Lesson Video: The Fundamental Forces Physics • 9th Grade

In this video, we will learn how to describe the properties of the four fundamental forces and which particles interact via each force.

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Video Transcript

In this lesson, we’re talking about the fundamental forces. There are four such forces, and we’re going to learn what causes each one as well as how the forces compare to one another. These four include gravity and electromagnetism. And as we get started, perhaps the most important as well as surprising thing we can say about them is that fundamental forces are caused by particle exchange. This means, for example, that if we have two objects, and say both of these objects have some amount of mass, then we know there will be a gravitational force on each object due to the other. We’ve often thought of this force as being due to one of these particles existing in a gravitational field created by the other.

If we get down to a very small scale though, subatomic distances, we find that what actually allows, say, the force of gravity to act between these two objects is the exchange of a certain kind of particle. So if we’re thinking of this object being attracted gravitationally by this one, we can say that that will only happen once this mediating particle is sent from the one to the other. We say that it’s that exchange that allows a gravitational force to exist between our two objects. Whatever the properties of this particle then, we hold that it is responsible for the gravitational force between massive objects.

Now, gravity, we can recall, is just one of the four fundamental forces. The other three, in no particular order, are the electromagnetic force, the weak nuclear force, and the strong nuclear force. Just like gravity, each of these fundamental forces are also caused by particle exchange. And interestingly, each force has its own specific particle that mediates that force’s interaction. This may raise the question though, just what sorts of particles are these that cause these fundamental forces?

Consider the kinds of subatomic particles we’re already familiar with. One such group of particles is the six kinds of quarks. These, along with their antiparticles, make up all hadrons. Another separate class of particles, of which there are also six, is the leptons. These include electrons, muons, tauons, and their corresponding neutrinos.

As it turns out, these various force carrier or force transmitter particles, we could call them, are neither quarks nor leptons. Rather, they belong to a third class called bosons. There are many different types of bosons, most of which have nothing to do with carrying forces from one particle to another. We’re going to focus though on the four that do. These include the photon, symbolized with the Greek letter lowercase 𝛾; a particle called a gluon, represented by a lowercase g; a pair of particles called W bosons, represented, respectively, by a W with a positive and a negative superscript; and lastly a Z boson, represented by a Z with a zero superscript, representing that it is neutrally electrically charged.

Now, since there are four fundamental forces and we have four bosons here, we might think that one goes to each force. But interestingly, here’s how it works out instead. Both the W and the Z boson are known to be particles responsible for the weak nuclear force. This means that it’s by the exchange of W and Z bosons that the weak nuclear force can act between particles.

The next boson up on our list is the gluon. And this is known to be the force carrier for the strong nuclear force. Recall that it’s the strong force that helps atomic nuclei, protons and neutrons, stay tightly packed together.

And then another sort of boson is the photon. Because we associate this particle with light, we might expect it to be the force carrier for electromagnetism, and that’s correct. It’s by the exchange of photons, which we now know are bosons, that charged particles are able to influence one another through this force.

Conspicuously absent from this list is a boson that mediates the force of gravity. Although a certain particle has been hypothesized as the gravitational force carrier, none has yet been experimentally discovered. This points to the fact that our physical model of how the universe works is not yet complete.

Now that we know the names of the bosons that serve as force carriers for three of the four fundamental forces, let’s compare these four forces one to another in terms of their relative strength as well as the distances over which they act. If we were to arrange these forces from strongest to weakest, then the strongest force of all is the strong nuclear force. Next in strength comes the electromagnetic force, then the weak nuclear force, and then way down at the bottom of our list is gravity. Gravity is actually much weaker than any of the other three.

A couple other interesting attributes to consider are the range over which a force acts, that is, the distance over which it acts, as well as whether that force attracts, repels, or both. Starting out with our strongest fundamental force, we know that the strong nuclear force is responsible for keeping atomic nuclei together. Even though this force is very strong, it only acts over a very short range, about equal to the diameter of an average-sized nucleus. Over these very small distances between particles, the strong nuclear force is able to overcome electrostatic repulsion between, say, adjacent protons in a nucleus.

When it comes to whether this force attracts or repels or both, the strong nuclear force is only attractive. If we then consider the electromagnetic force, we know that the mathematical equation for this force is expressed by Coulomb’s law. This law tells us that the electromagnetic force between two charged particles is proportional to one over the distance between their centers squared. In other words, no matter how big 𝑟 gets, up to the point that it’s infinitely large, there will still be some nonzero electromagnetic force between charged particles. We say then that the range of this force is unlimited; it’s infinite. And then, as far as attracting or repelling, we know the electromagnetic force can lead to both. That’s because two like charges repel one another, while two unlike charges attract.

Moving on to the weak nuclear force, it’s this fundamental force that’s responsible for nuclear decay processes. And like the strong force, it only acts over very short ranges. In fact, 10 to the negative 18th meters is smaller than the diameter of a single proton. This force, it turns out, is the shortest-range force of them all. And when it comes to whether it attracts or repels or both, because the weak nuclear force is responsible for nuclear decay, there isn’t a clear intuitive picture of which direction or directions in which it might act. For this force then, we won’t specify whether it’s attractive, repulsive, or both.

