### Gravitation Physics

on 21 Jan 2019, 10:30AM
• Gravitation
• Physics
• For Exams
General Knowledge...

1 Gravitation is the force of attraction between any two particles or objects in the Universe.

2 The force of attraction between any two objects is directly proportional to their masses and inversely proportional to the distance between them.

3 If a mass is small, gravitation is a weak force.

4 Gravitation force due to earth is called gravity

5 'G' is called gravitational constant whereas 'g' is an acceleration to gravity.

6 F= Gxm₁xm₂/d² Where 'F' is a force, m1 is mass of the first body

7 m₂ is mass of the second body

8 'G' is gravitational constant, 'd' is the distance between the two bodies.

9 Value of

10

11

12

13 F = ma (Newton's second Law)

14 a=GMe/R² 'a' is acceleration of an object falling freely towards earth depends upon the mass of the earth (Me) and the height of the object from the centre of earth. It does not depend upon mass of object, i.e. stones of 500 g and 150 g will reach earth at the same time if falling freely from the same height. 'R' is radius of earth. When object is on or near the surface of earth then d = R.

15 The case I: When an object is falling towards earth with an initial velocity (u), then final velocity 'v' after time 't' will be

16 v = u + gt
where 'g' is acceleration due to gravity
Height covered in 't' seconds, Similarly, or

17 When an object is falling from rest position means initial velocity u

18 Case III: When an object is thrown vertically upwards with initial velocity 'u', the gravitation acceleration will be negative (-g), then

19 Mass is the quantity of matter contained in it. It a scalar quantity, i. e. it has only magnitude but no direction.

20 It remains constant and does not change from place to place.

21 Mass of the body cannot be equal to zero.

22 It is measured by a physical or chemical balance. Its SI unit is kg.

23 Weight: The force with which an object is attracted towards the center of the earth

In case of earth a = g

24 Weight is a force and its SI unit is Newton.

25 It changes from place to place because of the value of 'g' changes

26 It is zero at the center of the earth where 'g' = 0

27 A relation between 1 kg Wt and Newton

28 So, the gravitational force which acts on an object of 1 kg mass is called is 1 kg Wt. g = M is mass of the earth, G is the Gravitational constant, 'g' is 9.8 ms⁻² which remains constant at the surface of the earth or closer to it. If we are moving away from earth 'g' is calculated by a given formula. 'd' is the distance from the earth.

29 The weight of an object on the moon is 1/6 the of the weight on earth. Thrust: The force acting on an object perpendicular to the surface is called thrust.

30 Pressure: It is defined as a force (effect of thrust) per unit area. P=F/a

31 SI unit of pressure is Newton m⁻² or Pascal (Pa).

32 Pressure depends upon (i) Force applied, (ii) Area of the surface over which force acts. Fluids (liquids and gases) also exert pressure on the base and walls of the container in which they are enclosed.

33 Buoyancy: It is the upward force experienced by an object when it is immersed into a fluid U = mg

34 It acts is upward direction and it depends upon the density of the fluid (liquid).

35 It the gravitational force of attraction of the earth on the surface of the object > Buoy force, the object sinks.

36 It the gravitational force of attraction of the earth on the surface of object ≤ Buoyant for the object floats.

37 That is why all pin sinks and boat/ship floats on the surface of the sea. Density is defined as the mass per unit volume. <
Its SI unit is Kg m⁻³

38 Archimedes Principle: 'When a body is fully or partially dipped in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
Applications:
(i) It is used to determine relative density of substances.
(ii) It is used in designing ships and submarines.
(iii) Hydrometers and lactometers are made on this principle.
It is a ratio hence it does not have any unit.

S.No. Mass Weight
1 (i) It is a quantity of matter contained in a substance. (i) It is a force by which earth attract the body towards it.
2 (ii) Its SI unit is kg. (ii) Its SI unit is Newton.
3 (iii) It remains constant (iii) It changes from place to place with change in value of 'g'
4 (iv) Mass is a measure (iv) Weight is a force acting vertically downward.
5 (v) It has magnitude only. (v) It has magnitude and direction
6 (vi) It does not depend upon weight. (vi) It is directly Proportional to mass.

