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Physics:Rocket!

  • 13-06-2008 8:39pm
    #1
    cartman444
    Closed Accounts Posts: 125 ✭✭


    hey i just cant seem to get my head round how a rocket works!lol

    it says in my book the burning gas has a very high velocity so a large momentum and therefore the rocket has an equal and opposite momentum in the other direction. but if the momentii(:D) are equal then in opposite directions does that not mean they cancel each other out and it shouldnt move!!


Comments

  • #2
    mike-gunners
    Closed Accounts Posts: 33 ✭mike-gunners


    no no.it means that the momentum is conserved.ie the momentum before is equal to the momentum after.


  • #3
    mac-1405
    Closed Accounts Posts: 21 ✭mac-1405


    cartman444 wrote: »
    hey i just cant seem to get my head round how a rocket works!lol

    it says in my book the burning gas has a very high velocity so a large momentum and therefore the rocket has an equal and opposite momentum in the other direction. but if the momentii(:D) are equal then in opposite directions does that not mean they cancel each other out and it shouldnt move!!

    Newton's Third Law: If an object A exerts a force on an object B then the object B will exert an equal but opposite force on the object A.
    So because the gas and fire and all that is exert a force on the ground the ground exerts a force on the rocket which makes it go upwards.
    Then since the gas has a high velocity and therefore a large momentum the rocket has a large momentum.(principle of conservation of momentum)

    Hope that helps


  • #4
    Mark200
    Registered Users, Registered Users 2 Posts: 3,536 ✭✭✭Mark200


    In other words ^ every action has an equal but opposite reaction. So if there is a force moving forward, then there must be a force moving backwards too as the force must have an equal but opposite reaction.


  • #5
    mac-1405
    Closed Accounts Posts: 21 ✭mac-1405


    Mark200 wrote: »
    In other words ^ every action has an equal but opposite reaction. So if there is a force moving forward, then there must be a force moving backwards too as the force must have an equal but opposite reaction.
    Yeh but thats the junior cert definition and you dont get full marks for it. Rocket fuel pushes down, rocket goes up (3rd Law) Rocket fuel was going down fast so rocket goes up fast(principle of conservation of momentum)

    There nice and easy to understand.


  • #6
    life_is_music
    Closed Accounts Posts: 259 ✭✭life_is_music


    rocket is pushing down, the ground can't be pushed down so therefore the rocket is raised up


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  • #7
    Peleus
    Registered Users, Registered Users 2 Posts: 784 ✭✭✭Peleus


    If its an equation they want use cons of monentum:

    m1u1 + m2u2 = m1v1 + m2v2

    momentum before = momentum after
    0 + 0 = (mass of rocket)(velcoity of rocket) + (mass of fuel)(velocity of fuel)

    (mass of rocket)(velcoity of rocket) = -(mass of fuel)(velocity of fuel)

    the minus shows that the velocity of the fuel and rocket must be in opposite directions.


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