We have studied the motion of objects earlier. What causes objects to move? When a 'push' or a 'pull' is applied to it by another object, it moves. Such an interaction between objects or between an object and its surrounding is called a force. To determine the force, we need to know how strong and in what direction it acts. Hence, force is a vector quantity and is expressed in units of newtons. For example, the gravitational force directed towards the Earth's center makes fruits fall on the ground. When more than one force acts on an object, their combined effect is the same as a single force equal to the vector sum of individual forces. This principle is referred to as the superposition principle of forces. Conversely, any force acting on an object can be resolved into its horizontal and vertical force components.