Momentum-impulse vs. rectilinear motion equations

AI Thread Summary
Using rectilinear motion equations requires a correct understanding of acceleration, which in this case is not simply gravity due to the system involving pulleys. The assumption that acceleration is gravity is incorrect because the objects are not in free fall; they are influenced by additional forces. To apply kinematics correctly, one must calculate acceleration using the sum of forces equals mass times acceleration (F=ma). The discussion emphasizes the importance of accurately determining the forces acting on the system to solve for acceleration. Understanding these principles is crucial for resolving the discrepancies between the two methods.
cipotilla
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Here we are again...We have apully system as seen on page 1 of the attachment. I tried using the rectilinear motion equations, assuming a=constant, but the book's solution used the momentum-impulse equations and got a different answer.

I don't understand why my approach is incorrect... Thanks.
 

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Nothing wrong with using kinematics with constant acceleration. But how did you solve for the acceleration?
 
I just assumed the acceleration was gravity...I'm guessing there's something wrong with that assumption...but I don't know what...
 
Its not gravity because its not free falling. I can't believe I made that mistake. I would have to use Sum of forces=ma and find the acceleration if i wanted to use kinematics, right?
 
The "acceleration due to gravity" applies to a freely falling object--where the only force acting is gravity. But these objects are attached to ropes and pulleys--they are not freely falling! Solve for the acceleration like in any other pulley problem.
 
cipotilla said:
Its not gravity because its not free falling. I can't believe I made that mistake. I would have to use Sum of forces=ma and find the acceleration if i wanted to use kinematics, right?
Exactly. (I knew you'd snap out of it. :smile: )
 

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