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Why do projectiles cause acceleration

  1. Nov 3, 2011 #1
    I am taking AP physics and have taken physics before but am puzzled. I started thinking about the classical tug of war question. I get that a team wins in tug of war not because the pull on each other but because they push on the ground. This is b/c of newton's 3rd and b/c there is a net force there is acceleration.

    But then I thought about why a bolder shot out of a big sling shot can knock over a wall. The boulder exerts a force on the wall, the wall exerts a force on the boulder.

    So there must be a net force but why is there a net force. B/c of acceleration / f = ma?

    If yes can you please explain it and not just say yes you're right. I think I'm right i just can't convince myself why this happens.
    I know its extremely conceptual but I'm just stuck.
  2. jcsd
  3. Nov 3, 2011 #2
    It has more to do with the stress from the force than newtons 3rd law.

    What you actually want is the amount of pressure the wall can handle (N/A). A strong force over a large area or a smaller force over a really small area would be enough.
  4. Nov 3, 2011 #3
    But how can the ball exert a force in the first place? Is that just because it has mass and acceleration
  5. Nov 3, 2011 #4


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  6. Nov 3, 2011 #5
    so the accleration of the ball is greater than the acceleration of the wall and so there is a net force? or what exactly happens in this situation
  7. Nov 3, 2011 #6
    The boulder is an object in motion, which (ignoring friction) will keep moving unless decelerated by a force.

    When the boulder hits the wall, the momentum of the boulder tries to accelerate the wall, and the wall tries to decelerate the boulder. That's where the forces come from. I'm a bit rusty so maybe not 100% perfectly correct, but that's the basic idea.
  8. Nov 4, 2011 #7
    I think your confusion comes from where the forces are acting, yes the two forces are equal, so the net force on the system (stress the entire system of ball and wall) is zero, however the force on the wall is not zero and the force on the ball is not zero; the wall only has a force from the ball, so it will accelerate, the ball only has a force from the wall, and so it will also accelerate. The fact that these two forces are equal and opposite shows that the system will have no acceleration, which just goes to show the conservation of momentum in a closed system.
  9. Nov 4, 2011 #8


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    When the ball is launched, the Earth gains some equal and opposite momentum. When the ball finally comes to a halt, after ploughing through the wall, The Earth loses that momentum.
    Energy has been transferred from the mechanism of the catapult to heating up the wall and ball, when it's all come to rest, finally.
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