Dynamics is the study of the relation between motion and the forces involved. Galileo was the first philosopher to discover the property of inertia. It is defined as the inability of the objects to change by themselves their state of rest or of uniform motion in a straight line. The particles or objects cannot accelerate or decelerate on their own. The agents which produce such movements are known as forces. A laptop kept on a table, cannot move on its own. Thus the tendency of any particle to remain in its state of rest or of motion is termed as inertia.
For instance, the athletes do take a long run before taking a long jump. Their bodies do get into a state of motion and due to inertia of motion, they will be able to complete the task in hand. Similarly, when any moving vehicle with passengers stops abruptly, the travellers would be jerked forward!. This is due to inertia. They tend to remain in their state of motion, though the vehicle has come to rest. Thus, the first law can also be known as the law of inertia.
In other words, according to the first law of motion, the vector sum of all the forces which are acting on an object should be zero, then only the object would not be accelerated. It remains at rest or moves with an uniform acceleration.
The magnitude of motion in a moving particle is defined as the momentum. It is the product of the mass of the body and its velocity. The second law states that the applied or external force is proportional to the rate of change of momentum. Greater the applied force, greater will be the change in momentum.
Impulse is a very huge force which acts for a very short interval of time. Thus the impulse of a force is equivalent to the change in the momentum. When a cricket ball strikes a bat, force is very high but it does act only for a very short time.
Sir Issac Newton assumed that action can be force, or momentum, or energy, or work. Further, he opined that the reaction must be equal and opposite to action. A standing person will be pulled downwards by gravitational force but the floor opposes this force by an equal force acting upwards. The law can also be proved by swimmers, who push the water backwards, flying birds, and aeroplanes. A laptop on a table remains stationary due to the equilibrium of upward and downward forces.