Gravitational forces | Forces at a distance | Middle school physics | Khan Academy

When you hear the word gravity, you probably just think of things falling, like an apple from a tree. But did you know it's also the reason why your lamp is staying on the floor? That's because gravity is so much more than things falling down. Gravitational forces are these invisible forces that pull objects together. So gravitational force is actually attracting the lamp to the floor, and these forces exist between all objects with mass.

So let's write these key points out about gravitational forces, which I'm going to use gf to represent. We said they are attractive forces and that they exist between all objects with mass—objects with mass. To explain this, we first need to remember a couple of things. Mass is how much matter objects have, and matter is the stuff an object is made of. Any object with mass generates a gravitational pull, so there is a gravitational force of attraction between every object.

The amount of gravitational force between two objects will depend on two things: the masses of the two objects and the distance between them. The mass of each object is proportional to the gravitational force. This means that the more mass an object has, the stronger its gravitational force. And now we can understand why gravity makes things fall. The Earth is massive—literally! It's almost 6 septillion kilograms. That's a 6 with 24 zeros after it, so it generates a huge attractive force.

For comparison, my lamp is only one kilogram, which is why if I jump, I fall towards the Earth and not towards my lamp. But we said the mass of the object is just one factor affecting the strength of its gravitational force. The other is the distance between objects. The more distance between the objects, the weaker the gravitational pull between them. For small objects without much mass, it doesn't take much distance for their gravitational forces between each other to be so weak that we don't notice them.

For something like the Earth, you have to go really far away to not be affected by its gravitational force of attraction. I mean, look at the moon—it's almost 240,000 miles away! That's almost 400,000 kilometers away, and it still feels effects from Earth's gravity. That's why it's orbiting us. But since the moon is also a pretty massive object, we do experience the effects of its gravitational pull on the Earth. This is why we have tides. The moon's gravitational force will pull on Earth's water, which results in us having high and low tides.

Now you might be wondering, if gravity can affect the moon or cause tides, how can we even move around? Why aren't we just face planted on the ground? Because Earth's gravity is pulling us towards it. It turns out that actually gravity is a pretty weak force. We only even notice its effects when an object is massive, like planets or stars, and the gravitational force on you is way weaker than most forces you exert every day. In fact, every time you pick up a glass of water, you're overpowering the entire mass of Earth. How cool is that?