The world is full of forces that are important for our everyday life – without friction we would be unable to drive cars on the road safely. Did you know that all forces can be classified into one of the following four categories: gravitational force, electromagnetic force, strong force and weak force. These make up the fundamental forces of nature and can also be thought of as interactions. These forces all have a particle associated with them. In this article, we will discuss the four different types.


Gravitational Force

This is the force that people are probably most familiar with and is commonly referred to as gravity. Out of all of the forces it is the weakest, with only 6×10-39 of the strength of the strong force. However, it has an infinite range. Gravity is the force of attraction between two objects, which depends on the distance between the two objects. This force exists between every single particle in the universe with every other particle, however weak it may be.

The gravitational force felt by each object due to its attraction to the other, based on Newton’s law of gravitation [1]

From the figure, it shows that the strength of the force depends on the masses of the two objects (m1 & m2) and the distance between them d (this is sometimes referred to as r instead). The further away the objects are, the smaller and weaker the force is. It is understandable then that due to its infinite range, it can correspond to quite weak forces.


Electromagnetic Force

This is also sometimes referred to as electromagnetism and is the category that electric and magnetic forces fall under. This is likely the type of force that people will understand and encounter the most after gravity: taking the example of driving again, friction is due to electrical interactions between the atoms of the car tyres and the atoms on the road. Magnetic forces, for example, the force between two magnets, occur due to electric charges in motion. For a long time, it was thought that electric and magnetic forces were completely separate from each other. However, it was discovered that they were both just components of the electromagnetic force, the Lorentz force. It also has infinite range but is much stronger than the gravitational force.


Strong Force

The last two forces one is probably less acquainted with. The strong nuclear force, or often shortened to the strong force, is the strongest of the four as the name would suggest. It is the force that allows larger particles to exist. It is responsible for binding clusters of quarks together, therefore allowing protons and neutrons to exist. Neutrons and protons then form the nucleus of an atom and are also held together by the strong force. Protons are positive particles and neutrons are neutral particles; as you may know from magnets, like tends to repel like. The positive protons experience an electric force that repels them from each other; if not for the strong force the nucleus would then break apart.

The strong force overcomes the electric repulsion experienced by the protons, keeping the nucleus together [2]

The strong force however has a very limited range in comparison to the gravitational and electromagnetic forces, roughly in the subatomic region. Therefore, this force only occurs when particles are no further than the length of a proton apart.


Weak Force

Finally, the weak force has the smallest range of all the forces. Unlike the name suggests, it is not actually the weakest (this is gravity), however it is the second weakest. The weak force is the cause of beta decay, a form of radioactivity. In the nucleus of a radioactive atom, a neutron is then converted to a proton, and two particles are released from the nucleus, an electron (a small negatively charged particle) and an antineutrino (nearly massless antiparticle). This is one form of beta decay and it occurs due to the weak force. The weak force is the reason why carbon dating exists, allowing archaeologists to determine the age of many organic-based objects that previously came from living organisms. Carbon-14 is an unstable nucleus due to the weak force and will decay into nitrogen over time. By measuring the fraction of carbon-14 remaining in the object, its age can be determined.


Comparing the forces

Now that we have described each force briefly, let us compare their strengths, ranges and discuss the particles that are responsible for the different type of interactions.

The four fundamental forces and their properties, in order of decreasing strength [3]

The comparative strengths can easily be seen, as well as the varying range and what this size actually compares to for the strong and weak forces. The mediating particle is the one that is involved to cause a certain interaction to occur. For the strong force, it is mediated by gluons, massless bosons with a spin of 1. All the forces in fact have bosons as their mediating particles, particles with integer spin (0, 1, 2, …). The electromagnetic force is mediated by photons, particles of light. The weak force has mediating particles of W and Z bosons. It is believed that the gravitational force has a mediating particle, the spin 2 graviton; however currently this is all theoretical as this boson has yet to be observed experimentally.


Combining all four?

As previously stated, originally electric and magnetic forces were thought to be separate, until it was discovered that they can be described by the electromagnetic force. Since the 20th century, the electroweak theory has been developed, that describes both the electromagnetic and weak forces as a single electroweak force. This theory has been tested rigorously and so far has passed all experimental testing. There has been a desire to create a grand unified theory, that would combine the electroweak force and the strong force. This ‘electronuclear’ force has yet to be observed, however there has been models created that predict its existence. There has also been great interest in a theory of everything that would combine all the forces. There is still much about the universe we are discovering and it will be interesting to see if such theories are ever proven to be true.


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