1:- What is gravitation?
Ans:- Gravitation is the universal force of attraction acting between all matters. It is very weakest known force in nature and thus plays no role in determining the internal properties of every day matter.
2:- What is gravity?
Ans:- Gravity is the gravitational force that occurs between the earth and other bodies. It is the force acting to pull objects towards the earth.
3:- What is the different between gravitation and gravity?
Ans:- Gravitation is a gravitational force that acts between all matters but gravity is a gravitational force that acts between earth and other objects. It helps the earth to attract any objects towards its centre.
4:- What is centripetal force?
Ans:- A force that acts on a body moving in a circular path and is directed towards the centre around which the body is moving is called centripetal force. In the absence of centripetal force the object flies of along the straight line. This straight line will be a tangent to the circular path.
Universal law of gravitation
Every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them. Let two objects A and B, the mass of object A is 'M' and the mass of object B is 'm'. Let the force of attraction between these two objects is F.
According to the universal law of gravitation, the force between two objects is directly proportional to the product of their masses. That is,
F ∝ M × m .................... eq 1
The force between two objects is inversely proportional to the distance between them, that is,
F ∝ 1/d2 ..................... eq 2
Combining eqs 1 and 2 we get.
F ∝ M × m/d2 ................. eq 3
or, F = G(M × m)/d2 ...... eq 4
where G is the constant of proportionality and is called the universal gravitation constant.
By multiplying crosswise, eq 4 we get.
F × d2 = G M × m
or, G = F d2 / M × m
The SI. unit of G is N m2 / kg2
The value of G was found out by Henry Cavendish by using a sensitive balance. The accepted value of G is 6.673 × 10-11 N m2 / kg2 .
5:- Write four importance of the universal law of gravitation?
Ans:- The universal law of gravitation successfully explained several phenomena which were believed to be unconnected:-
(1) The force that binds us to the earth;
(2) The motion of the moon around the earth;
(3) The motion of planets around the Sun;
(4) The tides due to the moon and the Sun;
6:- What is free fall?
Ans:- The earth attracts objects towards it. This is due to the gravitational force. Whenever objects fall towards the earth under this force alone, these types of fall is called free fall.
7:- What is acceleration due to gravity?
Ans:- When an object falls towards the earth, the direction of the object does not change but the velocity of the object changes. And changes in velocity is known as acceleration. this acceleration due to the earth's gravitational force. Therefore this acceleration is called the acceleration due to gravitational force or gravity. It is denoted by 'g'. And, the SI. unit of acceleration due to gravity is m/s2.
8:- What is the relation between G and g?
Ans:- G represents universal gravitation constant and g represents acceleration due to gravity . We know that force is the product of mass and acceleration due to gravity.
F = mg ............ (1)
And from universal law of gravitation
F = G (M×m)/d2 .............. (2)
From eq.(1) and (2) we get
mg = G (M×m)/d2
g = G× M/d2
Let an object be on the surface of earth than the distance 'd' will be equal to the radius of the earth that is represented by R.
therefore g = G× M/d2
9:- How can calculate the value of 'g'?
Ans:- We can calculate the value of 'g' by putting the value of 'G', 'M' and 'R' in a equation. The equation is g = G× M/d2 . It is the relation between G and g.
Where G = Universal gravitational constant
M = Mass of the earth
R = Radius of the earth
We know that
G = 6.7×10-11 N m2 / kg-2
M = 6×1024 kg
R = 6.7×106 m
And,
g = G× M/d2
= 6.7×10-11 N m2 / kg-2 × 6×1024 kg / (6.7×106 m)2
= 9.8 ms-2
Thus the value of acceleration due to the earth, g = 9.8 ms-2 .
10:- What is mass?
Ans:- Mass is the matter contained in a body. mass of an object is the measure of its inertia. If the mass of an object will greater the inertia will also greater. mass of an object is constant and does not change from place to place. It is denoted by "m".
11:- What is weight?
Ans:- The weight of an object is the force with which it is attracted towards the earth. It is denoted by "W".
