# Momentum-impulse vs. rectilinear motion equations

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.

#### Attachments

• P13 132.pdf
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## Answers and Replies

Doc Al
Mentor
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 dont know what...

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?

Doc Al
Mentor
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
Mentor
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. )