Ever thought about how much you weight? What if I told you you could lose weight by gonig into space - in fact, you would weigh nothing at all if you were in space. Want to put on weight? Why not just take a trip to Jupiter - you would weight 23 times more than you weigh on the earth! Trouble is that when you get back to the earth, your weight will be back to normal, so it's not a quick way to get hench... There is obviously something weird going on here, and it is with the use of the word weight. in science, it doesn't mean what you think it means - let's take a look at it in more detail.
Mass is the amount of matter in an object. Weight is a force acting on that matter. Weight is the result of gravity that attracts objects towards the centre of the Earth.
The strength of the weight as a force depends on the strength of the gravitational field around the Earth and other planets. The formula that shows the relation between mass, weight and gravitational field strength is:
weight = mass × gravitational field strength
Weight is measured in Newton (N), mass in kg and the gravitational field strength on Earth is 10 N/kg. On the Moon it is only 1.6 N/kg.
Gravity is very important, because it keeps the Earth's atmosphere in place, it secures the orbit of the planets around the Sun and keeps galaxies together. Tide is a result of gravitational pull between the Earth and the Moon, as you can see on this diagram.
Black holes have an extremely high gravitational pull, so nothing can escape them.
The worksheet 'Gravity and Satellites' includes more information on natural and man-made satellites and their uses.