Find mass of an object hanging on string

[angi-18]

please tell me how to find the mass of an object which is hang on the string?
I mean what would be the formula ?
i am also writing here my numerical question...
Q:two bodies A and B are attached to a string which passes over a pulley so that two bodies hang vertically.if the mass of body A is 4.8 kg find the mass of body B which moves down with an acceleration of 0.2m/s.s.(g=9.8m/s.s)

 

[grzz]

Make a diagram of the situation.

Look carefully at mass B. How many forces act on mass B?

 

[angi-18]

no force is acting on mass B the formula given in our textbook is:
α=(mB-mA/mB+mA) x g

i don't know how to arrange it to find the mass B...

 

[grzz]

We are trying to show HOW to get that formula.

Look at mass B. It has the Earth pulling it down - that is the weight mB x g of mass b - and the string pulling it up by a tension T.

Now use Fnet = ma for mass B.

 

[grzz]

Fnet = ma ...for mass B

mBg - T = mBa

Fnet = ma ....for mass A

T - mAg = mAa

If the tension T is eliminated from the above two equations then one gets the equation required.