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AravindG

  • 5 years ago

A boy standing on a stationary lift (open from above) throws a ball upwards with the maximum initial speed he can, equal to 49 m/s. How much time does the ball take to return to his hands? If the lift starts moving up with a uniform speed of 5 m/s and the boy again throws the ball up with the maximum speed he can, how long does the ball take to return to his hands?

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  1. AravindG
    • 5 years ago
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    u see i got first part

  2. AravindG
    • 5 years ago
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    10 s

  3. AravindG
    • 5 years ago
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    but i dont undestand y we get same answr in 2nd case

  4. JamesJ
    • 5 years ago
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    The position of the ball at time time is \[ y(t) = 49 t - \frac{1}{2}gt^2 \] \[ = 49 t - 4.9t^2 \] \[ = 0 \] when \( t = 0, 10 \) seconds, right. Now in the second case, the speed of the ball is (49 + 5) m/s = 54 m/s, so it's position is \[ y_{ball}(t) = 54 t - 4.9t^2 \] ... but ...

  5. JamesJ
    • 5 years ago
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    the position of the boy is \[ y_{boy}(t) = 5t \] Hence the question is: for what t is \[ y_{ball}(t) = y_{boy}(t) \] Solve that equation and you'll get back \( t = 0, 10 \) seconds.

  6. AravindG
    • 5 years ago
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    hw u got pos of boy as 5t??

  7. JamesJ
    • 5 years ago
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    because the lift is moving up at a constant velocity of 5 m/s

  8. AravindG
    • 5 years ago
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    wow i got the old eqn

  9. AravindG
    • 5 years ago
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    :)

  10. JamesJ
    • 5 years ago
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    right, exactly

  11. AravindG
    • 5 years ago
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    thx

  12. AravindG
    • 5 years ago
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    so shall i make it general

  13. AravindG
    • 5 years ago
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    that if someone is in a lift and the lift moves with const speed then time to throw and catch the balll is same as when lift is at rest??

  14. JamesJ
    • 5 years ago
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    Yes.

  15. AravindG
    • 5 years ago
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    k

  16. AravindG
    • 5 years ago
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    wt if it was not const speed?

  17. JamesJ
    • 5 years ago
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    Then you won't have equality.

  18. JamesJ
    • 5 years ago
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    Frames of reference moving at a constant speed are called inertial frames. The laws of Newtonian physics work the same in all inertial frames.

  19. AravindG
    • 5 years ago
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    u mean any system with a=0 is an inertial frame?

  20. JamesJ
    • 5 years ago
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    Yes

  21. AravindG
    • 5 years ago
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    wow

  22. AravindG
    • 5 years ago
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    wel james hav u seen a type of problem in which a rope is given and we can slide down that and the breaking tension is given and max a with which we can slide down??

  23. JamesJ
    • 5 years ago
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    Yes, but I'm going to go now. I'm sure gogind or jemurray or others can help you with that.

  24. AravindG
    • 5 years ago
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    oh bye

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