It is a tribute to Isaac Newton's genius that he summarized the physical principles of motion of bodies in three succinct laws, which we will elucidate here. Before we explain the actual laws themselves, it is crucial to understand the physical concept of a force. Put it simply, a force is either a pull or a push. We observe forces in everyday life. A glass falls on the floor and shatters into a hundred pieces. When the glass was falling towards the floor, there was a force of attraction (pull) between the glass and the Earth. At the instant when the glass hit the floor, there was a force of repulsion (push) between the glass and the floor, since the floor stopped the glass from moving further towards the earth. Moreover, various repulsive forces between the different parts of the glass came into action suddenly and the glass shattered into a hundred fragments.
Newton's three laws of motion are all about forces and how they affect the movement (or lack of movement) of objects. The first law states that objects do not change either speed or direction unless acted upon by an external force. A skater continues to move in his direction of skating at a constant speed and the only thing that stops the skater is the force of friction between the skate's wheels and the floor. This force of friction acts against the direction of movement of the skater and thus tends to slow down the skater.
Unfortunately for us, a perfectly frictionless milieu does not exist on earth. An aircraft cruising at 500 kilometers per hour has to overcome considerable friction (drag) from the air around it. Indeed, most of the thousands of liters of fuel that the airplane burns are used in overcoming this drag force. In outer space, however, there is no air and, therefore, no air resistance. So objects in outer space that are not near a heavenly body (e.g. a planet or a moon) continue to move in a straight line at a constant speed until they encounter proximity with another body.
Newton's second law of motion postulates two concepts. Firstly, the change in speed or direction of a body is in direct proportion to the force applied to the body that has caused this change. In simple words, the greater the force applied, the greater will be the acceleration or deceleration (i.e. change in speed) of the body on which the force has been applied. We observe this in everyday life. If we push an object with greater force, it will attain a greater speed. The second concept is that the more massive an object is, the lesser will be the change in speed. Or, to put it in different words, a greater force is required to achieve the same change in speed in a more massive object. Again, this is easily observable. For example, it is easy for me to push a family car, but when I push a truck with the same force, it hardly moves.
Have you ever wondered what it is that pushes a jet aircraft forward despite the fact that it is attached to nothing and is suspended in mid-air? Here comes Newton's third law of motion into action, which states that objects exert pull or push forces onto each other symmetrically. That is, if 'A' is pulling 'B', then 'B' is also pulling 'A' simultaneously with exactly the same amount of force. Likewise, if 'C' is pushing 'D', then 'D' is also pushing 'C' with the same quantum of force. In our example of the glass falling to the floor, it is not only the Earth that is pulling the glass towards itself. The glass is also pulling the Earth upwards with the SAME force. However, as required by Newton's second law of motion (explained above), the change in speed of the Earth as it moves upwards will be a zillionth part of the readily observable change in speed of the glass, since the Earth is so much more massive than the glass.
So, how do airplanes manage to move in the air? When the fuel burns in a jet engine, it causes the production of gases in high pressure, which are then released from the nozzle on the back of the engine. It can be said that the engine has exerted a push force on the gases. According to Newton's third law of motion, these gases simultaneously exert an equal push force on the engine in the opposite direction. Since the engine is attached to the airplane, the whole airplane moves forward.
It is these three Laws of Motion that have revolutionized the way we think about the movement of objects in our Universe and which have enabled us to send spacecraft and men to the Moon and beyond.