Short/long answer questions of chapter 2 (physical quantities and measurement) of class 8

 1. Define the term density of substance.

Ans. Mass per unit of a substance is called as its density.

2. Name the SI unit of density. How is it related to gram per centimetre cube?

Ans. The SI unit of density is kg per metre cube ( kgm-³).

It is related to gcm-³ as

1kgm-³= 10-³ gcm-³

3. The density of brass is 8.4 gram per centimetre cube. What do you mean by this statement?

Ans. The density of brass is 8.4 gram per centimetre cube. It means that 8.4 gram of brass occupies a volume of 1 cm cube.

Or 1 centimetre cube of brass has a mass of 8.4 g.

4 . Arrange the following substances in order of their increasing density:

Iron, cork, brass, water, Mercury.

Ans. Iron=7.8gcm-³

Cork=0.25gcm-³

Brass=8.4 gcm-³

Water=1 gcm-³

Mercury=13.6 gcm-³

Increasing order means we have to start from the lowest density to higher density.

Cork < water < iron < brass < Mercury.

5. How does the density of liquid or gas vary with temperature?

Ans. The density of a liquid or gas decreases with rise in temperature.

The density of liquid or gas increases with decrease in temperature.

Because mass of a substance remains same but with the change in temperature volume varies. Volume increases with increase in temperature and decreases with decrease in temperature because of thermal expansion.

6. A given quantity of a liquid is heated. Which of the following quantity will vary and how?

Ans. Mass remains same.

Volume increases with increase in temperature and decreases with decrease in temperature cause of thermal expansion.

Density decreases with increase in temperature and vice - versa.

7. Describe an experiment to determine the density of the material of a coin.

Ans. Put the coin on a digital balance and measure its mass.

2. Find the volume of the coin by the liquid displacement method.

3. After finding mass and volume of the coin. Use the formula of density as

Density=mass/volume.

Explain  activity 1 or activity 2 of page number 22 of your physics textbook.

8. Describe an experiment to determine the density of a liquid.

Ans.  the density of a liquid can be determined by  using a density bottle.

1. Mass of empty bottle M₁

2. Mass of bottle filled with water M₂

3. Mass of bottle filled with liquid M₃

These masses are measured by using beam balance.

4. The mass of liquid M = M₃ - M₁

5. Volume of liquid. V=M₂ - M₁

6. By using formula of density as,

      Density=mass of liquid/volume of liquid.

 ρ=M/V

  ρ=(M₃ - M₁) / M₂ - M₁

9. What is a density bottle? How is it used to find the density of a liquid?

Ans. Density bottle is a specially designed bottle  which is used to determine the density of a liquid. The stopper has a narrow hole through which it which allows the excess liquid to drain out.The bottle always contains the same volume of liquid each time when it is filled.

It is is used to find the density of a liquid by measuring mass and volume of the liquid. 

10. Define the term relative density of a substance.

Ans. Relative density of a substance is the ratio of density of a substance to the density of water at 4 degree Celsius. Or,

Relative density of a substance is the ratio of the mass of any volume of the substance to the mass of an equal volume of water.

11. What is the unit of relative density?

Ans.relative density is a unitless quantity. Relative density is a pure, it has no unit.

12. Distinguish between density and relative density.

Ans.1. density of a substance is the mass per unit volume of  that substance. Whereas relative density of a substance is the ratio of density of that substance to the density of water at 4 degree Celsius.

2. Density is expressed in gcm-³ or kgm-³. Whereas relative density has no unit.

Note write these differences in a vertical column method. One side write density  and other side write relative density.

13. Explain the meaning of the statement'relative density of aluminium is 2.7'.

Ans. It means that the density of  aluminium is 2.7 times more than the density of water at 4 degree Celsius.

14. How does the density of a body and that of liquid determine whether the body will float or sink into that liquid?

Ans.1.if the density of body is more than the density of liquid then that body will sink in it.

2.and if the density of body is less than the density of the liquid than in that case the body floats in that liquid.

15. A cork piece floats on water surface while and iron nail sinks in it. Explain the reason.

Ans. A cork piece  floats on water surface while an iron nail sinks in it because the average density of cork is less than the density of water where as the average density of iron is more than the density of water.

