- #1

- 10

- 1

- 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?