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8th Class Science Force and Pressure Buoyancy


Category : 8th Class

*        Buoyancy


When an object is immersed in liquid an upward force acts on it, which is called buoyancy. The buoyant force does not depend upon the weight of the object or the shape of the object, it only depends on the weight of the liquid displaced by the object. This principle is applicable to all the object of any density. If the density of the object is more than the density of the liquid, the object will sink. But if the density of the liquid is more than the object, the object will float on the surface of the liquid. On the other hand, if the density of the liquid and objects are equal, the object will float with half part immersed in the liquid.





        Find the weight of an object whose volume 45 m3 and density 2.4 gm/m3.

(a) 0.108kg

(b) 1.1 kg

(c) 2.4kg

(d) 3.2kg

(e) None of these


Answer: (a)


Weight of an object is given by,

W = m x g

We can find the value of W by substituting the other parameters and we get the weight to be 0.108 kg.



        Thomas throws a silver coin in the sea. The density of the coin is 0.985 gm/m3. Will the coin float on the surface of water or it will sink?

(a) Sink

(b) Float

(c) Float half immersed

(d) can't say

(e) None of these


Answer: (b)


Since the density of the coin is less than the density of water, hence it will float on water.



*           Amount of Buoyancy

The amount of buoyancy of a body is determined by the body's specific gravity. Specific gravity is the ratio of the weight of a body to the weight of the water it displaces. Pure water has a specific gravity 1. The specific gravity of other objects is determined by finding the ratio of the density of the object to the density of the water. Objects with the gravity of less than 1 will float and the object with a specific gravity greater than 1 will sink.

Archimedes' Principle.gif

Archimedes' Principle


From the above figure, we can observe that when a weight is hanged with the help of a spring balance, the weight is more in air and less when it is dipped into the liquid. This is because, when the weight is immersed into the liquid, the buoyant force acts on it in upward direction, which reduces its weight. The decrease in the weight is equivalent to the weight of the liquid displaced by the object immersed in it.



*            State of the Buoyancy


*             Positive Buoyancy

If an object floats, it means the object displaces an amount of water that weighs more than the weight of the object. In other words, the object is less dense than the water.



*          Negative, Buoyancy

If an object displaces the amount of water whose weight is less than the object, it will sink. This means that the object is denser than the water and the water does not provide sufficient buoyancy to make it float.




*           Neutral Buoyancy

If an object is neutral, that is, it floats half immersed in the liquid, this means the object displaces the amount of water that weighs the same as the weight of the object.







         The pressure of the liquid is not same at each point in the sea. If a diver dives into the sea, at which point the diver will feel the maximum pressure?

(a) At the top

(b) At the bottom

(c) In the middle

(d) Same at all points

(e) None of these


Answer: (b)


We know that pressure increases with the depth of the sea, so diver will feel maximum pressure at the bottom of the sea.



         A small boat carrying loads does not sink in the river, at the same time an iron pillar of same weight sinks into the river. The reason for this is_______.

(a) Positive buoyancy

(b) Negative buoyancy

(c) Neutral buoyancy

(d) All of these

(e) None of these


Answer: (a)


The reason for this is the total downward weight in case of boat is less than that in the case of iron pillar. Therefore, it sinks.



*           Center of Buoyancy

The center of buoyancy is a point on the object, around which the buoyant force is balanced. For example, in human body the lungs provide a large buoyant volume of air, the center of buoyancy is usually located in the chest. The center of mass is below the center of buoyancy. When a person of average build tries to float horizontally on the back with arms along the sides of the body, the center of mass is nearly level with the center of buoyancy. Many people have more body weight than their legs and hips, because of the high proportion of muscle tissue at that point their center of mass is near the hips. When they try to float in a horizontal position, gravity pulls their hips and legs downward, while the buoyant force of the water pushes the chest area (center of buoyancy). Thus, the centre of the buoyancy of any individual lies in and around waist and chest of the individual, depending upon his/her weight.






       A box floats in water with 60 % of its volume below water surface. What is the density of the box? Take the density of water to be 1000 kg/m3.

(a) 0.6 kg/m3

(b) 60 kg/m3   

(c) 100 kg/m3

(d) 600 kg/m3

(e) None of these


Answer: (d)



         Archimedes principle has wide application. It can be used to find the volume of the solid, which is immersed in the liquid or the relative density of the liquid. Can Archimedes principle be used for the measurement of density of solid objects?

(a) Yes

(b) No

(c) Few selected solid

(d) Can't say

(e) None of these


Answer: (a)



  • The water pressure 10,000 m below the surface is equivalent to seven elephants standing on a dinner plate.
  • The pressure at the centre of the earth is about 400 billion Pa.
  • The pressure of a shark bite is about 30 million Pa.
  • The vacuum in the best laboratory is 1 trillionth Pa.
  • The pressure of sunlight is about 3 millionths Pa.



  • Pressure is defined as the force per unit area of the surface.
  • Liquid and gas exert pressure on the wall of the container.
  • The atmospheric pressure is the pressure exerted by the air molecule on us.
  • The force acting normal to the surface is called thrust.
  • Buoyancy is the upward force acting on the object, which is fully or partially immersed in water.
  • Archemedes principle states that force acting on an object is equivalent to the weight of the liquid displaced by the object.
  • Relative density of the liquid is the density of one liquid with respect to the liquid.
  • Sharp knives require less force to cut vegetables than a blunt knife.
  • Pressure of the liquid increases with the depth.
  • Pressure decreases with the height.

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