Question

How can gravitation be a weak force when it keeps all the planets and sun together

Answer 1

Gravity is a weak force when compared with the other forces in nature (see other answers about all of that). However, the magnitude of the force is directly related to the masses of the objects involved - so objects with a greater mass create a greater gravitational force on other objects. In technical terms we say that the gravitational force and mass of the objects involved is directly proportional.

Now, when incredibly massive objects are involved, such as planets, stars, galaxies and so forth, then the force of gravity is proportionally great. Thus, what we feel on earth as gravity (as opposed to the jumping around in the weak gravity of the moon) is what a massive object like the earth is able to exert on a tiny object like a human being on the surface.

This might sound like nothing special, but if you think about it it takes an object the mass of the earth to exert a force on you which you can counteract simply by jumping. As mentioned in the other answers, getting completely free of the earth's gravity pull and getting out into space is another matter and requires a much greater force (and a good thing too! or we would jump right off the planet), but if gravity was any stronger we would not even be able to move ourselves or anything.

You yourself, as an object with mass, exert a gravity field on other objects. You exert a pull on the earth in the same way the earth exerts a pull on you. However, due to the relative masses of the objects (the earth and you), what the earth "feels" is negligible.