# Application of Newton's third law

By newton's third law , we know that every action has an equal and opposite reaction .We move left when we push slightly on the right and move forward as we push backward. But how do rockets, satellites etc change their direction in the space as they have nothing to push on?

Crudely, just consider the burned rocket fuel that is ejected out of the back of the rocket due to chemical reactions. The rocket exerts a strong backward force on the burned rocket fuel. According to Newton's third law, required reaction is that the burned rocket fuel exerts an equal forward force on the rocket. This force accelerates the rocket forward.

Keep in mind that Newton's third law says nothing about pushing against something, the rocket does not need to push against "a medium" to accelerate forward.

• This is fine in case of moving forward ..... But how does it change direction ( where is newtons 3 rd law used in changing direction ? ) – Saravanan Ramesh Sep 1 '14 at 14:29
• One simple explanation would be, the outlet of burnt fuel can take on different angles(mechanically can turn e.g.), hence change the direction of the propulsion forces. – Phonon Sep 1 '14 at 14:32
• Hey @phonon ! It would be kind of u if u tell abt this one also .... Is mass lost due to friction ? – Saravanan Ramesh Sep 1 '14 at 14:40
• I don't understand your question! What friction? If you're talking about the loss of fuel as the rocket burns it away for propulsion, then I can answer. Main type of engines I've heard of, are the "liquid engines", and there's no friction involved. Instead in a liquid rocket, the propellants, the fuel and the oxidizer, are all stored separately in liquid form and are pumped into the combustion chamber of the nozzle where burning occurs, temperature and pressure shoot up to very high values, finally ejection occurs. (loss of mass: fuel burnt&ejected). – Phonon Sep 1 '14 at 18:59
• Coming apart from this rocket thingy ......I was actually told by one of my teachers that if we drag a block initially at rest on a table , to some specific distance and drag it back to it's initial position , it is an irreversible change as there is loss of mass due to friction .. This was a bizarre situation for me as I never heard such a thing . how can it be an irreversible change ? What's ur opinion abt this ? – Saravanan Ramesh Sep 2 '14 at 14:46

Rockets fire matter backwards, which, together with conservation of momentum (i.e. Newton's third law), leads to them being propelled forward.

It works like in the video below, where soda is matter and the bottle is the rocket :)

bottle analogy

As the net external force acting on the rocket-fuel system is 0 N, the system's linear momentum is constant, the same before and after the burning and ejection of some fuel. Note that linear momentum is a vector quantity with a magnitude equal to mass x speed. A change in one direction of part of the system results in a change of equal magnitude in the opposite direction

If part of the system's mass increases speed in one direction, another part will gain speed in the opposite direction. Fuel accelerated to a given speed in one direction results in the rocket and its remaining fuel gaining speed in the other direction.

Nothing external to the rocket-fuel system is required. In short, the rocket and the non-ejected fuel require nothing to push off from.