The four forces are Gravity, Electromagnetism, the Strong Force and the Weak Force. Clearly gravity isn't the force in question, firstly gravity is only ever attractive, secondly gravity is very very weak and only comes into play when you consider celestial size bodies. The Strong and Weak Forces only every occur on the atomic scale, and are very rarely encountered in day to day life. This leaves one force to be the force preventing you from putting your hand through a table, electromagnetism.
As you will know, everything around us is made of atoms. Atoms are made of very small nuclei (which, in turn, are made of positivly charged protons and electrically neutral neutrons) with tiny negatively charge electrons orbiting around them. Now when you try to get close to the table, the electrons orbiting the atoms in your hand, come in very close proximity to the electrons orbiting the atoms in the table. You can bring your hand very close to the table, until you think you are touching it. In fact, you aren't actually in contact with the table. The electrons in your hand are being repelled by the electrons in the table, creating a very strong repulsive force, which you think of as the 'hardness' of the table. It can be quite strange to think you aren't actually touching the table, you're nanometers away from it, and you can't get any closer! You can't put your hand through the table because the really strong electromagnetic force is causing the electrons in your hand and the table to repel! When you try to get closer, the table will move away, and this is due to the repulsion of the electrons being enough to cause a change in momentum of the table!
This is the case in every day life. Electromagnetism is the most common force we encounter (apart from gravity keeping us on the planet). And it can become quite infuriating if you think about it too much and realise you are never actually, truly touching anything!
Answered by: Simon Hooks, Physics Undergrad Student, Imperial College London
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