In physics, a force is anything that causes a free body with mass to accelerate. The net (or resultant) force is the sum of all the different forces acting on a body.

The simplest way to describe force is to say that is is a `push' or a `pull'. The push or pull on an object may cause either deformation or may change the state of motion of the object under consideration.

If we leave aside the deformation aspects, then force can be considered to produce change in the state of the motion of the object i.e. velocity. We have, though, experienced in real life that a 'push' or 'pull' does not always manifest in the change of motion. The reason is simple. A change in the state of motion requires a net force. For example, if the force is great enough to overcome friction the object being pushed or pulled will move. So long as the forces on an object are balanced (i.e. net force is zero), the state of motion described by "velocity" will remain same.

Force is a vector quantity defined as the rate of change of momentum induced in a free body by the force, and therefore has a direction associated. The SI unit for force is the newton.

Many forces exist: Coulomb's force (the force between electrical charges), gravitational (force between masses), magnetic force, frictional forces, centrifugal, impact force, and spring force, magnetism, tension, chemical bonding and contact force to name a few.

Only four fundamental forces of nature are known: the strong nuclear force, the electromagnetic force, the weak nuclear force, and the gravitation. All other forces can be reduced to these fundamental interactions

The modern quantum mechanical view of the first three fundamental forces (all except gravity) is that particles of matter (fermions) do not directly interact with each other but rather by exchange of virtual particles (bosons) (as, for example, virtual photons in case of interaction of electric charges). According to general relativity gravity results from the curvatures of spacetime.

Isaac Newton's laws of motion were first published in his work Philosophiae Naturalis Principia Mathematica (1687). Newton used them to prove many results concerning the motion of physical objects. In the third volume (of the text), he showed how, combined with his law of universal gravitation, the laws of motion would explain Kepler's laws of planetary motion.

Newton's second law is a mathematical definition of force, first proposed by Newton himself (thus the name).

1: "An object at rest or traveling in uniform motion will remain at rest or traveling in uniform motion unless acted upon by a net force." (inertia)

2: "The rate of change of momentum of an object is directly proportional to the force acting on the object." (F= ma)

3: "All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction." (To every action, there is an equal and opposite reaction).

In physics, gravitation or gravity is the tendency of objects with mass to accelerate toward each other. Gravitation is one of the four fundamental interactions in nature, the other three being the electromagnetic force, the weak nuclear force, and the strong nuclear force. Gravitation is the weakest of these interactions, but acts over great distances and is always attractive. In classical mechanics, gravitation arises out of the force of gravity (which is often used as a synonym for gravitation). In general relativity, gravitation arises out of spacetime being curved by the presence of mass, and is not a force. In quantum gravity theories, either the graviton is the postulated carrier of the gravitational force, or time-space itself is envisioned as discrete in nature, or both.

The gravitational attraction of the Earth endows objects with weight and causes them to fall to the ground when dropped. Moreover, gravitation is the reason for the very existence of the earth, the sun and other celestial bodies; without it matter would not have coalesced into these bodies and life as we know it would not exist. Gravitation is also responsible for keeping the earth and the other planets in their orbits around the sun, the moon in its orbit around the earth, for the formation of tides, and for various other natural phenomena that we observe.