Scientists have described four different kinds of forces in nature:
The two forces with which you are most familiar are the ones which you can see around you in your daily life; electromagnetism and gravity. You may be surprised that we've described gravity as being the weakest force ... surely a force that can hold massive planets in orbit must be a very strong one!
Electric and magnetic forces result from a property of particles called electric charge. Charged particles exert electric forces on each other, and moving charges produce magnetic fields; magnetic and electric forces are considered to be different aspects of the same force.
- Strong Nuclear
This is the strongest of the four forces; it's the force that holds the particles in the nucleus of an atom together. It has to be much stronger than the electromagnetic force, because it is able to overcome the force of the positively charged protons, which try to repel each other. However, this very strong force acts over only very small distances ... the size of a nucleus.
- Weak Nuclear
This is a force which is involved in the radioactive decay of some nuclei; it is similar to the electromagnetic force, which leads scientists to believe that all four forces may really be different aspects of one single force.
This is the weakest of the four forces. It exists between all objects, and can act over long distances. For example, it holds the earth in orbit around the sun, and all the stars together in our galaxy.
In fact, gravity is much weaker than electromagnetism. The reason gravity seems so strong is because we see around us the effects of the gravity caused by the entire planet earth.
Consider this: you could hold in your hands two magnets which attract each other so strongly that you are unable to pull them apart with all your strength. That's a lot of force for two little pieces of metal the size of your hand.
On the other hand, while the force of gravity from the planet below you pulls you to its surface, you can very easily separate yourself from the earth ... by jumping.
Some people erroneously believe that there is no force of gravity once you reach space, and that's why astronauts 'float' in orbit. This is totally false. The force of gravity gets smaller the farther from earth you get, but very slowly as we're used to measuring distances. At orbital heights, it's still just about as strong as it is down on the surface. It's still a quite respectable size even at the distance of the moon. The further from the earth you get, the smaller the force of gravity, but it never gets to zero.
Let's look at the situation a slightly different way. Imagine you are wearing boots with magnets on them, and are standing on a small metal plate.
It isn't hard to imagine that these small pieces of metal will prevent you from lifting your feet at all ... you are 'glued' to the metal. The magnetic force in the metal is strong enough to keep you completely immobile.
If you can manage to get some friends to free you and then lift you up high enough, the force will no longer be strong enough to be felt. The magnetism acts strongly only when your boots are very close to the plate.
Now imagine you're just standing on the ground. Despite the fact that the gravity from the entire planet earth is pulling you down, you can easily separate yourself from the ground by jumping. You're strong enough to separate yourself from the surface of the earth despite the entire planet's gravitational force.
This force may be weak, but it acts over large distances. No matter how far up you get, the force of gravity can still pull you back to the ground.
To understand why astronauts seem to float, visit our 'Weightlessness' page.
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