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Newtons third law

  1. Dec 5, 2007 #1
    I have spent a ton of time researching my question but i can't find the answer
    i know that for every action there is an equal and opposite reaction
    i know if you have a tire on the ground and when it starts spinning it pushes so that the concret will go the other way but the ground it to strong so then the tire goes forward in other words being the reaction.
    My question is when you have a rocket why does it go up?
    i know that the air or what every coming out pushes the rocket forward, but can someone explain it to me just like the example i just gave you.
     
  2. jcsd
  3. Dec 5, 2007 #2

    dst

    User Avatar

    It pushes out gases with a certain momentum and is propelled due to conservation of that momentum. Let's say the total momentum of the system = 0. Then a rocket throwing out stuff with -500kg.m.s^-1 will also experience a change of momentum of +500kg.m.s^-1, and (-500) + (500) = 0, so momentum is conserved (it MUST be conserved). Then, the impulse on the rocket (fΔt) is equal to that momentum (+500kg.m.s^-1). From then on, it's a simple case of working out how that force will affect a rocket of whatever mass.

    Don't think of it as a tire against the ground. Imagine you are in a railcar, filled with sandbags, on a frictionless railway. If you start from rest, and throw a sandbag out, you'll throw the sandbag out but the sandbag will also push you in the opposite direction, in much the same way as if you pushed off a wall - the difference being, the sandbag presents less resistance. The faster you lob the sandbags, the greater the momentum, and hence propulsion force. Lobbing two sandbags at the same time also increases the momentum.

    The rocket, is effectively "pushing" against the gases. The gases, although they may not look like it, do present a huge opposing force.
     
    Last edited: Dec 5, 2007
  4. Dec 5, 2007 #3
    Think of it this way. A ball dropped exerts a force on the Earth, but why does only the ball drop? well the forces maybe equal but their acellerations are not. The less massive object does most of the acelleration. write out newtons second law for the ball acting on the earth, and the earth acting on the ball, it is simple algebra.
     
  5. Dec 5, 2007 #4
    i am still rereading but what does (f triangle t) mean
    and what does this "^" mean
     
  6. Dec 5, 2007 #5
    "^" exponential power ex: x^2 means x-squared
     
  7. Dec 5, 2007 #6
    the reaction/action is a pair that occurs simultaneous.
    Frictional forces are acting between the tire and earth, the less massive object the tire is being accellerated.
     
  8. Dec 5, 2007 #7

    dst

    User Avatar

    Δ = greek letter delta, and denotes a difference in something. In this case, force multiplied by the difference in time (i.e. the time taken for the momentum to finish changing).
     
  9. Dec 5, 2007 #8
    Think of a rocket like a balloon. When you let it go, the air comes out and it goes shooting around the room. As the balloon gets smaller it pushes the air out. That escaping air pushes on the inside of the balloon (to get out) so for opposite reaction the balloon moves the opposite direction of the escaping air.

    Same thing with a rocket. The hot gas goes shooting out the back, right? To escape, it pushes just as hard on the rocket to get out as it escapes, so the rocket goes the other way.

    A big ton of generalizations I realize, but it gives you a picture you can see in your mind that is accurate and then later you can put the math in to figure out why some rockets are better than others and stuff, just like some cars are better than others even if they all have tires that go round.
     
  10. Dec 6, 2007 #9

    Dale

    Staff: Mentor

    D_H recently made an excellent tutorial on rockets. https://www.physicsforums.com/showthread.php?t=199087 The basic point is that rockets work via conservation of momentum. From a 3rd law perspective the rocket pushes on the exhaust and the exhaust pushes an equal and opposite amount on the rocket.
     
  11. Dec 6, 2007 #10
    hey thanks for your helps guys
    this was the post that helped me the most but now i see why it happens
     
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