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Inclined plane and velocity

  1. Mar 5, 2013 #1
    1. The problem statement, all variables and given/known data
    If the incline plane is 10 m long, and it is elevated at an angle of 30 degrees: If a 5kg object takes t1 seconds to go down the ramp and a 10kg object takes t2 seconds to go down the ramp... What is the relationship between t1 and t2?


    2. Relevant equations
    F = ma
    Fx of slope = mgsinx
    Fnorm = mgcosx


    3. The attempt at a solution
    If the incline plane is 10 m long, and it is elevated at an angle of 30 degrees... Then this means the x component of acceleration down the ramp is 4.9 m/s^2 meaning that a 5 kg object and 20 kg object will take the same amount of time to go down this 10 m ramp.

    This is correct, right?

    Secondly:

    Also, if you throw an object up, when Vf = 0 it will start accelerating downwards.

    Does this mean the velocity increases, or decreases? I answered in class that it depends on relativity, being as its a vector quantity (i.e. up can be negative or down can be negative). The teacher said this is wrong, and the velocity has to increase. Why?
     
  2. jcsd
  3. Mar 5, 2013 #2
    Yes, that is correct.

    It's possible that your teacher was thinking about the magnitude of the velocity (also known as speed) which does increase as the object starts falling downwards.
     
  4. Mar 5, 2013 #3
    Thank you.

    Also, it does increase if it falls downwards. But, what if you make downwards negative and upwards positive?

    Then say it will go from 0 to -10m/s to -20m/s etc. Would this be considered decreasing?

    Or is the magnitude of the quantity the absolute value of it?

    magnitude velocity = |V|



    Ah okay, thank you
     
    Last edited: Mar 5, 2013
  5. Mar 5, 2013 #4
    Magnitude is the absolute value, so, yes, it will always be increasing.

    If your answer was "it depends on relativity, being as its a vector quantity (i.e. up can be negative or down can be negative)", it's absolutely right.
    But if instead your answer was "it depends on relativity, being as its a vector quantity", he might have ruled it out because it's incomplete.
     
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