To every action there is an equal and opposite reaction
object 1 on object 2 is equal in magnitude and opposite in direction to the force exerted by object 2 on object 1. Thus, an isolated force cannot exist in nature.
This law has already been mentioned with reference to the force with a table exerts on a book placed upon it and also in connection with the weight o a body in an accelerating elevator ( see post :Newton’ second law of motion…. ). We shall now discuss another aspect of the law.
It’s to be noted that momentum is a vector quantity, i.e., it has direction as well as magnitude. The momenta of the bullet and gun are equal but in opposite directions. Consequently, the sum total of their momenta is zero. This illusrates an important principle arising out of Newton’s second and third laws which is known as the
In other words, Whenever two or more particles in an isolated system interact, the total momentum of the system remains constant . So, the total momentum of an isolated system at all times equals its initial momentum. We know that Newton’s cradle illusrates the Law of the conservation of momentum.
Rocket propulsion
Let’s now think of a toy rubber ballon which has been blown up with air, as you can see in the picture above, and its mouth secured with string. If the string is now removed so that the air can escape rapidly, the ballon will, if released, dart round the room until all air has gone. It behaves as a simple rocket.
The rockets which form a familiar feature fo firework displays contain solid chemicals which burn to produce a high-velocity blast of hot gas. Space rockets which are designed to travel large distances have tanks of liquid fuel together with a supply of either liquid oxygen or some other liquid which produces oxygen to enable the fuel to burn.
In either case, a chemical reaction takes place inside the rocket and creates a large force which propels the gaseous products of combustion out through the tail nozzle, with tremendous velocity. The reaction to this force propels the rocket forwards. Although the mass of gas emitted per second is comparatively small, it has a very large momentum on account of its high velocity. An equal momentum is imparted to the rocket, the rocket also builds up a high velocity.
Jet engines
An aircraft jet works on the same principle as a rocket, the difference between them is concerned only with the method of obtaining the high velocity gas jet. The figure shows, in simplified form, the construction of an axial-flow gas turbine or jet engine. The fuel used in these engines is kerosene – paraffin-. This is sprayed through burners into combustion chambers, where it burns in a blast of compressed air and produces a high-velocity jet of gas which emerges from the exhaust nozzle.
The air supply is in front of the engine and compressed by a turbine compressor. A turbine is simply a special kind of fan having alternate sets of fixed and rotating metal blades. The compressor itself is driven by another smaller turbine worked by the exhaust jet.
In order to start the engine it is, of course, necessary to have a powerful starter motor to impart an initial rotation to the compressor and a hot electric spark to egnite the kerosene inside the combustion chamber.
Jet engines of this type are used for aircraft which fly through the atmosphere, but they cannot be used to propel vehicles into outer space where is no air supply. Space rockets, on the other hand, can travel the earth’s atmosphere, since they carry their own oxygen supply.