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bioagentX

Bungie Jumping

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I''m trying to understand conceptually the act of bungie jumping. For example, I''m confused as to why when someone bungie jumps, they eventually stop moving. I mean, I know its because the rope pulls on the person''s feet, but I''m thinking about the problem in terms of forces. When the person jumps off of the object and the cord is being stretched, he has two forces acting on him, mg and the force that the bungie acts on him. What I''m trying to understand is why the bungie cord decelerates him because if he pulls on the cord in one direction, and the cord pulls on him in the other direction equally, wouldn''t those two forces cancel out leaving only the force of gravity and thus accelerating him at 9.8 m/s^2? I know my question seems confusing but if anyone could try and decipher what I said and give me some understanding I''d greatly appreciate it. Thx, --BioX

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Guest Anonymous Poster
The simple answer is the cord pulls on him with a bigger force than he pulls on the cord.

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Guest Anonymous Poster
Or I should say with a bigger force than gravity pulls him down.

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Guest Anonymous Poster
Not quite....
The answer is this:

The bungee jumper exerts a force on the ROPE with his weight. The rope exerts a force on the JUMPER with its tension. Thus, the two equal and opposite forces are acting on 2 different objects, so they cannot possibly cancel out.

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Let me rearrange your text a bit:

When the person jumps off of the object and the cord is being stretched, he has two forces acting on him, the gravitational force mg=m*9.8m/s² and the force that the bungie acts on him.
If gravity pulls on him in one direction, and the cord pulls on him in the other direction equally, those two forces cancel out leaving no force at all.

Sidenote: I hope you´re aware that F=0 doesn´t nessecarily mean the movement has stopped. For this to happen you need extra terms (forces) that take into account any kind of friction.


@AP3:
The forces acting on the rope are rather irrelevant (unless you like complicating things).


[edited by - Atheist on February 26, 2004 9:44:08 PM]

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I believe you guys are forgetting friction. If there was no friction in the air, and from the rope and the jumper moving him/herself, then there would be some kind of continuous motion. The stopping of the jumper does, indeed rely on friction.

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You think it''s friction?
I think its just the decceleration that the rope provides when it is stretched out. If you''ve ever been jumping, you know that the cord isn''t stretching the whole time. There''s a section of freefall, then the rope is taut, at which point it begins applying an acceleration that changes the velocity of the jumper until they are moving in the opposite direction. Without the freefall, the tether would slowly stretch out until the rope reaches the tension that balances it with gravity. i.e. you''d go down and stay there.

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I dont know though, the momentum of the jumper falling decreases every time he goes up and falls again, and the kinetic energy is what pulls the rope tight, and when the jumpers momentum is low, there is not as much energy from the jumpers fall pulling on the rope. onece the momentum is low enough, friction finishes it off i think, because if there was no friction, there would be tiny movements up and down forever.

ps. sorry, athiest, i diddnt notice

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@bjmumblingmiles:
Neither true. the freefall changes the actual (quantitative) movement a bit but qualitatively (the oscillating) it remains the same.

>> Without the freefall, the tether would slowly stretch out
yes, so your downward accelleration a would be a>0, hence speed v>0.
>> until the rope reaches the tension that balances it with gravity.
That´s correct. At this point you´ll have a=0 and still v>0 (v=vmax to me exact).
>> i.e. you'd go down and stay there.
Nope, since v>0 you´ll move further downwards but have a<0 from this point on which will cause your speed to decrease until v=0 but a!=0 (a<0). Hence, the (qualitatively) same procedure will start again and never end.

Maybe I´ll write out the equations; can´t get to sleep anyways.

EDIT:
@Whelzorn: Now it was me who was late cause I wanted to respond to the reply above yours . You probably couldn´t have noticed since I edited it in about 3min (which one easily needs to write a text) before you posted.




[edited by - Atheist on February 26, 2004 10:15:43 PM]

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