Ch. 10 Gravitation Extra Questions

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Q. Define density and relative density. Give their S.I. units.

 A. Density: It is the mass of substance per unit volume. S.I. unit of density is kg/m³.
Relative density: It is the ratio of the density of a substance to that of water.

 Q. Define thrust and pressure. Give their S.I. units.

 A. Thrust: It is the force acting on an object, perpendicular to its surface. S.I. unit of thrust is N (Newton).

 Pressure: It is the perpendicular force acting on a unit area. S.I. unit is Pascal (Pa) i.e. N/m².

 Q. Explain why a sharp knife cuts better than a blunt knife.

 A. The area on which the force acts decreases if we use a sharp knife and thus the pressure and effect of force increases (P∝1/A). Hence a sharp knife cuts better than a blunt knife.

 Q. Explain why tanks are made to run over broad steel tracks?

 A. Broad steel tracks increase the area of contact between the tank and the ground. This reduces the pressure on the ground and prevents the tank from sinking into the ground. (P∝1/A).

 Q. Explain why dams have a very broad base.

 A. Broader base increases the area of contact between the dam and the ground. This reduces pressure on the ground and thus prventing the dam from sinking into it. (P∝1/A).

 Q. Write down the factors on which upthrust depends.

 A. 1. Upthrust ∝ Volume/ density of object
2. Upthrust ∝ Density of fluid
3. Upthrust ∝ 1/ Temperature (i.e. upthrust is inversely proportional to temperature)

 Q. An object when weighed by a spring will weigh more in air or vacuum. Justify.

 A. An object would weigh more in air because air's density is very low and thus there is less upthrust to counter the weight. In vacuum, the object will weigh even more because there is no fluid present and thus there is no upthrust. (Upthrust ∝ Density of fluid)

 Q. A cubical box of mass 4g with side of 2.5 cm is lying on the ground. Calculate the pressure exerted by the box on the ground.

 A. m = 4
Side (a) = 2.5 cm
Area of contact = a²
                         = (2.5)²
                         = 6.25 cm²
Force exerted by the box = mg
                                         = 4 x 10
                                         = 40
Pressure = Force ÷ Area
                = 40 ÷ 6.25
                = 16 Pa

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