We know that
F = ma .......................... ( 1 )
acceleration due to gravity and acceleration are equivalent
Therefor
a = g ............................ ( 2 )
Put the value of eq (2) in eq (1) we get
F = mg ......................... ( 3 )
and weight is also a type of force
therefor
F = W ......................... ( 4 )
Put the value of eq ( 4 ) in eq ( 3 ) we get
W = mg
Weight = mass × acceleration due to gravity
The SI unit of weight is Newton.
12:- What is the weight of an object on the moon?
Ans:- The weight of the object on the moon is the force with which the moon attracts that object the mass of the moon is less than the mass of the earth. Therefore the gravitational force of moon is less than the gravitational force of the earth. So, that the weight of an object on moon is less than the weight of same object on the earth.
13:- What is the relation between weight of the object on the moon and weight of the object on h earth?
Ans:- Let weight of an object on the moon = Wm
Mass of the moon = Mm
Mass of the object = m
Radius of the moon = Rm
So, Wm = G(Mm×m)/Rm2 ............... eq. (1)
And, let weight of an object on earth = We
Mass of the earth = Me
Mass of the object = m
Radius of the earth = Re
So, We = G (Me×m)/Re2 ............... eq. (2)
And mass of the earth = 5.98×1024 kg
Radius of the earth = 6.37×106 m
Mass of the moon = 7.36×1022 kg
Radius of the moon = 1.74×106 m
Put the value of mass and radius of the moon in eq. (1) we get.
Wm = G(Mm×m)/Rm2
Wm = G(7.36×1022 kg×m)/(1.74×106 m)2
We = 2.431×1010 G×m ............. eq. (3)
And Put the value of mass and radius of the earth in eq. (2) we get
We = G(Me×m)/Re2
We = G(5.98×1024 kg×m)/(6.37×106 m)2
We = 1.474×1011×G×m ............ eq. (4)
Divide eq. (3) by eq. (4) we get.
Wm / We = (2.431×1010 G×m)/(1.474×1011×G×m)
Or Wm/ We = 0.165
Or Wm/ We = 1/6
Or Weight of the object on the moon / Weight of the object on the earth = 1/6
Or Weight of the object on the moon = 1/6× weight of the object on the earth.
Thrust and Pressure
14:- What is thrust?
Ans:- The force acting on a object perpendicular to the surface is called thrust.
15:- What is pressure?
Ans:- The thrust on unit area is called pressure.
Pressure is direct proportional of thrust
pressure ∝ thrust .......... eq. (1)
And, pressure is inverse proportion to area.
pressure ∝ 1/area ............ eq. (2)
From eq. (1) and (2), we get.
pressure ∝ thrust × 1/area
pressure = k × thrust × 1/area
the value of constant k in SI. unit is 1.
So, pressure = thrust/area
The SI. unit of thrust is 'N'
The SI. unit of area is 'm2'
So, the SI. unit of pressure is Nm-2
In honour of scientist Blaise Pascal, the SI unit of pressure is called Pascal, denoted by 'Pa'.
16:- Why a nail has a pointed tip?
Ans:-
17:- Why knives have sharp edges?
Ans:-
18:- Why buildings have wide foundation?
19:- What is buoyant force?
Ans:- The upward force exerted by the water on the object is known as up-thrust or buoyant force. The magnitude of buoyant force depends on the density of fluid. Buoyant force is directly proportional to the density of fluid. If the density of fluid will increase then the buoyant force will also increase.
20:- What is density?
Ans:- The mass of an object per unit of volume is called density.
21:- Why object float or sink when placed on the surface of liquid?
Ans:- The floating and sinking of an object depends on the density of objects and the density of the liquid. If the density of object is greater than the density of liquid than the object will sink. While the density of object is smaller than the object will float in liquid.
22:- Why does a nail sink in water while a cork floats in water?
Ans :- The density of nail is greater than the density of water therefor nail will sink in water. While the density of corck is smaller than the density of water so, corck will floats on water.
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