Force is an action that causes a free object with nonzero, finite mass to accelerate, relative to a non-accelerating frame of reference. Examples of force include the thrust of a rocket engine, the impetus that causes a car to speed up when you step on the accelerator, and the pull of gravity on your body. Force is the effect that produces the phenomenon of weight for objects with nonzero, finite mass.
The standard unit of force is the newton. One newton is equivalent to one kilogram-meter per second squared (kg · m/s 2 or kg · m · s -2). Alternatively, the dyne is sometimes used to express force; one dyne is equal to one gram-centimeter per second squared (g · cm/s 2 or g · cm · s -2). To convert from newtons to dynes, mulitiply by 100,000 (10 5). Conversely, multiply by 0.00001 (10 -5).
Force is defined in mechanical terms, but it can result from the action of electric fields, magnetic fields, thermal heating, particle bombardment, and various other phenomena. In all cases, force has the property of imparting acceleration to particles or objects. If an object is anchored so it cannot move, a force against it produces pressure.
See a video about the four fundamental forces of physics: