Force
Force is defined as rate of change of momentum of a body . F = dp/dt. For non-relativistic speeds, the momentum is given by p = m v, giving F = m a.[1] In these expressions, F stands for the total vector sum of all forces, m for the mass of the object, a for its acceleration expressed as a vector, p stands for momentum vector and v for velocity vector. The expression F = m a is Newton's Second Law, which was stated first by Sir Isaac Newton.
When the velocity of the object approaches the speed of light, these expressions need to be modified to account for so-called "relativistic effects". Newtonian mechanics is then no longer a good approximation, and one should use the description given by Einstein's Theory of Relativity.
There are four known types of forces occurring in nature[2]:
- Electromagnetic force
- Gravitational force
- Strong force
- this is the force that keeps atomic nuclei together.
- Weak force
- this force is (amongst other things) involved in beta decay, in which a neutron in an atomic nucleus is changed to a proton, emitting an electron and a neutrino.
It should be noted that the latter two forces have an extremely short range (on the order of femtometers), and that a classical (Newtonian or relativistic) description of these forces is not possible. They can only be desribed using quantum field theory, a relativistic version of quantum mechanics.