What causes forces? model 5 ext
Animation showing repulsive force from exchange of a ball
Physicists think that all forces are caused by the exchange of particles. Imagine two jugglers playing on a frozen lake. When they start throwing their batons at each other, they will be pushed apart. The batons carry momentum and energy from one juggler to the other and as they are caught this momentum pushes each juggler away from the other across the slippery ice.
The electromagnetic force
Animation showing repulsive force from exchange of a virtual photon
Electrons exchange virtual photons. Note that they will be sending out and absorbing other virtual photons in every direction.
In the same way, the electromagnetic force is carried by particles called virtual photons. Two negatively charged electrons will exchange these virtual photons and push each other apart. Notice that they are sending virtual photons out in every direction. The intensity of the virtual photons gets smaller with distance, because they are spreading out. This is why the force gets weaker with distance. It is even possible to explain why the force decreases with the square of distance.

The electromagnetic force is described by the theory of quantum electrodynamics (QED), which was developed by Richard Feynman. What about the other forces?

The force of gravity is another inverse square force, therefore physicists think that there is an exchange particle that causes it. This particle would be radiated by anything with mass and its intensity would decrease with the square of distance. Physicists have already named this particle the graviton but have not found one yet.
Strong nuclear force
The strong nuclear force acts between quarks (and between hadrons which are made up of quarks). The particle that carries the force is called the gluon (the physicists who labelled this particle decided against a poetic Greek name).
The need for the strong nuclear force
Weak force
The weak force is the interaction that explains beta decay - the decay of a neutron into a proton, an electron and an anti-neutrino. There are three particles that carry the weak force, they are the Z0 (zed-zero ), W+ (double-you-plus and W- (double-you-minus). By now physicists could not think of any more inventive names and stuck with the letters.
The need for the weak force
Force Affects Particle
with charge.
Quarks &
gravity Anything with

Quarks &
strong Quarks Gluon
weak Quarks &
W+, W-,
Table 1. The gauge bosons - force carrying particles.
A family of force carriers.
The force-carrying particles all belong to a family called the gauge bosons. They are summarised in Table 1
The story so far
The four forces are carried by exchanging particles
The exchange particles are called guage bosons
Question M10

a) What are guage bosons?

b) Which force is carried by the gluon?

c) Which guage boson is though to exist but hasn't been discovered yet?

The story so far
  • There are four main forces: gravity, electromagnetic, strong nuclear and weak
  • Gravity and electromagnetism are long range forces that get weaker with distance
  • The strong nuclear force and the weak force are short range and only have a noticeable effect within the nucleus