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Momentum-impulse vs. rectilinear motion equations

  • Thread starter cipotilla
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  • #1
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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 dont understand why my approach is incorrect... Thanks.
 

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  • #2
Doc Al
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Nothing wrong with using kinematics with constant acceleration. But how did you solve for the acceleration?
 
  • #3
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I just assumed the acceleration was gravity...I'm guessing there's something wrong with that assumption...but I dont know what...
 
  • #4
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Its not gravity because its not free falling. I cant 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?
 
  • #5
Doc Al
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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.
 
  • #6
Doc Al
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Its not gravity because its not free falling. I cant 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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