# Homework Help: Calculating mass of a block

1. May 4, 2010

### miktalmyers

Calculating mass of a block....

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

2. Relevant equations

Force equations/Kinetic equations

3. The attempt at a solution

FA = mA * 9.8 m/sec2

FA = FB
where FB is the force acting through the string from A to B

The forces on B are friction from the table, friction from C, and the force it takes to move B's mass at 0.8 m/sec2
FB = FfB + FfC + (1.4 kg * 0.8 m/sec2)

FfC = 0.3 * ( 0.6 kg * 9.8 m/sec2 )

FfC = 1.764 N

FfB = 0.25 * [ (1.4 kg + 0.6 kg) * 9.8m/sec2 ]

FfB = 4.9 N

FB = 1.764 N + 4.9 N + (1.4 kg * 0.8 m/sec2)

FB = 7.784 N

FA = 7.784 N

7.784 N = mA * 9.8 m/sec2

mA = 0.794286 kg ~ 794 grams

2. May 4, 2010

### miktalmyers

Re: Calculating mass of a block....

No one?

3. May 4, 2010

### PhanthomJay

Re: Calculating mass of a block....

This is not FA (the string tension), this is the weight of block mA. FA must be less than the weight of block mA, or else it can't accelerate downward.
yes, this is correct.
stay away from pseudo forces, the horizontal forces on B are friction from the table, friction from C, and the tension force FB
yes...