Dynamics is the study of the relationship between motion and the forces involved. Galileo was the first philosopher to discover the property of inertia. It is defined as the inability of objects to change by themselves their state of rest or uniform rectilinear motion. Particles or objects cannot speed up or slow down on their own. The agents that produce such movements are known as forces. A laptop placed on a table cannot move on its own. Thus, the tendency of any particle to remain in its state of rest or motion is called inertia.

For example, athletes do a long run before taking a long jump. Their bodies go into a state of motion, and due to the inertia of motion, they will be able to complete the task at hand. Similarly, when any moving vehicle with passengers comes to an abrupt stop, the travelers would be pushed forward! This is due to inertia. They tend to remain in their moving state, even though the vehicle has stopped. Therefore, 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 forces acting on an object must be zero, then only the object would not accelerate. It remains at rest or moves with a uniform acceleration.

The magnitude of motion in a moving particle is defined as momentum. It is the product of the mass of the body and its speed. The second law states that the applied or external force is proportional to the rate of change of momentum. The greater the applied force, the greater the change in momentum.

Momentum is a very large force acting over a very short time interval. Thus, the impulse of a force is equivalent to the change in momentum. When a cricket ball hits a bat, the force is very high but it acts only for a very short time.

Sir Issac Newton assumed that action can be force, momentum, energy, or work. Furthermore, he opined that the reaction should be equal and opposite to the action. A standing person will be pushed downward by the force of gravity, but the ground opposes this force with an equal force acting upward. The law can also be tested by swimmers, who push the water back, flying birds, and airplanes. A laptop on a table remains stationary due to the balance of upward and downward forces.