One very important application of Newton's third law is in the movement of rockets. A rocket has on board a fuel and oxidant as the amount of oxygen reduces as the rocket rises higher and is zero in outer space.
When the fuel is burnt using the oxidant, it exits the rocket at a very speed. The third law of gravity which states that for every action there is an equal and opposite reaction applies here. The gases exiting at a very high speed result in the rocket being pushed in the opposite direction. As the mass of the gases is not very high, but they are being expelled at a very high speed, it results in the heavy body of the rocket moving forward.
It is Newton's Third Law that has enabled our expeditions into outer space.
According to the first law of newton,eveybody continues to be in its state of motion or uniform motion in asraight line , unless compelled bysome external to act or otherwise. This imlies that the acceleration of the body is zero when the net forces is zero. The acceleration could be there if only there is some external force is acting on the body.
In case of a spaceship when it is in space free from all gravitational forces, if suddenly the rockets are turned off, the spaceship will not have acceleration, but still it continues in motion with a uniform velocity. To bring it to the zero velocity we must give it a breaking force by turning on the rocket.
Sometimes we feel that an object is at rest and we may feel that there is no force acting on it. An example is a book on the table.It is at rest because it has two equal forces acting in opposite directions- force of its own weight due to gravity pulling it down and another force of normal reaction by the table acting on the book.