Ever thought about how much you weigh? What if I told you you could lose weight by going 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 weigh 23 times more than you weigh on the earth! Trouble is that when you get back to Earth, your weight will be back to normal, so it's not a quick way to change your body shape... 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 Newtons (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 also secures the orbit of the planets around the Sun and keeps galaxies together. Tides are a result of the gravitational pull between the Earth and the moon, as you can see in this diagram.
Black holes have an extremely high gravitational pull, so nothing can escape them.
Now on to some questions!