 #1
kasnay
 10
 0
 Homework Statement:

A 3.5kg block is on a tabletop and is attached by a string to hanging block of mass 2.8 kg. The blocks are released from rest and allowed to move freely. If the table has a coefficient of friction of 0.4
A) Find the Acceleration
 Relevant Equations:
 fnet x and fnet y
I have a conceptual question about this problem.
I can write the 3.5 kg block equation as Fnet(block 1)=(Force of tension)(Force of friction)=m1a
I can write the 2.8 kg block as Fnet(block 2)=(Force of tension)(Force of gravity2)=m2a
My question is this
If I set the forces of friction equal I get the wrong answer. However If I assume the fnet of block 2 is negative (because its going to fall) I get the correct answer.
Why do I need to assume the second block fnet is negative? Shouldnt the math already account for what needs to happen?
I can write the 3.5 kg block equation as Fnet(block 1)=(Force of tension)(Force of friction)=m1a
I can write the 2.8 kg block as Fnet(block 2)=(Force of tension)(Force of gravity2)=m2a
My question is this
If I set the forces of friction equal I get the wrong answer. However If I assume the fnet of block 2 is negative (because its going to fall) I get the correct answer.
Why do I need to assume the second block fnet is negative? Shouldnt the math already account for what needs to happen?