Force
Force is defined as rate of change of momentum of a body <math>\vec F = {d \vec p \over dt} </math>. The SI unit of force is the newton and the US customary system unit is the pound.
The momentum of an object is given by <math> \vec p = m \vec v</math>. For non-relativistic speeds, <math> \vec F = m \vec a </math> when mass can be assumed to be constant.[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
- The force that acts on objects with electric charge.
- Gravitational force
- The force that attracts any two objects with mass.
- Strong force
- The force that keeps atomic nuclei together.
- Weak force
- This force is 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 described using quantum field theory, a relativistic version of quantum mechanics.