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Need Help

  1. Mar 24, 2009 #1
    Need Help !!

    1. The problem statement, all variables and given/known data
    Two bodies, A and B arrive at the base of a fricitonless inclined plane with equla momenta of magnitude p. mb=3ma. The ratio of the max. distance Db that B travels up the plane to the max. distance Da the A travels up the plane is : Db=Da ? is that correct ?

    2. Relevant equations

    p=mv
    v=d/t

    3. The attempt at a solution
    I am not sure I subs. back and then cancled every thing and that was the result.
     
  2. jcsd
  3. Mar 24, 2009 #2

    LowlyPion

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    Re: Need Help !!

    If the momentum is equal and one mass is 3 times the other what does it say about the relationship of their speeds?
     
  4. Mar 25, 2009 #3
    Re: Need Help !!

    In fact, there is no thing about the speed, I typed the question as it is ..

    Thank you
     
  5. Mar 25, 2009 #4

    LowlyPion

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    Re: Need Help !!

    You don't need to know the absolute speed of either. But you do know something about their relative speeds as a consequence of knowing they have the same momentum.
     
  6. Mar 25, 2009 #5
    Re: Need Help !!

    Sorry, I didn't get you, would you tell a bit more ?
     
    Last edited: Mar 25, 2009
  7. Mar 25, 2009 #6

    LowlyPion

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    Re: Need Help !!

    You know momentum is the same. But the masses are different. That means the speeds are different.

    So how are the speeds different?
     
  8. Mar 25, 2009 #7
    Re: Need Help !!

    Well, then what does it has to do with their distances ?
    Do you mean to say that will different distances as a result of that?

    Or,, in other words how to prove that?
     
  9. Mar 25, 2009 #8

    LowlyPion

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    Re: Need Help !!

    You think something that is slower at the bottom of a ramp will go as far as something that is faster?

    I guarantee that it will not.

    Regardless of the mass, the acceleration from gravity down the ramp is the same whether slow or fast. So clearly then something going faster will go farther. It's simple kinematics.
     
  10. Mar 25, 2009 #9
    Re: Need Help !!

    It is a muliple choisse ques. We have to choose one and prove it

    Her are the choises:
    Db=1/9Da

    Db=1/3Da

    Db=Da

    Db=3Da

    Db=9Da


    I thought about the 3rd choise because of the equal mometom, and I am not sure, What do you think? and how to prove ?
     
  11. Mar 25, 2009 #10
    Re: Need Help !!

    How did you know that one is faster? The question says that they will go down the incline with we know which is the faster?
     
  12. Mar 25, 2009 #11

    LowlyPion

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    Re: Need Help !!

    Momentum P is a constant

    P = Ma*Va = Mb*Vb

    And you know that Mb = 3*Ma

    So what must Va be relative to Vb ?
     
  13. Mar 25, 2009 #12
    Re: Need Help !!

    Yes, what am trying to do is exactky same to what you did, but I think that their velocities will be the same and therefore the distance will be the same?

    ISn't it???
     
  14. Mar 25, 2009 #13

    LowlyPion

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    Re: Need Help !!

    No. Why are you resistant to just doing the math? Substitute 3*Ma for Mb. What does that tell you about the velocities?
     
  15. Mar 25, 2009 #14
    Re: Need Help !!


    Or do you think that I should use conservation of energy?

    Or I choose the second choise
     
  16. Mar 25, 2009 #15

    LowlyPion

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    Re: Need Help !!

    I think you should just do the math and get the right answer.
     
  17. Mar 25, 2009 #16
    Re: Need Help !!

    I will get equal velocities and then equal distancse
    Right?
     
  18. Mar 25, 2009 #17
    Re: Need Help !!

    Please, I getting confused more and more ...

    The question is asking about the distances not the velocities..

    ???????

    I will get equal velocities and then, what math do mean ??
    Do mean that v=d/t ?? What has to do with gravitation
     
  19. Mar 25, 2009 #18

    LowlyPion

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    Re: Need Help !!

    The answer is no. You do not get the same velocities. You keep repeating that and it's still no.

    The momentum is the same.
    The masses are different.
    The velocities MUST be different.

    I've written the equations. I've suggested what to substitute to solve for what the velocity must be.

    What I won't do is just give you the answer.
     
  20. Mar 25, 2009 #19
    Re: Need Help !!


    Do you mean that we will subs. of the masses after we write the equation you just gave me..
    And then we get that
    Vb= 1/3 Va

    and Now, do we use that v=d/t to find the distance?

    Thank you :)
     
  21. Mar 25, 2009 #20

    LowlyPion

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    Re: Need Help !!

    Almost. Now that you know what the velocity is, and you understand that acceleration (actually deceleration) down the incline is the same then how will that velocity difference translate into the distance until it stops going higher?

    The equation for that is

    Vf2 = Vi2 + 2*a*x

    Note that a is -, and Vf is where it reaches 0.

    What is the effect in x, when V is 3 times larger?
     
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