## RaphaelFilgueiras 3 years ago cool question.An helium ballon is fixed in a wagon floor with a massless thread,the wangon is completely closed.The thread stay in vertical position while train moves with contant velocity.If the train is accelarated forward ,we can say,in relation to the the train the ballon will: a)move back and the tension in thread will increase b)move back and the tension in thread not change c)move forward and the tension in thread increase d)move forward and the tension in thread not change e)remains in vertical position

1. RaphaelFilgueiras

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2. Jemurray3

I'm not sure where traction fits in to this...

3. Algebraic!

think he means Tension.

4. Jemurray3

Oh, I see. Well, what are your thoughts?

5. RaphaelFilgueiras

it's tension like @Algebraic! said

6. Algebraic!

my thoughts are too despicable to be aired in this forum.

7. Jemurray3

@RaphaelFilgueiras Do you think it goes forward or backward?

8. experimentX

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9. RaphaelFilgueiras

10. Jemurray3

Oh, okay :) And @experimentX consider the acceleration as a gravitational field pointing towards the back of the car.

11. experimentX

I was thinking that the baloon will experience backward with respect to the frame.|dw:1348770823195:dw|

12. Jemurray3

Not quite. The air inside the car feels that acceleration too, and because it is heavier than the helium inside the balloon, the balloon actually feels a force toward the front.

13. Algebraic!

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14. experimentX

Woops ... never considered that fact.

15. Jemurray3

@Algebraic! that is true except the balloon goes to the front, not the back.

16. experimentX

Clever problem indeed!!

17. RaphaelFilgueiras

18. Algebraic!

hmm, how?

19. RaphaelFilgueiras

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20. Jemurray3

Walter Lewin actually does a demonstration of this effect in his 8.01 course, if you want to see it for yourself.

21. Algebraic!

I think I remember seeing it, actually. I just don't remember the logic.

22. RaphaelFilgueiras

the ballon will suffer a aparent g in gap direction

23. experimentX

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24. RaphaelFilgueiras

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25. RaphaelFilgueiras

the buyont force will be in same direction of gap

26. experimentX

helium is tricky gas. should have noted.

27. Algebraic!

pressure differential. well, I feel dumb. http://www.youtube.com/watch?v=Vi8t0x6zZNY&feature=player_detailpage#t=1524s

28. experimentX

haven't taken this course yet. is it first year mechanics?

29. Algebraic!

It's not calc. based year one physics?