law of conservation momemtum ><?!! (1 Viewer)

[lufia]

Hey i need sum help plzz.
what is the law of conservation of momemtum? i did some research and ??!
How does this relate to how a rocket work in lift off apart from newtons 3rd law.



From what i learned, for every force there is an equal and opposite force. But isnt this the same theory as Newtons 3rd law?

What the :?F!?S!, :S
helpz plz, maybe gimmie a link or explain will help heaps.

 

[zeropoint]

It just means that the sum of the momenta in a system of interacting particles remains constant over time. A rocket uses internal energy to impart momentum to stream of exhaust gasses. Conservation of momentum says that

[momentum of rocket] + [momentum of exhaust] = [constant],

think of the momenta as 1-dimensional vectors pointing along the x-axis. If the propellant is ejected to the left (say), then the momentum of the exhaust is a negative number, so the momentum of the rocket must increase in order for the total momentum to be conserved. This causes the rocket to acquire a velocity to the right as we would expect by Newton's 3rd law. Obviously the situation is identical if the rocket is pointing to the left, except the signs of the momenta in the equation are reversed.

If you have studied calculus then you can calculate the change in momentum due to a tiny packet of exhaust in a tiny interval of time and calculate the resulting force on the rocket by Newton's second law. The result is

[thrust] = [mass flow rate] * [exhaust velocity]

Hope this helps.

 

[Riviet]

From what i learned, for every force there is an equal and opposite force. But isnt this the same theory as Newtons 3rd law?

Yes, that's Newton's 3rd Law. It is separate from the law of conservation of momentum but is applied in a rocket launching, ie the earth pushes on the rocket adn the rocket pushes down on the earth.

 

[zeropoint]

Actually Newton's third law and conservation of momentum are almost equivalent, with conservation of momentum being slightly more general. In fact, Newton's third law can be derived using conservation of momentum.

 

[lufia]

thx zeropoi

 

[Mountain.Dew]

for a mathematical proof involving momentum in rockets, u might have a look at this:

http://community.boredofstudies.org/showpost.php?p=2111468&postcount=4