Newton Law Questions

### Have a look on

1Who defined the law of gravitation?

b) Archimedes
c) Galileo

4 SI unit of acceleration

b) m/s⁻²
c) kg/s²
d) kgs²

7SI. Unit of pressure

a) Newton

c) Nm² Kg⁻²
d) m/s²

10 If the distance between two masses is doubled, the gravitational attraction between them

a) is doubled
b) becomes four times
c) is reduced to half

13 Which of the following has no unit?

a) Force
b) Density
c) Acceleration

16 S.I unit of Gravitation constant (G)

a) Nm² Kg²

c) Nm⁻² Kg²
d) m² Kg⁻²

19 Theory of motion of planets was given by

b) Copernicus
c) Newton
d) Galileo

22 As we go from the equator to North pole the value of 'g', the acceleration due to gravity.

a) Remains the same

c) Decreases
d) None of the above

25 The tides in the sea are primarily due to

a) The gravitational effect of the Sun on the Earth

c) The gravitational effect of Venus on the Earth
d) The atmospheric effect of the Earth

### Most important

2 In the Earth, the weight of a body is maximum at the

a) South Pole
b) Increases

d) Surface

5 The apparent weight of a person in a lift which is moving down with uniform acceleration _____.

a) Same as the weight when the person is stationary
b) Greater than the weight when the person is stationary

d) Twice the weight when the person is stationary

8With reference to gravity, what is G called?

a) Zero
b) v/r

d) v2/r

11The average kinetic energy of the molecules of an ideal gas is directly proportional to

a) Acceleration due to gravity
b) Gravitational force

d) Gravitational attraction

14Which of the following is Moon's gravitation?

a) Moon's gravitation = 1/6th Mars gravitation
b) Moon's gravitation = 1/8th of Earth's gravitation

d) Moon's gravitation = 1/8th of Mars gravitation

17Who fist determined the value of G gravitational constant?

a) R. R Heyl
b) Boyle

d) Poynting

20Law of gravitation applies to _____.

a) The earth and the moon

c) The planets around the sun
d) The earth and the objects of earth

23At which of the following an object is maximum?_

a) At equator
b) At tropic of Capricorn

d) At tropic of Cancer

26The sensation of weightlessness in a spacecraft in an orbit is due to the

a) Acceleration in the orbit which is equal to the acceleration due to gravity outside
b) Presence of gravity outside but not inside the spacecraft

d)Fact that spacecraft in the orbit has no energy

### Questions

3 The apparent weight of a man in left is less than the real weight then-

a) The lift is going up with uniform speed
b) The lift is going down with uniform speed.
c) The lift is going up with acceleration.

6 What is the minimum escape velocity of a rocket to be launched into space?

a) 5 km/sec.

c) 6 km/sec.
d) 15 km/sec.

9 Time period of revolution for a Geo stationary satellite is____.

b) 365 days
c) 30 days
d) Continuously changes

12 A body has a weight of 10kg on the surface of the earth. What will be its weight when taken to the center of the earth?

b) 10
c) 5
d) 20

15 Calculate the force of gravitation between the earth and the sun, given that the mass of the earth = 6 × 10 kg and of the Sun = 2 × 10 kg. The average distance between the two is 1.5 × 10 m.

b) 4 x 10²²N
c) 4 x 10²⁰N
d)3.56 x 10²⁰

18 A boat will not submerge when it displaces water equal to its own

b) Weight
c) Surface area
d) Density

21 An object weighs 10 N when measured on the surface of the earth. What will be its weight when measured on the surface of the moon?

a) 1.8 N

c) 10 N
d) 0N

24A force of 20N acts upon a body whose weight is 9.8 N. What is the mass of a body and how much is its acceleration?

a) 10 kg, 20m/s2
b) 20 kg, 10m/s2

d) 1 kg, 10m/s2

27The mass of an object _____

a) Changes from place

c) Is equal to its weight
d) Is greater at mountains