Hi. It’s Mr. Andersen and this AP Physics essentials video 55. It is on gravitational
forces. Remember there are four fundamental forces in the universe. We have gravity, this
attraction between all objects that have mass. Electromagnetism and finally the strong and
the weak nuclear force. Now how are gravitational forces different from the other three? First
of all they are always attractive, pulling material together. They also work at all scales
from the very very small to the very very large. And finally they are incredibly weak
compared to the other forces. But as the size gets larger then those forces become the dominant
force. And so a gravitational force exists between any objects that have mass. So if
these two objects have mass there is going to be a gravitational force between them.
And so if we let them go, there is going to be an acceleration towards each other. You
can see the bigger object is not moving as fast just because it has more mass. And so
gravitational forces are always attractive. In fact you are connected to all mass in the
universe. There is a slight gravitational attraction between everything in the universe.
And so also you should know that they are relatively weak. Compared to the other fundamental
forces they are very small. But as the scale gets larger, and that is another neat thing
about gravity, it operates at all scales, as the scale gets larger, even though these
forces are weak, they become the dominant forces. And so Newton was the first one to
really understand or quantify gravitational force. Again what we do is take the mass of
the two objects, mass 1 times mass 2. We then divide it by the radius or the distance between
those two objects. And then we multiply it times the gravitational constant. Now the
important thing to note is that that gravitational concept is an incredibly small number. And
so if we are multiplying small masses, for example atoms times each other, we are going
to get forces that are incredibly small. They do not really dominate at that level. And
as the masses get larger then we are going to start gravity take over. Now this is just
a model of how gravity works. Remember Einstein’s general theory of relativity does a better
job of explaining this, where everything that has mass leads to curvature of space-time.
And then objects are just following this curve space-time. Now gravity compared to the other
forces is incredibly weak. And let me kind of show you how that works. Let’s just say
gravitational force is a force of 1. How much stronger is that weak force going to be? It
is going to be 29 times the force of gravity. What about electromagnetism? It is going to
be 36 times. And of course strong force is going to be the greatest force. It is going
to be 38 times that of gravity. So let me give you an example of that. You are sitting
on the earth or you are balancing your computer on a table and it does not just move through
the table even though gravity is acting on it. So that electromagnetism of those atoms
is actually what is holding those objects in place. Now as the scale gets larger and
larger and larger, even though this gravitational force is small, it is going to become the
dominating force. Let me show you how that works. So here again is Newton’s law of
universal gravitation. And so if we were to plug in some masses of some different objects,
so if we are at the level of an atom, it is going to have a mass of 1 times 10 to the
negative 26 kilograms. It is incredibly small. So think about just multiplying that times
another atom. Even though the distance together might be really really close, this gravitational
force is going to be incredibly small. Let’s move to the size of a baseball. So it is 1.5
times 10 to the negative 1 kilograms. Let’s say we take 2 baseballs and move them a meter
apart. There is going to be a gravitational force between the two, but it is going to
be on the order of 1 times 10 to the negative 12 newtons. So it is not very large at all.
But if we move up to the level of the earth, which has a mass of 5.9 times 10 to the 24
kilograms. Now gravity starts to take over. So for example if we were to plug in the value
of the earth here as 1 mass. A baseball here as the other mass. So it is just sitting on
the surface of the earth, then we were to use the radius of the earth to figure out
the distance between those two objects, we would find that that force now is going to
be around 1.5 newtons. It is going to be a trillion times that force of just having two
baseballs together. So you can see as we get the size scale of the sun, it is the order
of 10 to the 30 kilograms. Or the universe, 10 to the 42 kilograms, that gravity really
starts to take over at this scale. And so did you learn to articulate situations when
gravity is the dominant force? Again that is going to be when we get into the macroscopic
level. And then finally do you remember that these forces are all varying in their strengths?
And gravity is going to be the weakest of all of the forces by orders of magnitude.
I hope so. And I hope that was helpful.