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DLS

  • 3 years ago

At the same instant that a 0.50-kg ball is dropped from 25 m above Earth, a second ball, with a mass of 0.25 kg, is thrown straight upward from Earth’s surface with an initial speed of 15 m/s. They move along nearby lines and pass without colliding. At the end of 2.0 s the velocity of the center of mass of the two-ball system is:

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  1. mayankdevnani
    • 3 years ago
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    options??

  2. DLS
    • 3 years ago
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    A. 11 m/s, down B. 11 m/s, up C. 15 m/s, down D. 15 m/s, up E. 20 m/s, down

  3. DLS
    • 3 years ago
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    V(a)=20m/s V(b)=-5m/s

  4. mayankdevnani
    • 3 years ago
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    Since this is multiple choice, lets use 10 m/s^2 as the acceleration due to gravity.

  5. DLS
    • 3 years ago
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    so 0.50*20+0.25*(-5)/0.75

  6. mayankdevnani
    • 3 years ago
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    The trick is to look at the momentum of the system after 2 seconds. So for the heavy ball, after two seconds, it has a velocity of -20 m/s.

  7. DLS
    • 3 years ago
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    10+1.25/0.75 (wil we have a minus sign or are we considering just the mag?

  8. mayankdevnani
    • 3 years ago
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    Thus it has a momentum of -10 kg*m/s.

  9. DLS
    • 3 years ago
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    u took downwards negative?

  10. mayankdevnani
    • 3 years ago
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    yaa, i took downwards negative. The light ball will have a velocity of -5 m/s (15 m/s going up minus 20 m/s due to gravity gives 5 m/s downward). So it has a momentum of -1.25 kg*m/s.

  11. mayankdevnani
    • 3 years ago
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    In both cases, the negative sign indicates the momentum is downward, and the sum is -11.25 kg*m/s.

  12. mayankdevnani
    • 3 years ago
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    Find the velocity of the system by dividing by the total mass (0.75 kg)?? then solve it and you get your answer.

  13. DLS
    • 3 years ago
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    i took downwards positive and upwards negative,im getting something different

  14. mayankdevnani
    • 3 years ago
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    no, always took downwards negative and upwards positive

  15. DLS
    • 3 years ago
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    v=0+10*2 v=20m/s downwards and v=15-20 v=-5m/s upwards

  16. mayankdevnani
    • 3 years ago
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    no!!!!

  17. mayankdevnani
    • 3 years ago
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    Find the velocity of the system by dividing by the total mass (0.75 kg) and you get 15 m/s down. so your answer is option C) 15 m/s up.

  18. DLS
    • 3 years ago
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    wait u messed up!!

  19. mayankdevnani
    • 3 years ago
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    no,, 15 m/s down.

  20. DLS
    • 3 years ago
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    im taking downwards negative and upwards positive so for the 0.25 kg ball, v=15+20=35? we are taking upwards positive right

  21. mayankdevnani
    • 3 years ago
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    right

  22. DLS
    • 3 years ago
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    0.25*35=8.75

  23. DLS
    • 3 years ago
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    -20+8.75

  24. DLS
    • 3 years ago
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    oh :o

  25. DLS
    • 3 years ago
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    -11.25/0.75=-15m/s

  26. DLS
    • 3 years ago
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    i see! :D

  27. mayankdevnani
    • 3 years ago
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    okk

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