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Momentum and movement basic GCSE Quest

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  1. Mar 10, 2012 #1
    Mr Green, mass 80 kg, stepped off a boat of mass 35 kg at a speed of 3m/s. how fast did the boat move in the opposite direction?

    i have no idea how to work it out?
     
  2. jcsd
  3. Mar 10, 2012 #2

    PeterO

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    I think you will find that Mr Green and the boat are assumed to be at rest initially, so the total momentum was - and will remain - zero.
     
  4. Mar 10, 2012 #3
    ok thanks but how do i answer the question?
     
  5. Mar 10, 2012 #4
    i still don't understand i don't understand what formula to use as well.
     
  6. Mar 10, 2012 #5

    PeterO

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    If you define the direction Mr Green goes as positive, he will have some positive momentum after he leaves the boat.
    To maintain a total of zero, the boat must have an equal sized, but negative momentum - so it will be travelling at an appropriate speed in the negative direction (ie the other way)
     
  7. Mar 10, 2012 #6
    i think i remeber my teacher saying that it's: 80 + 35= 115
    115 x 3 = 375
    375/35 = 1.3...m/s

    is that correct?
     
  8. Mar 10, 2012 #7

    PeterO

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    Sadly, no.

    When you added the masses and multiplied by 3 (presumably the speed of Mr Green), you calculate the momentum if both he and the boat were travelling at that veocity; only he is.
     
  9. Mar 10, 2012 #8

    PeterO

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    answer these in order:
    What is Mr Green's mass?

    What is Mr Green's velocity?

    What is Mr Green's momentum?

    What is the boat's momentum?

    What is the boat's mass?

    What is the boat's velocity?
     
  10. Mar 10, 2012 #9
    you are very kind for helping me.

    80kg

    240 kg m/s

    105 kg m/s

    35kg

    no idea for velocity i know formula is f=d/t but you aren't given time or distance.
     
  11. Mar 10, 2012 #10

    PeterO

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    For that final velocity - think about how you calculated the momentum of Mr Green - and don't forget to use + and - signs correctly.
    Velocity is a vector; so can be positive or negative.

    EDIT: Hint - answers 3 and 4 add up to zero
     
  12. Mar 10, 2012 #11
    is the answer 240/35?
     
  13. Mar 10, 2012 #12
    did i get the question right?
     
  14. Mar 10, 2012 #13

    PeterO

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    Certainly is.
     
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