# Selina Solutions Class 9 Physics Solutions for Exercise 5B in Chapter 5 - Chapter 5 Upthrust In Fluids Archimedes Principle And Floatation

Question 27 Exercise 5B

A body of mass 70kg, when completely immersed in water, displaces 20,000 cm3 of water.

Find: (i) the weight of the body in water and (ii) the relative density of material of the body.

Mass of the body = 70kg

Volume of water displaced = 20,000 cm3 or 0.02 m3

(i) To find the weight of the body submerged in water

Mass of the solid immersed in water= mass of the water displaced

= volume of water displaced x density of water = 0.02 x 1000 = 20kg

Weight of the body, W = mg

Weight of the body in water = (70 x 9.8) – (20 X 9.8) = 50 kgf

(ii) Density in C.G.S = mass/volume = 70 x 1000/ 20000 =3.5 g cm3

Relative density of the solid = density in C.G.S without unit = 3.5

Video transcript
hello everyone welcome to lido this is priya and here we have an exciting question to solve let us look into the question a body of mass 70 kgs when completely immersed in water displaces 20 000 centimeter cube of water find the weight of the body in water and the relative density of material of the bottle water so first we try to solve the first one the weight of the body in the water before that as what we know from this question is we know the mass of the body is 70 kgs mass of body is 70 kgs and the second thing what we know is the water has displaced with twenty thousand centimeter cube what we know in the second volume of water volume of water which is displaced equal to 20 thousand centimeter cube and in meters when we have to write it will be 0.02 meter cube now to find out the weight of the body submerged in water we have a formula mass of solid immersed in the water equal to mass of mass of water displaced that means volume of water displaced volume of water displaced into density of water density of water as we know the volume of water displaced it is 0.02 and the density of water is 1000 so we get 20 kgs weight of the object w equal to mg weight of the object w equal to mg so weight of the body in water weight of body in water equal to 70 into nine point eight ninety nine point eight is a gravity minus twenty into 9.8 so as a result we'll get 50 k g f so weight of the body in water is 50 kgf now we need to find out the second one the relative density of material of the body density in cg is density in cgs equal to mass by volume we know the mass it is 70 into 1000 by we know the volume 20 000 when we simplify this we'll get 3.5 centimeter gram 3.5 gram per centimeter cube now relative density of water the relative density of solid relative density of solid equal to density in cgs cgs without unit we don't use unit here without unit that means three point five so this is the relative density of solid hope you understand this video if you like my video please subscribe to nido channel and if you have any doubts you can keep a message in comment box thank you
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