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Physics: Newton's Second Law 38 Views
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Description:
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.
Transcript
- 00:02
Newton's second law- we're free fallin for you.
- 00:08
Tom Petty song... [mumbling.]
- 00:22
[mumbling]
- 00:26
[mumbling]
- 00:38
alright moving on here we go let's talk about fallin. it's probably safe to say [man rides motorcycle]
Full 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
- 01:40
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]
- 02:11
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
- 02:23
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
- 02:37
these are all lightweight things and as my toes will tell you something heavy
- 02:42
like a bowling ball definitely doesn't seem
- 02:44
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
- 02:53
freefall means only gravity is acting on an object. but we've got air all around [man smiles underneath parachute]
- 02:59
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
- 03:27
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
- 03:37
doesn't all that force mean more acceleration? we can figure this one out
- 03:42
at the bowling alley. after we're done hiring all our trophies.. you know. then we
- 03:46
can hit the lanes and start knocking down some pins. we'll take out purple
- 03:49
penny and rack up our first strike of the evening. okay okay gutter ball. we can
- 03:55
still pick up the spare .but this time we'll roll a penny down there- well at [bowling pins knocked down]
- 03:59
least that one didn't end up in the gutter and knock over any pins either
- 04:03
though. now before we get kicked out of the bowling alley for throwing around
- 04:06
spare change, which one took more force to get moving? the bowling ball or the
- 04:11
penny? yeah the bowling ball after all it has
- 04:13
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
- 04:29
the ball as it falls. well Newton's second law boils down to this force
- 04:34
equals mass times acceleration. so what are the forces being exerted on our
- 04:40
bowling ball and on our penny? well the Penny's 2.5 grams but we want
- 04:44
to convert that to kilograms to make sure we're using the same units that
- 04:48
make up a Newton. so we've got point zero zero two five kilograms times 9.8 meters
- 04:53
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
- 05:01
three kilograms times gravity we find gravity generates a force of about 72 [equation]
- 05:06
Newton's which is a pretty big difference in force, and which is why the
- 05:10
bowling ball is even knocked down any pins. while the penny just bounces off
- 05:15
of them as they laugh at it angrily. but there's another aspect to this whole
- 05:19
gravity thing that we need to understand. gravity is related to mass and distance.
- 05:24
the closer two objects are to each other the more force gravity exerts. and the
- 05:30
more mass an object has the more gravitational force it has. and we can't
- 05:34
forget that every object has its own little pull of gravity. yep even a penny
- 05:38
creates a teeny tiny gravitational field. and the bowling ball creates one that's
- 05:43
a little less teeny-tiny. of course both of these things are miniscule compared
- 05:47
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
- 05:55
each other like a virtual hug. now check out my trusty parachute here.
- 06:00
a parachute only works because of drag which we can also call air resistance.
- 06:05
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]
- 06:27
that usually only comes into play when the object is a dog .but it does also
- 06:32
happen on our motorcycle from time to time. so you told you. now there is at least one
- 06:37
place on earth where we can get rid of drag. NASA has a huge vacuum chamber in
- 06:44
Sandusky Ohio. they use it to test equipment that's going to be used in
- 06:48
outer space. and if you want actual proof that a bowling ball and a feather will
- 06:54
fall at the same, time well check out this video right. here you can go.
- 06:58
so acceleration is the result of unbalanced forces. but what does an
- 07:03
unbalanced force you look ?like it can be hard to actually see a force like drag.
- 07:08
no one ever says look up in the air it's a bird it's a
- 07:12
plane! no it's drag. yeah they don't say that. but we do have
- 07:17
one tool that can help us visualize forces the good ol FBD or Free body
- 07:22
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
- 07:37
for our FBD is to draw something basic to represent our object. we'll make our
- 07:41
hungry friend into a square like this. and we put a dot in the middle there to
- 07:46
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
- 07:56
very beginning of the fall the moment gravity kicks in well there wouldn't be
- 08:01
any drag. but as soon as oh let's call him Willie
- 08:04
to avoid any lawsuits .as soon as Willie not Wiley but Willie .starts to gain
- 08:10
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
- 08:26
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
- 08:47
velocity happens there won't be any more of it.
- 08:49
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
- 09:00
is really clever from the answer there. how about an FB D for us on our super
- 09:06
bad motorcycle picking up speed before we hit the ramp. Oh what would that force
- 09:10
diagram look ?like so we can draw the ground first a plane a horizontal line
- 09:14
will work just fine and then we can simplify ourselves. looks like well
- 09:19
really capture my best side there yeah. now let's give ourselves a center of
- 09:24
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]
- 09:36
vector. and we've got the normal force from the ground pushing us up then we've
- 09:41
got our engine working in the forward direction we'll call that F sub a for
- 09:46
applied force. and any time you've got motion in any direction
- 09:50
you've got friction working the other way, so that's F sub F. like we said we're
- 09:55
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
- 10:05
but we know that we are accelerating. so here we go.
- 10:11
all right well now as we're flying over these buses let's take a minute to
- 10:16
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
- 10:30
we've always got the big G to deal with. straight down. for that vector but the
- 10:35
normal force vector isn't straight up. the normal force is always perpendicular [lever used to illustrate forces]
- 10:39
to the surface the object is on. so it'll be going up at an angle this away. and
- 10:46
here's where things might get a little tricky. our motion is going down the ramp
- 10:50
in this direction. any time we have motion we've got friction acting in the
- 10:55
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
- 11:04
brakes and holding on for dear life. you definitely won't be given any gas. but we
- 11:08
do have acceleration it's just negative acceleration .so if the positive
- 11:13
direction is the direction of motion this negative acceleration will be in
- 11:17
the opposite direction. like the friction. well here the brakes are providing that
- 11:21
negative acceleration so we can consider that our applied force. we don't want it [diagram explaining forces]
- 11:26
to overlap with the friction vector so we'll draw it okay. about that one. all
- 11:30
right and we've only got the best brakes on our bike so we'll assume that the
- 11:34
braking force is stronger than friction. remember diagrams were meant to help us
- 11:38
think about what forces are acting on an object. if we have to improvise a little
- 11:42
bit well that's just fine we just need to make sure our diagram is clear and
- 11:46
understandable. okay coming in for landing now we're sure
- 11:49
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.
- 12:04
Newton told us a long time ago that force equals mass times acceleration and
- 12:08
that's never gonna change .a law is a law after all .so even as I'm lying in my
- 12:14
hospital bed drinking all of my meals through a straw there's physics going on [man in hospital bed]
- 12:18
all over the place. but don't worry I'll be back on my feet soon you can't keep a
- 12:22
daredevil down. I wonder if you could attach some rocket
- 12:25
boosters to this bed and clear the grand canyon.
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