The forces between the particles Strong and weak nuclear force

Fundamental particles interact with each other via the strong and weak nuclear forces and the electromagnetic force (gravitational attraction is too weak to play a significant role). The strong force binds nucleons together in the atomic nucleus and is also responsible for the interaction mechanism through which new particles are created. The force has a typical reaction time of 10-22 s, which is approximately the time it takes a particle travelling near the speed of light to cross the diameter of a nucleus. Particles which are subject to the strong nuclear force are classified under the generic name of hadrons (from the Greek 'aSpoa, meaning 'robust'). The weak force which is responsible for radioactive beta decay is also responsible for the decay of relatively long-lived particles, such as the decay n^ with mean life 2.6 x 10-8 s. Both the strong and weak forces have a short range and do not extend beyond the nuclear diameter. The magnitude of the strong force exceeds that of the weak by a factor of about 1012. To put this into perspective, let us assume that an ant can push with a force equivalent to a weight corresponding to 0.1 g. Compare that with a scrum of eight rugby players pushing with a total force equivalent to a weight of about 1000 kg. The ratio of these two forces is a factor of about 107, still far short of the ratio of the strong to the weak nuclear force. In fact the equivalent ratio is the force of about 100,000 scrums divided by the force of an ant!

Worker ant. Courtesy of Wales vs Australia, Cardiff,

USDA APHISPPQ Archives. November 2006.

Comparing forces exerted by ants and rugby players:

100,000 scrums _ Strong nuclear force 1 ant Weak nuclear force

There are some particles which are completely insensitive to the strong nuclear force. These are called leptons. The photon is also a member of this 'club of total immunity'.

Club of Immunity from the Strong Nuclear Force (members' list, 1964)

Name Photon


Electron Muon


e m me 2G7 me

Electromagnetic force

Particles which are electrically charged also experience the electromagnetic force. Just as leptons are immune to the strong nuclear force, electrically neutral particles do not feel electric or magnetic forces. Electromagnetic forces are about 100 times weaker than the strong nuclear force, and are subject to the laws already well documented by Clerk Maxwell in the 19th century. Electric and magnetic fields are used to accelerate and guide the protons in accelerator rings. They are also used to separate and control secondary beams of new charged particles produced at the target.

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