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

A metal cube of edge 5cm and density 9.0 gcm-3 is suspended by a thread so as to be completely immersed in a liquid of density 1.2 cm-3. Find the tension in the thread.

(Take g = 10 m/s2)

[Hint: Tension in thread = apparent weight of the cube in liquid

Given:

Density of metal cube = 9.0 gcm-3

Density of liquid = 1.2 gcm-3

Side of the cube = 5cm

 Volume of the cube = 5 x 5 x 5 = 125cm3

To find the weight of the cube:

Mass of the cube = volume of the cube x density of the cube

= 125 x 9 = 1125 g

∴ Weight of the cube = 1125 gf

The weight of the cube acts downwards

Upthrust acting on the cube = weight of the liquid moved

= volume of the cube x density of the liquid x g

= 125 x 1.2 x g = 150gf

Upthrust on the cube acts in the upward direction

Tension in thread = total force acting in the downward direction

= weight of the cube acting downwards – upthrust acting on the cube

= 1125 – 150 = 975gf or 9.75N

Video transcript
"hello everyone welcome to vidu this is priya and here we have an interesting question to solve let us look into the question a metal cube of edge 5 centimeter and density 9 gram per centimeter cube is suspended by a thread as to be completely immersed in a liquid of density 1.2 centimeter cube find the tension in the thread take gravity equal to 10 meter per second square so what's a hint given here is tension in thread equal to apparent weight of the cube in liquid so let us see what are the values given the question it has given the value of density of the metal cube which is 9.0 gram per centimeter cube and it has also given the density of liquid which is 1.2 gram per centimeter cube and the side the side of the cube also given and we need to find out the volume of the cube we have a formula volume of q equal to 5 into 5 into 5 which is 1 25 centimeter cube so which gives a value of 125 centimeter cube and to find the weight of the cube we have a formula mass of the q equal to volume of the queue volume of q into density of the q we know mass of the cube equal to volume of the cube into density of the cube and when we substitute the value 125 in volume of the cube and we know the density of the cube which is 9 when we multiply these two 120 1 25 into nine which gives one thousand one hundred and twenty five grams so this is the weight of the cube the weight of the cube acts downwards the weight of the cube acts downwards so up thrust acting on the queue equal to weight of liquid weight of liquid you want as we know abstract acting on the cube equal to weight of the liquid which has spelled our mode that implies volume of the cube into density of the liquid density of the liquid into gravity so we know volume of the cube is 125 and density of the liquid is 1.2 and we know the gravity is 10 which gives an answer 1 50 g f a thrust on the on the cube acts in the upward direction this is for downward direction and when we see the uprest of the cube on upward direction let us see a thrust off the cube acting in the upward direction if it is a upward direction tension in thread tension in thread equal to total force total force acting in the downward direction so tension in thread is a total force acting in the downward direction so we need to measure weight of the cube weight of the cube acting downwards minus upthrust acting on the cube so to find out the tension in the thread we know total force acting in the cube downward direction will be will be the tension in thread so we get this by taking weight of the cube acting downwards minus up thrust acting on the cube we know the weight of the cube is 1125 and we know the upthrust acting on the cube which is 150 so total will get 975 g f or we can also write this as 9.75 newton hope you understand this video if you like my video please subscribe to our channel and if you have any doubts you can keep a message in comment box thank you"
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