If an object does not change its position with respect to time and the surroundings, it is said to be at rest, else it is said to be in motion. Force is that which changes or tries to change the state of rest or of motion of an object by a push or pull. The magnitude of force on an object is given by the product of the mass of the object (m) and its acceleration (a). Mathematically it is expressed by the equation, F = ma. CGS unit of force is dyne and the SI unit is newton (N). The line along which a force acts on an object is called the line of action of the force. The point where the force acts on an object is called the point of application of the force. When a number of forces act simultaneously on an object then their equivalent is the net force on an object. If the net force is zero the forces are said to be balanced which results in zero acceleration, else the forces are said to be unbalanced which results in acceleration of the object. The force that opposes the relative motion between the surfaces of two objects in contact and acts along the surfaces in contact is called the force of friction or simply friction.