Lastly, we come to gravity, which as we said is by far the weakest of these four forces. This may seem strange when we consider that as we make astronomical observations, gravity seems to be the dominant force. But we can recall that both the strong and the weak nuclear forces are very range limited and also that very large macroscopic objects, such as moons or planets or stars, are typically electrically neutral. And so for very large masses over very large distances, gravity is the most influential force.

Just like with electromagnetism, the force of gravity is proportional to one over 𝑟 squared, where in this case 𝑟 is the distance between the centers of mass of two masses. So gravity, too, has an infinite range and will only become zero when two masses are literally infinitely far apart. And in terms of attracting or repelling or both, we know that the gravitational force is always attractive.

Now that we know about how the four fundamental forces compare with one another, as well as the specific particles that carry these forces, which we learned are called bosons, let’s get some practice with these ideas through an example.

Which of the following symbols represents a gluon? Uppercase G, lowercase 𝛾, uppercase 𝛤, lowercase g, lowercase y.

Okay, to answer this question, we can be helped by remembering that a gluon is part of a class of particles called bosons. Specifically, it’s one of the four bosons that are known as force carriers. These are bosons that particles exchange so that forces can be experienced between the particles. The force carrier bosons are the photon, the gluon, the W boson, and the Z boson.

We want to know which of these five symbols is typically used to represent the gluon. A photon, we know, is represented by the symbol 𝛾. And so we can cross off option (B) on our list. The W boson consists of a particle and antiparticle pair represented with a capital W and a plus and a minus sign superscript. And a Z boson is represented by a capital Z with a zero superscript. None of these last three symbols appear on our list. So we’re not able to cancel out any more options.

To answer this question, we’ll simply need to recall that a gluon is represented by a lowercase g. We do see that symbol on our list as option (D). And so our answer is that a lowercase g is the symbol that represents a gluon.

Let’s look now at a second example exercise.

Which of the following symbols represents a photon? Lowercase 𝜐, 𝜈, lowercase y, uppercase Y, or uppercase 𝛶, lowercase 𝛾.

Alright, a photon, we can recall, is a massless particle that serves at least two functions. First, it’s the name we give to the smallest possible packet of light energy. So when electromagnetic radiation or light is transmitted from one location to another, we say that that happens via photons. But then a second function of photons is to serve as what’s called a force carrier. If we have two electrically charged objects, say this one and this one, then the way that there’s an electromagnetic force from one object to the other is via or by the exchange of photons.

In this case, the photon is not transmitting light but rather transmitting force, electromagnetic force. Anyway, the symbol we use to represent this particle is the lowercase Greek letter 𝛾. Among our answer options, we see that as choice (E), and so that will be our answer. It’s lowercase 𝛾 that represents a photon.

Let’s look now at one last example exercise.

List the four fundamental forces from greatest relative strength to lowest relative strength.

Okay, to begin putting these forces in order of strength, let’s start by recalling what they are. In no particular order, the four fundamental forces, that is, the forces behind all other forces we observe, are gravity, the electromagnetic force, the weak nuclear force, and the strong nuclear force. We want to put these forces in order from the strongest, we’ll say that’s number one, to the relatively weakest, number four.

One of the nice things about the names that have been given to these fundamental forces is that the strongest force actually has that word in its name. The strong nuclear force, which is responsible for holding the nuclei of atoms intact, acts over only very short ranges, about the diameter of an average-sized nucleus, but over those distances is more powerful than any other force.

The next strongest fundamental force is the one that the strong nuclear force needs to overcome to keep the protons in a nucleus together. These positively charged particles naturally want to repel one another via the electromagnetic force.

Now, if we’re unsure of which force goes here in the third slot, whether gravity or the weak nuclear force, it can help us to remember that the force of gravity is actually by far the weakest of these four forces. This can seem strange because, to our eyes, gravity may be the most apparent of these forces. But nonetheless, from a strength perspective, it is the weakest, by a lot. This means that our third spot will be occupied by the weak nuclear force, the force responsible for nuclear decay processes. And so from strongest to weakest, the four fundamental forces are the strong nuclear force, the electromagnetic force, the weak nuclear force, and gravity.

Let’s summarize now what we’ve learned about the fundamental forces. In this lesson, we learned that the fundamental forces are mediated or carried by particles called bosons. These bosons are the photon, which mediates the electromagnetic force; the gluon, which mediates the strong nuclear force; and the W and the Z bosons, which mediate or carry the weak nuclear force.

We noted that the boson thought to mediate the force of gravity is hypothetical and hasn’t yet been discovered. Along with this, we learned that of the four fundamental forces, the strong nuclear force is the strongest. Then comes the electromagnetic force, then the weak nuclear force, and finally gravity. We learned the ranges over which these forces act and for three of the forces whether they attract, repel, or both.

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