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What is the time needed for A to move 50cm downward?

  1. Jul 4, 2005 #1
    https://www.physicsforums.com/attachment.php?attachmentid=4204&stc=1
    Both mass of A and B is 0.49kg. A ring of 0.001kg is added on A to let A begin to move downward.
    (a) what is the time needed for A to move 50cm downward?
    (b)What is the time needed for A to move another 50cm downward if the ring was taken off imediately after first 50cm?


    I can find the the answer of (a) but i dont know how to solve the 2nd question since I have no idea at all about how to find out the acceleration after first 50cm. Can anyone help me?
     

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  3. Jul 4, 2005 #2
    I assume you did some free body force diagrams and analysed them for the first part?
    A had the extra weight and so due to f=ma began to move.
    I assume you then used constant acceleration formulae to calculate the time it took for A to move 50cm.

    Now....

    After the first 50cm, the weight is removed. What does this mean in terms of the equating of forces?... Does A still accelerate? Does it stop dead? Does it keep the same velocity?
    Think about Newton's First Law : An object will remain at rest or at constant velocity unless acted upon by an external force.
     
  4. Jul 5, 2005 #3
    A is still moving ...is it still accelerate with the same acceleration as previous?
    How does Newton`s First Law relate with this question?
     
  5. Jul 5, 2005 #4

    HallsofIvy

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    "An object will remain at rest or at constant velocity unless acted upon by an external force."

    After the additional weight is removed, there is no (net) external force- A will continue to move with no acceleration but at the speed it had at 50 cm.
     
  6. Jul 12, 2005 #5
    Thanks. Is the answer equal to 5s?
    Assume s=0.5
    u= the speed at first 50cm
    a=0
     
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