# Homework Help: Tension Force (should be easy)

1. May 10, 2012

### Angry Citizen

1. The problem statement, all variables and given/known data

The weights of the blocks are W_A = 120 lb and W_B = 20 lb, and the surfaces are smooth. Find the acceleration of block A.

[A picture is shown. Block A is connected via rod to a pulley, and Block B is hanging from the pulley via cord.]

2. Relevant equations

F=ma

3. The attempt at a solution

This is embarrassing. I feel like I'm having a brain fart. I did free body diagrams showing that T-W_B=(M_A)a, and that T=(M_B)a, and substituted them into equations, but the book says it's the wrong answer; I should be getting 4.02 ft/s^2, but I'm getting something different. Any help please?

2. May 10, 2012

### PhanthomJay

You ll have to attach the pic or better describe the setup. 3 pulleys? What side is rod connected? How many ropes?

3. May 10, 2012

### Angry Citizen

Here you go.

#### Attached Files:

• ###### Untitled.jpg
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4. May 10, 2012

### PhanthomJay

I don't know what answer you are getting, but your signage is wrong when looking at
Block B. also make sure your mass unit are in slugs.

Edit: you also used M_B in your second equation when you should have used M_A.

Last edited: May 10, 2012