Why do some objects accelerate and others not? How can an object traveling in one direction be accelerated in another direction? What are the causes of the increase in speed of a moving object? These and other questions can be answered when we extend our discussion of the motion of objects to another branch of mechanics known as Dynamics (from the Greek word dynami meaning power). In dynamics, one discusses the cause of motion and how they affect motion. In the study of kinematics, we only need to know the position of the object at a given moment. In dynamics, the pushes or pulls that cause resistance to movement are studied.

It is our daily observation that we need force to make a body move.

For example, we must kick a soccer ball to make it move and push or pull a cart to get it from one place to another.

Therefore, we can say that force is that agent which can produce motion in a body, but sometimes force may not succeed in producing motion in a body.
For example, if a child pushes a bus at rest, it will remain at rest even though the child’s push tends to produce motion in a body. Therefore, it must be said that force is that agent that tends to produce movement in a body. Even this definition is not complete because force can also be used to stop motion in a body.

It is a matter of common observation that when we catch a moving cricket ball, we apply a force in an opposite direction to the moving ball, thus stopping the ball from moving.

Therefore the complete definition of force would be, force is an agent that changes or tends to change the state of rest or the movement of a body.