How to Calculate Mass of a Block Using Force Equations?

[miktalmyers]

Calculating mass of a block...

Homework Statement

Homework Equations

Force equations/Kinetic equations

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

 

[miktalmyers]

No one?

 

[PhanthomJay]

Homework Statement

Homework Equations

Force equations/Kinetic equations

The Attempt at a Solution

FA = mA * 9.8 m/sec2

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.

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

yes, this is correct.

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

stay away from pseudo forces, the horizontal forces on B are friction from the table, friction from C, and the tension force FB

FB = FfB + FfC + (1.4 kg * 0.8 m/sec2)

yes...

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

I'm not checking your numbers, but your equations are OK

7.784 N = mA * 9.8 m/sec2

mA = 0.794286 kg ~ 794 grams

This is your error as i noted above; the weight mA acts down, and FA acts up, solve for mA using Newton 2.