Pressue in liquid 9th class (lec 4)

 video lecture for Pressure in a liquid

Question 1: What do you understand by buoyancy? Question 2: Why does an object float or sink when placed on the surface of the water?

Question 3: An object is put one by one in three liquids having different densities. The object floats `1,2,3` with `1/9`,`2/11` and `3/7` parts of their volumes outside the liquid surface in liquids of densities `d_{1}` `d_{2}` and `d_{3}` respectively. Which of the following statement is correct?

(a) `d_{1} > d_{2} > d_{3}`

(b) `d_{1}>d_{2}< d_{3}`

(c) `d_{1}<d_{2}>d_{3}`

(d) `d_{1}<d_{2}<d_{3}`

Question 4: An object weighs `10 N` in air. When immeped fully in water, it weighs only `8 N`. The weight of the liquid displaced by the object will be

(a) `2N`

(b) `8 N`

(c) `10N`

(d) `12 N`

Question 5: (a) A cube of side `5 cm` is immersed in water and then in saturated salt solution. In which case will it experience a greater buoyant force. If each side of the cube is reduced to `4 cm` and then immersed in water, what will be the effect on the buoyant force experienced by the cube as compared to the first case for water. Give reason for each case.

(b) A ball weighing `4 kg` of density `4000 kg m s^{-3}` is completely immersed in water of density `103 kgm^{-3}`. Find the force of buoyancy on it. (Given `g = 10 m s^{-2}` .)

Answer Key

Q1:  When an object is immersed in water, the upward force exerted by the water on it is called buoyancy.

Q2:  Answer- (i) If the density of an object placed on the surface of the water is less than the density of water, then that object will float.

(ii) If the density of an object placed on the surface of the water is greater than the density of water, then the object will sink. (iii) If the density of the object is equal to the density of water, then the object will float in the middle Will remain.

Q3:  

(d) When an object floats in liquid, a buoyant force acts on it, which is equal to the weight of liquid displaced by that object. As more part of object’s volume floats outside the liq^O&ftore is the buoyant force and higher is the density of liquid.

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Q4:  Solution:

(a) Weight in air `= 10 N` Weight in water `= 8 N`

The weight of the liquid displaced by the object = Loss in weight of the object `= 10N-8N = 2N`.

Q5:  Solution:

(a) The cube will experience a greater buoyant force in saturated salt solution than in water as the density of saturated salt solution is greater than that of water.

As buoyant force = weight of liquid displaced by the object =Vag (a is density of liquid) As each side of the cube is reduced to `4 cm` from `5 cm`, so volume of cube decreases and hence the buoyant force also decreases.

(b) Here, Mass of ball, `m = 4 kg` Density of ball, `ρ= 4000 kg m^{-3}` Density of water `σ=103 kg m^{-3}`

Volume of ball `=\frac{m}{ρ}=\frac{4kg}{4000kgm^{-3}}=10^{-3} m^{3}`

So, force of buoyancy on the ball

`=Vσg=10^{-3}\times10^{3}\times10=10N`.