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

[frozen7]

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 don't 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?

 

[tyco05]

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.

 

[frozen7]

A is still moving ...is it still accelerate with the same acceleration as previous?
How does Newton`s First Law relate with this question?

 

[HallsofIvy]

A is still moving ...is it still accelerate with the same acceleration as previous?
How does Newton`s First Law relate with this question?

"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.

 

[frozen7]

Thanks. Is the answer equal to 5s?
Assume s=0.5
u= the speed at first 50cm
a=0