3.1 Concept of Force
In kinematics, we studied the motion of objects. If the position, velocity, and acceleration were known at any time, then the position and velocity of the moving body at another time could be completely described. But one of the things left out of this discussion was the cause of acceleration produced in the body.
If a stone is dropped from a height, it is accelerated downward because the Earth exerts a force of gravity on the stone that pulls it down. When we drive a car or motorcycle, the engine exerts a force which produces acceleration. We observe that whenever there is acceleration, there is always a force present to cause that acceleration. Dynamics is concerned with the forces that produce changes in the motions of bodies.
A common concept of a force is a push or a pull that starts, stops, or changes the magnitude and direction of velocity of a body. We come across many forces in our daily life. Some of them we apply to other bodies and some are acting on us. For example, when we open a door, we push or pull it by applying force. When we are sitting in a car, we push against the seat as the car turns around a corner.
Force transfers energy to an object. For example, a man who moves a wheelbarrow with its load first applies force to lift it and then applies force to push it. He applies different amounts of force on each handle when turning the wheelbarrow around the corner in order to keep it from tipping over.