Protons and neutrons are collectively known as nucleons, and there is less energy per nucleon in helium than there is in hydrogen, in carbon compared with helium, in oxygen compared with carbon, and so on all the way up to iron. Provided they can overcome the mutual repulsion caused by the fact that they have positive electric charge, nuclei will stick together and release energy in the process. What holds them together, in spite of their positive charge, is a force known as the strong nuclear force, which is more powerful than the electric force but only has a very short range; so nuclei only fuse when they get very close together, which happens under the conditions of extreme density and high temperature inside stars. But when they do get close, they grab hold of each other eagerly. The strong force is, logically enough, the strongest of four forces that affect material things. There is another force which operates only on the scale of nuclei and particles, known as the weak force, and then there are the two forces familiar from everyday life, electromagnetism and gravity. Even though it is the most obvious force we experience in everyday life, gravity is by far the weakest of the four. The reason why it is so important to us is that our weight is caused by the pull of the entire Earth, almost six million billion billion kilograms of matter (6 × 1024  kg), acting together. It takes the gravitational pull of all that mass put together to hold us down on the surface of the Earth with the weight we feel. This can be put in perspective by comparing the strength of gravity with the strength of the electromagnetic force - or with one aspect of electromagnetism, the electric force. Both forces obey an inverse square law, which means that the force between two objects decreases as 1 divided by the square of the distance between the two objects, so we are comparing like with like. Whatever the distance between them, the electric force of repulsion between two protons is 1036  times stronger than the strength of the gravitational attraction between the same two protons. On the nuclear and atomic scales, gravity is utterly insignificant, and molecules are held together by electric forces without any complications caused by the gravitational interactions between atoms. These electric forces can, of course, produce an attraction, not just a repulsion, which is what holds electrons and nuclei together in atoms, and holds atoms together to make molecules. It is the electric forces operating between a few atoms (few compared with the number of atoms in the entire Earth) that hold an apple to a tree by its stalk, resisting the gravitational pull of all of the atoms in the Earth put together. There is a competition between electric forces trying to hold things together at this level, and gravity trying to break things apart. When the apple gets heavy enough and does fall, the gravity of the Earth wins this particular battle, but only by exerting literally all of its strength on one single apple. This only happens because gravity differs from the electric force in one important way. Electric charge comes in two varieties, positive and negative, which cancel each other out. There is no overall electric charge on an atom, and there is no overall electric charge on the Earth. But gravity always adds up; the more atoms you have in an object, the stronger its gravitational pull. By the time you get to an object the size of our Moon, or a planet, its self-gravity is powerful enough to pull all the matter together into a spherical shape, but individual atoms retain their identity within the sphere; by the time you get to an object the size of the Sun, its self-gravity is powerful enough to crush atoms in the centre of the sphere and press nuclei close enough together for fusion to occur. It is the strength (or weakness) of gravity that determines how big a star is, and how quickly it burns its nuclear fuel.Question 8According to this passage, an apple stays on an apple tree because:The entire Earth exerts its gravitational pull on the apple.Gravity is a very insignificant force.Gravity is weaker than the electric force.Gravity breaks things apart, while electric forces hold them together.