16. Which of the following with sink or float on water? Density of water=1gcm-3

A) body A having density of 500 kg per metre cube.

B) body B having density of 2520 kg per metre cube.

C) body C having density of 1100 kg per metre cube.

D) body D having density 0.85 g cm-³.

Ans. Density of water in SI unit is 1000kgm-³.

Since the density  of body B & C is more than the density of water, so both will sink in water.

On the other hand the density of body A & D is less than the density of water so these two bodies will float in water.

17. State the law of floatation?

Ans. According to the law of floatation,the weight of a floating body is equal to the weight of the liquid displaced by its submerged part.

18. The density of water is 1.0 gram per centimetre cube. Density of iron is 7.8 gram per centimetre cube. The density of mercury is 13.6 gram per centimetre cube. Answer the following:

A) will a piece of iron float or sink in water?

Ans. A piece of iron sinks in water because its density is more than the density of water.

B) will a piece of iron float or sink in Mercury?

Ans.a piece of iron will float in Mercury because its density is less than the density of Mercury.

19. The diagram given below shows a body floating in three different liquids A,B & C at different levels.

a) in which liquid does the body experience the greatest bite force?

b) which liquid has the least density?

c) which liquid has the highest density?

Ans. a) the body experience the same buoyant force in all three liquids.Because  for a floating body the buoyant force is equal to the weight of the body. Since three bodies are floating show buoyant force in all three cases would be same.

b) liquid A has  has the least density. Because less the density of liquid,  then larger portion of the floating body is inside the  liquid. By law of floatation the weight of a floating body should be equal to the weight of the liquid displaced by its submerge part.

Weight of the liquid displaced depends on the the volume submerged, density of liquid and acceleration due to gravity.

c) liquid C has the highest density because in that the submerged part of the body is very less .

20. For a floating body, how is its weight related to the buoyant force?

Ans. For a floating body the weight of the body should be equal to the the buoyant force or weight of the liquid displaced by it submerged part.

21. Why does a piece of ice float on water?

Ans.a piece of ice float on water because its density is less than the density of water.

22. Explain why an iron needle sinks in water but a ship made of iron floats on water?

Ans.an iron needle sinks in water but ship made of iron floats on water because the average density of iron needle  is more than the density of water where as the average density of ship made of iron is less than the average density of water so it floats on the surface of water.

23. Is it easier to swim in sea water than in river water? Explain the reason.

Ans.it is easier to swim in seawater than in river water because the density of seawater is more than the density of river water due to the presence of salt in the sea water.the weight of a man gets balanced by the less immersed part of his body in sea water as compared to that in river water.

24. Icebergs floating on sea water are dangerous for ships. Explain the reason.

Ans. Icebergs floating on sea water are dangerous for shipsbecause an iceberg floats in sea water with its large portions  submerged inside the water and only a little portion of it is above the surface of water. Ship can collide with invisible part of iceberg under the surface of water.

25. Explain why it is easier to lift a stone under water than in air.

Ans. It is easier to lift a stone under water than in air because when the stone is inside the water then a upthrust acts on it which helps us in lifting the load in upward direction. Although upthrust acts in air as well as in water but since the density of water is more compared to the density of air so buoyant force is more in water.

26. What is a submarine?how can it be made to dive in water and come to the surface of water?

Ans.a submarine is a fish shaped water tight- boat provided with several ballast tank in its front and rear parts. It can travel underwater like a ship.

1. To make the submarine dive, the tanks are filled with water so that the average density of the submarine becomes greater than the density of seawater and it sinks.

2. If a submarine is to rise,water from the ballast tanks is forced out into the sea by allowing the compressed air to enter the tank. Now the  average density  of submarine is less than that of sea water. So it it goes up and come to the surface of water.

27. A balloon filled with hydrogen rises in air explain the reason.

Ans. When light gas like hydrogen or helium is filled in a balloon, the weight of air displaced by the inflated balloon (upthrust) a more than the weight of the gas filled balloon and it rises up under the influence of the net upward force.