Force
Force is defined as rate of change of momentum of a body (F = dp/dt). A unit of force is thenewton.
The momentum is given by p = m v. For non-relativistic speeds, 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.
There are four known fundamental 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.