Definition

A force is a push or a pull, and forces can make things start moving, stop, speed up, slow down, or change shape. Forces have both size and direction and are measured in newtons (N), where one newton is the force needed to give a $1\ \text{kg}$ mass an acceleration of $1\ \text{m/s}^2$. Contact forces such as friction and an applied push require touching, while non-contact forces such as gravity, magnetism, and electric force act at a distance. At the deepest level a force is a vector interaction between two objects, and Newton's second law in its general form says the net force equals the rate of change of momentum, $\vec{F}_{\text{net}} = d\vec{p}/dt$, which reduces to $\vec{F}_{\text{net}} = m\vec{a}$ for constant mass. The newton is a derived unit, $1\ \text{N} = 1\ \text{kg}\cdot\text{m/s}^2$, and all everyday forces trace back to two fundamental interactions, gravity and electromagnetism.