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A ball throw up and down !

  1. Sep 16, 2007 #1
    The problem statement, all variables and given/known data
    #1
    a) A ball is throwing straight upward, Is that motion positive or negative acceleration/velocity/displacement ?
    b) A ball is falling down (what go up must come down), Is that a positive or negative acceleration/velocity/displacement ?

    #2
    a) A car is moving forward then the driver step on brake, Is that motion positive or negative acceleration/velocity ?
    b) A car is moving backward then the driver step on brake, Is that motion positive or negative acceleration/velocity/displacement ?
    c)A car is moving backward then the driver step on gas, Is that motion positive or negative acceleration/velocity/displacement ?

    My answers
    #1
    a) positive acceleration, negative velocity (the velocity is decreasing), positive displacement
    b) negative acceleration, positive velocity (the velocity is increasing), negative displacement
    #2
    a) negative acceleration (the car is slowing down), negative velocity,
    b) same a
    c) i got confused
     
  2. jcsd
  3. Sep 16, 2007 #2

    learningphysics

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    careful... positive velocity just means it is moving foward or upward... negative velocity just means it is moving backwards or downwards... it doesn't meant that the speed is decreasing (that gives information about acceleration).

    If an object is moving forward and slowing down... that means velocity is positive and acceleration is negative. Also, if an object is moving backwards and speeding up backwards... acceleration is again negative.

    Give these another shot.
     
    Last edited: Sep 16, 2007
  4. Sep 16, 2007 #3
    #1
    a)positive velocity, negative acc
    b)negative velocity, negative acc
    #2
    a)positive velocity, negative acc
    b)negative velocity, positive acc
    c)negative velocity, negative acc
    Is those right ?
     
  5. Sep 16, 2007 #4

    learningphysics

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    All correct! But you didn't write the displacements.
     
  6. Sep 16, 2007 #5
    oh i forgot, Is the sign of displacements the same with velocity ? positive - up, back
    negative back and down
     
  7. Sep 16, 2007 #6

    learningphysics

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    Yes, in these cases it is.
     
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