# Find mass of an object hanging on string

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)

## Answers and Replies

Make a diagram of the situation.

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

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

i dont know how to arrange it to find the mass B...... 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.

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.