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Relative density of a substance is the density of that substance with respect to water.

R . D. = density of liquid / density of water  = d_m / d_w
         =  loss of weight of a solid in milk / loss of weight of that solid in water.

Let us take a clean ball or cube of volume V_s cm^3, density d_s  and weighing m grams.  Let us take a utensil of milk and then a tub of water.  Let us get one spring balance to measure the weight/mass of the ball.  Tie a string the ball or cube and attach it to the spring balance.  Please find a ball or cube having a density more than milk and water.  Like any metal.

1. Measure the mass/weight of the ball in air.  Call it W_a.

2. Measure the mass/weight of the ball, dipping the ball completely inside milk.  Call it W_m.

3.  Measure the mass / weight of the ball by taking it out from the milk vessel and putting it in the tub of water, completely immersing it in water.  Call it W_w.

     Relative density of milk = d_m =  (W_a  - W_m) / ( W_a - W_w)


 W_a - W_m =  [ V_s * d_s * g -  (V_s * d_s * g  - V_s * d_m * g ) ]
 W_a - W_w    = [ V_s  * d_s * g  -  (V_s * d_s * g - V_s * d_w * g ) ]

           hence the ratio gives  d_m / d_w

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