Momentum-impulse vs. rectilinear motion equations

[cipotilla]

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.

 

[Doc Al]

Nothing wrong with using kinematics with constant acceleration. But how did you solve for the acceleration?

 

[cipotilla]

I just assumed the acceleration was gravity...I'm guessing there's something wrong with that assumption...but I don't know what...

 

[cipotilla]

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?

 

[Doc Al]

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.

 

[Doc Al]

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. )