Full transcript
0:00most people are familiar with the
0:01concept of gravity especially here on
0:03earth it's this force that's directed
0:06towards the center of the planet and
0:08acts on the objects here on earth so
0:11when I throw a ball into the air the
0:13force of gravity is acting on it the
0:14entire time so as it's going up the
0:18force of gravity slows it down and
0:20eventually causes it to accelerate back
0:22down to earth
0:24so it's Newton's second law force is
0:26causing an acceleration on a certain
0:28mass it that same force of gravity acts
0:31on us though here on earth as well
0:33that's why we're staying on the planet
0:35it's pulling us towards the center of
0:37the planet and only the force of the
0:40ground keeps us from falling towards the
0:42very center so hopefully a familiar
0:45process but what is actually causing
0:47gravity well its mass its mass and
0:52Newton actually came up with a
0:55gravitational law the universal law of
0:57gravitation this describes gravity here
1:00on earth and between any objects and
1:04that's an important distinction because
1:06what this law shows is that any two
1:09objects with mass actually exert a
1:12gravitational force on each other so
1:16technically you and if you're sitting in
1:19a chair right now you and the chair
1:21exerting force of gravity on each other
1:23so would you and anyone else in the room
1:26it's just that the amount of force is
1:29you should be probably very weak if it's
1:31say between two people because your
1:33masses are so small and compared to the
1:35mass of Earth your gravitational force
1:38is very very small and almost
1:41non-existent just on it doesn't match up
1:44it's not gonna make a big contribution
1:47because the force from the earth is so
1:49much larger because of it's much much
1:51much much much larger mass okay but
1:55let's look more at this formula so we
1:56have two masses mass one mass two well
1:59the force also depends on this R which
2:03is the distance between the two masses
2:05and you can see it actually depends on
2:08the inverse of the square again so in
2:10other words the bigger the distance
2:13separ
2:13two masses the week of the force and
2:15this force drops off drastically with
2:18the distance squared okay so let's think
2:21about this
2:22so according this formula depends on the
2:24masses so if I double the mass the force
2:27would also double that's assuming I
2:30double one mass if I double both well
2:31that would be four times because we get
2:33a 2 and a 2 which become four times as
2:35great of the previous mass what happens
2:38if I double the distance since it
2:40depends on the distance squared it
2:42actually have to take that number and
2:43square it so two squared is four
2:45well since it's inversely related or
2:47because the R is on the bottom in this
2:49formula we know the force would become
2:51four times weaker so I'm not gonna
2:55actually ask you to calculate the force
2:56in any case because there's also this
2:58big G this G is a constant a number a
3:01really ugly number with a lot of units
3:04that you don't even need to know so
3:05you're not gonna be asked to calculate
3:06it but you do need to understand the
3:08relationship again if I triple the mass
3:10the force becomes tripled if I double
3:13the distance yeah it becomes four times
3:15weaker if I tripled the distances become
3:18nine times weaker so you do want to
3:21understand how this formula works and
3:23how it relates to mass and distance and
3:26it makes some sense that explains why we
3:29exert a good force on the moon and it
3:32orbs around us as compared to it being
3:34sucked away by the Sun the Sun is way
3:37more mass than we do but it's so much
3:39further from the moon than we are so
3:42even though we have less mass we exert
3:44more of a force than the Sun on our moon
3:46and the same with us same reason you
3:49know flying off Earth and go towards the
3:51Sun is because the force from Earth is
3:54much stronger because we're so much
3:56closer to it we're on the earth so
3:59that's why it depends on mass and
4:02distance and this actually describes the
4:06force of both masses this is going back
4:10to Newton's third law when mass one and
4:13pulls on mass to mass two technically
4:16pulls on mass one so when you have the
4:19force of gravity from Earth acting on
4:21you technically you're exerting a force
4:23of gravity on the earth as well
4:26it's just that your and your gravity
4:29remembers gonna be equal but opposite
4:33but technically the mass of the Sun is
4:37so much larger that we go back to
4:39Newton's second law F equals MA and so
4:42the acceleration caused on you is very
4:45large because your mass is not so large
4:48but the mass of the earth is so much
4:51larger that the acceleration is
4:52negligible so that's why technically
4:54yeah we all exert forces on the earth
4:57but they're very very very negligible
5:01don't really cause any kind of
5:02acceleration because the mass of the
5:04Earth is so large but when two objects
5:06are orbiting each other they are
5:08exerting forces on each other so as the
5:10Sun pulls on us we technically pull on
5:13the Sun and this force of gravity also
5:20ends up explaining why objects orbit
5:23each other in the case of the Sun we
5:26have this nice image here to help us out
5:27is at well any given time our planet has
5:30its certain inertia inertia to stay in
5:33the direction it's going so this moment
5:35right here
5:36we're consider the planet has its
5:38velocity in this direction if we're
5:40taking a snapshot of time our planet
5:42orbiting the Sun well at this exact
5:44moment it happens to be going in this
5:46direction the thing is the Sun is
5:50actually pulling on it it's actually
5:53pulling on the planet because of the
5:56gravitational pull and technically the
5:58planet pulls on the Sun but again it's
5:59much less of an impact year because the
6:02Sun is so much more mass but this force
6:05can be thought of as having two
6:07components one component that's actually
6:09a change of speed which is why the speed
6:12of the planet changes throughout its
6:14orbit but it also has a component that
6:17would be pulling it in this direction
6:19changing its direction remember
6:21acceleration can change speed and
6:23direction so because of the sun's pull
6:27it's actually gonna modify the path the
6:29planet takes this is similar to if you
6:32had something attached to a swing or a
6:34string and we're spinning it around the
6:36string is actually gonna keep it in a
6:39circular orbit
6:40because it's pulling inward so the
6:42planet even though its velocity right
6:45now is going in that direction and
6:46because it's inertia would keep us going
6:48in that direction but there's an outside
6:50force from the Sun the gravitational
6:53force causing it to start spiraling
6:56along on this path and that's why we
6:59orbit around the Sun is pulling on us
7:01and changing the path we take and that's
7:04the exact same explanation for why the
7:06moon orbits us our gravitational pull is
7:09pulling on it causing it to start
7:11curving around in this orbit same as if
7:15we launch something in a space a
7:16satellite and it's orbiting us the same
7:19idea it's always this fight between
7:22inertia and gravity gravity pulling and
7:25causing us to modify our path if the Sun
7:29wasn't there the plan would just fly off
7:30in this direction
7:31but since it is it pulls us around and
7:34around in a loop