We know of only four fundamental forces in our universe. The strongest is called the strong nuclear force and it is the force that holds protons and neutrons together in the nucleus. Next is the electromagnetic force, which is 137 times weaker than the strong nuclear force. This is what holds the electrons and protons together in atoms and also the force that binds atoms to each other to make molecules and compounds. Ten thousand times weaker still is the weak nuclear force which causes free neutrons to eventually decay into protons and electrons and is responsible for some low-energy forms of radioactivity. Finally, at the incredibly miniscule level of 1040 times weaker than the strong nuclear force, is gravity. Gravity is so weak that it takes masses as big as planets to produce appreciable effects. By comparison, you do not fall through the ground because the electromagnetic force creates repulsive forces between the atoms on the soles of your shoes and the atoms on the ground that are 1040 times larger than the gravitational force. The ratio between the electromagnetic force and gravity is why a concrete road is hard and stiff instead of mushy! 
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