10:07

and ten kilogram meters per second Invited by point One

10:09

seconds gives us a force of eighty one hundred newton's

10:12

out No wonder mike still down for the count after

10:14

seeing that nasty collision the grounds crew takes pity on

10:16

poor mike They cover the outfield wall with a few

10:19

layers of bubble wrap because bubble wrap sauce that makes

10:22

a change in momentum take a longer amount of time

10:24

So now instead of going from full speed to no

10:26

speeding point one seconds it'll taking half a second Well

10:30

how does that affect the force of the impact Wow

10:33

reduces it to one fifth of what it was Well

10:36

the new forces sixteen hundred twenty newtons which might not

10:39

feel great but it's a lot less likely to put

10:41

you on the permanent disabled list but this is why

10:44

most safety features in your car are meant to give

10:47

your moment and more time to reach zero that's what

10:50

airbags do people and crumple zones crumple zones air part

10:54

of your car that our mental collapse on impact by

10:57

collapsing they're distributing the mo mentum change in a crash

11:01

over a longer period of time than a rigid structure

11:04

would see knowing newtonian physics can help save lives And

11:08

just in case you want to prove it for yourself

11:09

go stand on your bed and then fall over on

11:12

your mattress kind of fun right now Do that on

11:14

the floor a lot less fun right Don't actually do

11:17

this by the way our lawyers just made us say

11:19

that Sorry Consider it more of a thought Experiment people

11:21

Physics is great but not worth breaking ribs over All

11:24

right well just remember even though the real world situations

11:27

are more complicated than ideal closed systems we still have

11:30

the tools to make sense of them The more equations

11:32

we know the more we're able to mess around with

11:34

them to make them do What we want him to

11:35

do so don't be afraid to breaking equation down to

11:38

its components and then build those back up in different

11:40

ways And don't let all this force of impact stuff

11:43

scare you too much Sure it can be a mildly

11:46

terrifying thing to think about a big change in mo

11:48

mentum over a super short period of time But we

11:50

can do things to slow that impact down Like you

11:53

know when i lay down a bunt i'm increasing the

11:56

time that force is applied to work in my favor

11:59

And hey maybe instead of always trying to hit a

12:02

home run i'll see if they have any of that

12:03

bubble wrap The left over there Yeah they'll never see 00:12:06.739 --> [endTime] this one coming

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