# Gravitation (Floatation) : Practice Questions

1 Mark Questions

Q.1 Â Define thrust.

Q.2 Â State a condition for an object to float when placed on the surface of water.

Q.3 Â A coin sinks when placed on the surface of water. Give reason.

Q.4 Â State and define SI unit of pressure.

Q.5 Â How is pressure related to the thrust exerted on a surface?

Q.6 Â Why do buildings have wide foundation?

Q.7 Â How does the density of fluid affect the magnitude of buoyancy acting on an object immersed in it?

Q.8 Â State Archimedes principle.

Q.9 Â Give two applications of Archimedes principle.

Q.10 Â Give the SI unit of relative density.

Q.11 Â Relate relative density of a substance to its density.

Q.12 Â Density of a solid is 7.5 ${\rm{g /c}}{{\rm{m}}^3}$ . What is its relative density?

Q.13 Â State the SI unit of buoyancy.

Q.14 Â School bags have broad straps.

Q.15 Â Dams have broad foundation.

Q.16 Â It is easier to cut an apple with sharp edge of knife.

Q.17 Â A mug appears lighter inside water.

Q.18 Â A solid exerts pressure of 20 Pa on a surface of 2 ${m^2}$. Find its weight.

Q.19 Â Relative density of an object is 1.35. Will it float or sink in water?

Q.20 Â Mark the direction of weight of the body and upthrust acting on it, in the following diagram:

Q.21 Â "A coin sinks in water because upthrust acting on it is greater than its weight." Is the statement correct? If not, correct it.

Q.22 Â An object is put one by one in three liquids having different densities. The object floats with 1/9, 2/11 and 3/7 parts of their volumes outside the liquid surface in liquids of densities d1, d2 and d3 respectively.Â  Arrange the densities in ascending order of their magnitude.

Q.23 Â An object weights 10 N in air. When immersed fully in water, it weights 8 N. Find the weight of liquid displaced by the object.

Q.24 Â State the principle of floatation for an object of weight W immersed in a fluid.

Q.25 Â Relate the SI unit of density with its CGS unit.

Q.26 Â A crumpled sheet of paper falls down faster than a plane sheet, Why?

Q.27 Â A body is immersed once in each of the following media:
mustard oil
water
glycerine
petrol
In which ease will there be maximum apparent loss of weight and why?

Q.28 Â Differentiate between force and thrust.

SolutionsÂ

1. Â Force acting perpendicular to a surface is called thrust.

2. Â A body will float if its density is less than that of water.

3. Â The coin sinks as its density is greater than water.

4. Â Pressure is defined as thrust per unit area. The unit of pressure is Pascal. One pascal is the pressure exerted when 1 N force acts as a surface area of 1${m^2}$ .

5. Â Pressure = Thrust/Area

6. Â Buildings have wide foundations to reduce the pressure on ground. Since larger the area, lesser will be the pressure.

7. Â Weight of fluid displaced = $\rho$gV where $\rho$ is the density.Â Hence, buoyant force increases with increasing density of fluid.

8. Â Archimedes Principle: When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

9. Â It is used inÂ  (i) designing ships andÂ  (ii) submarines.

10. Â Relative density has no units.

11. Â Relative density =${{{\rm{Density}}\,{\rm{of}}\,{\rm{substance}}} \over {{\rm{Density}}\,{\rm{of}}\,{\rm{water}}}}$

12. Â Relative density = ${{{\rm{Density}}\,{\rm{of}}\,{\rm{substance}}} \over {{\rm{Density}}\,{\rm{of}}\,{\rm{water}}}}$

$= {{7.5} \over 1} = 7.5$

13. Â Newton.

14. Â Pressure = Force/Area.

Broad straps have greater area and therefore exert lesser pressure.

15. Â Greater the area, less is the pressure exerted. The broad base of dams reduce pressure on the ground.

16. Â Sharp knives have very small area on which the force acts. Hence , pressure is much more and can easily cut an apple.

17. Â The mug appears lighter as the buoyant force acting in the upward direction reduces the effective weight of the mug when placed in water.

18. Â Weight = Pressure X Area

= 20 X 2 = 40 N.

19. Â As relative density is greater thanÂ 1 , the object will sink.

20.Â

${F_B}$ = Buoyant force

W = Weight of object

21. Â The statement is wrong. The correct statement will be " A coin sinks in water because upthrust acting on it is less than its weight".

22. Â ${1 \over 9} = 0.11\,;\,{2 \over {11}} = \,0.18\,;\,{3 \over 7} = 0.43$

There are the volume outside of liquids of densities d1, d2, d3 respectively. This means buoyant face is maximum for d3 and minimum for d2. As buoyant forceÂ  is proportional to density, d1< d2<d3.

23. Â Weight of object in air = 10 N

Weight of object in fluid = 8 N

Therefore, Loss of weight = (10 - 8) N = 2 N

Buoyant force = Loss of weight = 2 N

24. Â Archimedes Principle states when a body is immersed fully or partiallyÂ  in a fluid, it experiences an upward force that is equal to the weight of the fluidÂ  displaced by it.

25. Â 1000 kg/${m^3}$ = $1g\,m/c{m^3}$

26. Â Air resistance on sheet of paper will be more then the crumpled sheet as its surface area is greater. The net force acting on it is nearly zero so it moves down slowly.

27. Â Glycerine has highest density so maximum apparent loss will be seen when body is immersed in glycerine.

28. Â Force can have any direction, but thrust is that force which is acting perpendicular to a surface.