Force

Force, in physics, any action or influence that accelerates, or changes the velocity of, an object. Force is a vector, which means that it has both direction and magnitude. When several forces act on an object, the forces can be combined to give a resultant, or net force. The resultant force acting on an object, the object's mass, and the acceleration of the object are all related to each other by Newton's second law of motion, named after the 17th-century physicist and mathematician Isaac Newton. The second law of motion states that the acceleration an object experiences multiplied by the mass of the object is equal to the resultant force acting on an object. Thus, if a given force acts on two objects of different mass, the object with a larger mass will have a lower acceleration. See Mechanics.

An object experiences a force when it is pushed or pulled by another object. For example, shoving a stationary shopping trolley applies a force that causes the shopping trolley to accelerate. An object can also experience a force because of the influence of a field. For example, a dropped ball accelerates towards the ground because of the presence of the gravitational field; electrical charges attract or repel each other because of the presence of an electric field.

Usually, several forces act on an object at once. If multiple forces combine to give a resultant force that is zero, then the object will not accelerate; the object will either remain motionless or continue moving at a constant velocity. For example, if a person pushes a shopping trolley with a force equal in magnitude to the force of friction that opposes the trolley's motion, the forces will cancel, giving a resultant force of zero. As a result, the trolley will move down the aisle with a constant velocity. If the person suddenly stops pushing, the only force acting on the trolley is the frictional force. Since the resultant force is no longer zero, the trolley accelerates: its velocity drops to zero. (An acceleration such as this, in which speed is reduced, is also known as deceleration.)

In SI units, the unit of force is the newton, which is the force that imparts to an object with a mass of 1 kg an acceleration of 1 m/sec².

Forces acting at the molecular and atomic level are also known as interactions. See Elementary Particles.