Newton's laws of motion
Man pushing a wheelbarrow to demonstrate Newton's three laws of motion. The first law states that an object will either remain at rest or move at a continuous speed, unless acted upon by a force. The second law states that the force of an object is equal to its mass times its acceleration. The third law states that when one object places a force on a second object, the second object places a force of equal strength in the opposite direction on the first.
When we discussed linear motion, we described terms such as displacement, velocity and acceleration. But we did not ask: what causes motion? Surely objects in motion are moving due to something? We also said that no object really is at rest. What is the meaning of all this? What exactly is a state of rest ? To answer these important questions, we have to first understand a physical entity called force. The relationship of motion to the forces that cause it is the subject of Dynamics whereas Kinematics is the language of describing motion. We use two concepts, force and mass to analyze the principles of dynamics. Force is measured with a unit called the 'Newton'. One newton is the amount of force required to give a 1 kg mass an acceleration of 1 m/s2. The strength of a force is always expressed in terms of its magnitude and direction in which it is acted upon an object. Hence force is considered as a vector quantity. The resultant force is the single force that has the same effect on the object as all the individual forces acting together.
These principles can be summarized in three statements called Newton's laws of motion. The first law states that when the net force on the body is zero, its motion does not change. The object will either remain at rest or move at a continuous speed, unless acted upon by a force. The second law relates force to acceleration when the net force is not zero. The force of an object is equal to its mass times its acceleration. The third law is a relationship between the forces that two interacting bodies exert on each other. When one object places a force on a second object, the second object places a force of equal strength in the opposite direction on the first. These laws are a synthesis of ideas, observations and results of various experiments conducted by Newton and many other scientists before him including Copernicus, Brahe, Kepler and Galileo. Newton's laws are the foundation of classical mechanics and they help us understand most of the familiar kinds of motion. Newton's laws need to be modified in situations involving speeds close to the speed of light or very small size such as subatomic particles.