 So next we're going to talk about the forces that can act on particles. The first is something we're very familiar with on a day-to-day basis and that's the gravitational force. So the gravitational force is what basically keeps us walking on the Earth's surface. All particles experience the gravitational force. Now this might seem confusing to you because up until now you've learned that the gravitational force between two bodies is proportional to the product of their masses. Well as it turns out a massless particle can experience the gravitational force. It just doesn't actually create any gravitational force in and of itself. This is actually the reason why black holes are black. Photons, massless particles, can get sucked in by the gravitational force exerted by the black hole. The next force is the electromagnetic force. This is the force that keeps atoms and molecules together. Now the electromagnetic force only acts on charged particles. So for example protons and electrons are both charged particles because protons are positively charged and electrons are negatively charged. They're going to be attracted to each other and the the attraction is due to the electromagnetic force. Now the next force that particles can experience is called the strong force. This is the force that keeps atomic nuclei together. Now this force is called the strong force because over distances of about the size of a typical nucleus that's 10 to the minus 15 meters the force is even more powerful than the electromagnetic force. Basically this is the reason why protons which are positively charged can all live together within the atomic nucleus. They can be bound together despite the fact that they are experiencing a repulsive force due to the electromagnetic force. Only certain particles can experience the strong force. We call these particles hadrons. Now there's one other force that we didn't mention. This force is known as the weak force. The weak force is kind of a misnomer. The reason is that the weak force is actually one of the strongest forces but it acts over the shortest range. So it acts only over a distance of 10 to the minus 18 meters. So it's even shorter range than the strong force. Now the weak force is responsible for turning protons into neutrons and vice versa in processes like nuclear beta decay. The weak force acts on all particles. We have a special name for particles that don't experience the strong force but do experience the weak force. We call these particles leptons. Later on we'll learn that these forces are actually mediated by certain particles and that's one of the reasons why the ranges over which these forces interact are so different.