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Force Problem -- projectile launcher fires from a platform on a frictionless platform

  1. Mar 14, 2016 #1
    1. The problem statement, all variables and given/known data
    A projectile launcher fires a projectile horizontally from a platform which rests on a flat, icy, frictionless surface. Just after the projectile is fired and while it is moving through the launcher, the projectile has an accelleration of 25m/s^2. At the same time, the launcher has an accelleration of 0.25m/s^2. The mass of the projectile is 0.20kg. Calculate the mass of the launcher.

    Given: acceleration of projectile: 25m/s^2[forwards]. Accelleration of launcher: 0.25m/s^2[backwards]. mass of projectile- 0.20kg

    2. Relevant equations

    Fnet=ma
    3. The attempt at a solution
    I have no clue how to solve this. I checked the solution, and according to that, you have to solve for Fnet of the projectile using Fnet=ma, and the net force of the launcher is equal in magnitude but opposite in direction to that of the projectile. Then you use Fnet=ma for the launcher and solve for mass. I don't understand why the net force of the launcher is equal in magnitude but opposite in direction to that of the launcher. Can someone explain?
     
  2. jcsd
  3. Mar 14, 2016 #2

    haruspex

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    Newton... action and reaction... any memories stirring?
     
  4. Mar 14, 2016 #3
    As haruspex has said, you need to know about Newton's third law
     
  5. Mar 14, 2016 #4
    It just says that any action force will produce a reaction force, what does that have to do with net force? The net force inolves gravity in this case, and since the masses of these 2 obects are different, how can you assume that their net forces are equal in magnitude?
     
  6. Mar 14, 2016 #5
    right, my mistake,it must be conservation of momentum
     
  7. Mar 14, 2016 #6
    We haven't learned that yet, so I don't know how we would be expected to solve this/
     
  8. Mar 14, 2016 #7
    i will try figuring a solution
     
  9. Mar 14, 2016 #8
    No, i was right, we must use newton's third law, the mass is different but acceleration is different too
     
  10. Mar 14, 2016 #9
    Can you please explain further?
     
  11. Mar 14, 2016 #10

    haruspex

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    Yes. That is, the force body A exerts on body B is equal and opposite to the force body B exerts on body A.
    The net force on a body is from summing all forces acting on the body. Any reaction on it from another body contributes.
    There is a gravitational force, but there is also a normal force, which is here guaranteed to be exactly what is need to prevent it falling through the ice, and no more. Thus, can assume they will cancel.
    What does that leave for the net force on the platform?
     
  12. Mar 14, 2016 #11
    The force that makes the projectile move also affects on the launcher
     
  13. Mar 14, 2016 #12

    Are the projectile and the launcher in contact? I don't think so. What normal forces ?
     
  14. Mar 14, 2016 #13
    Is there a normal force on each object?
     
  15. Mar 14, 2016 #14
    It is the force that perpendiculars to the ground
     
  16. Mar 14, 2016 #15
    But they're not both in contact with the ground
     
  17. Mar 14, 2016 #16

    haruspex

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    By what magic does the projectile move?
    By what magic does the platform avoid falling through the ice?
     
  18. Mar 14, 2016 #17
    I think god can do that but not human and especially that we are under the influence of physics laws
     
  19. Mar 14, 2016 #18
    Could the inner diameter of the launcher not be wide enough so the projectile is not in contact with the launcher? In that case, it would not be in contact with any surface- no normal force
     
  20. Mar 14, 2016 #19

    haruspex

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    Action and reaction does not necessarily consist of contact forces. What force made the projectile move? What was the launcher's role in that?
    If the projectile is not in contact with the launcher at all while moving through the muzzle, what is stopping it from falling?
     
  21. Mar 14, 2016 #20
    The launcher exerted a force on the projectile, making it move. Does it have to be in contact with the launcher in order to not fall or could the force exerted on it horizontally be large enough to allow the projectile to only move horizontally(at least, initally)?
     
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