Action-Reaction pairs

  1. Action-Reaction pairs... :(

    Consider bungee jumping from the bridge at Royal Gorge Colorado (the worlds highest bridge). Describe the forces acting on the jumper and on the bungee at some point where there is tension in the bungee, but the jumper is still going downward. Which forces are third law pairs?

    So I was thinking on the person, it would be gravity and drag forces, and then gravity and tension forces on the rope. Does that sound correct?
  2. jcsd
  3. The does sound good, though you may want to think about the drag force and how it works.
  4. Doc Al

    Staff: Mentor

    Those aren't 3rd law pairs.

    Hint: For every force identify the two bodies that are interacting.
  5. Well a drag force is usually horizontal to an object, so the force I'm thinking of is lift force then, right? Or actually lift will act perpendicular to motion and in this case it would be acting either left and right, but there are no left and right forces acting on either the rope or person so it can't be that. The drag force will act in the opposite direction of motion, and since motion is down, it will act up canceling gravity out, right?
  6. Ok gravity is interacting with the rope and person. So that would be 3rd law pairs then. That seems like the only 3rd law pairs then since tension isn't on the person, it's more on the rope.
  7. Actually now I was thinking...tension would be exerted on both the person and rope since the person is falling and when the person falls to the point where the rope is stretched out as far as it can go, tension from the person is exerted onto the rope and vice versa. Am I thinking of that correctly?
  8. Doc Al

    Staff: Mentor

    When you say "gravity is interacting" with the person, what are the two bodies involved? (What produces the force of gravity?)
  9. What produces the force of gravity would be the mass of the person and acceleration.
  10. The person is one of the masses; what is the other?
  11. the mass of the rope?
  12. When you jump, gravity pulls you back down. Back down to what?
  13. Doc Al

    Staff: Mentor

    That's how you calculate the force of gravity. But what's the other object that creates the gravitational attraction? Hint: It's big.
  14. mass of the earth
  15. So the two bodies involved in the force of gravity are?
  16. earth and the person...i am so over-thinking lol
  17. ok so the force of gravity acting on the earth and the person is a third-law pair then...

    now what about tension acting on the person and the rope?
  18. Think about tension in terms of Newton's Third Law.
  19. Ok so Newton's third law states that, when two objects interact with one another, the force that the interaction will exert on the first object will be equal in magnitude and opposite in direction to the force that it exerts on B.

    ok so if the rope is stretched out as far as it can go, the force of tension will act on the rope in the upward direction and it will act on the person in the downward direction...but that won't work because the upward force is going to be larger in magnitude which is why the person is brought back up...

    wait...tension is on the bridge and the rope...not the person...?
  20. Doc Al

    Staff: Mentor

    When you say "force of tension" you mean: the force that the rope exerts upwards on the person. So the 3rd law pair to that force is: the force that the person exerts downward on the rope.

    Going back to the original post, here are the forces on the falling jumper:
    (1) The rope pulling up
    (2) Gravity pulling down
    (3) Air resistance pushing upward (if you want to consider it)

    Now find out what bodies are exerting each of those forces (it should be easy) and then write down the 3rd law pair for each force.
    Last edited: Nov 2, 2006
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