## Chapter 5

## Gravitation

#### Table of content:

Number | Name |

1 | Law of gravitation |

2 | Gravitation field |

3 | Mass of earth |

4 | Variation of g with altitude |

5 | Artificial satellites |

6 | Geo stationary satellites |

#### Chapter short Questions

##### Q.1 Does an apple attract the earth toward it?

Ans. Yes, apple attract the earth but this force is very very small so it is unable to pull the earth.

##### Q.2 With what force an apple weighing 1N attract the earth?

Ans. Apple weighing 1N attract the earth with a force of 1N.

##### Q.3 Does the weight of an apple increase, decrease or remain constant when taken to the top of a mountain?

Ans. As we go to the mountains, value of a decrease. So weight of the apple decrease.

#### Exercise short Questions

##### Q.1 What is the meant by force of gravitation?

Ans. In the universe, there exist a force between the bodies due to which everybody of the universe attract every other body. This force is known as force of gravitation.

##### Q.2 Do you attract the earth or the earth attract you? Which one is attracting with a large force? You or earth?

Ans. We attract the earth and Earth attract us but Earth attract us with large force because the mass of the earth is large.

##### Q.3 What is a field force?

Ans. The force which is acting on the body by another body whether body is in contact with other body or not. It is non contact force. The gravitational pull of the earth acting on the body whether the body is in contact with the Earth or not is called field force.

##### Q.4 Why earlier scientists could not guess about the gravitational force?

Ans. Earlier scientists could not guess about the gravitational force because of its low value. Scientists of old age have not sensitive instrument to detect that force.

##### Q.5 How can you say that gravitational force is a field force?

Ans. The gravitational force exists around the Earth and is acting on the bodies whether the bodies are in contact with the earth or not. So, we can say that gravitational force is a field force.

##### Q.6 Explain, what is meant by gravitational field strength?

Ans. In gravitational field, the gravitational force acting per unit mass is called the gravitational field strength. It becomes weaker and weaker as we go away from the object applying the gravitational force. Its value is 10Nkg-1

##### Q.7 Why law of gravitation is important to us?

Ans. Law of gravitation is important to us because it is used to calculate force of attraction between two masses. It is used to calculate the mass of earth. Motion of artificial satellite around a planet is accordance law of gravitation.

##### Q.8 Explain the law of gravitation?

###### Ans. Law of gravitation:

Everybody in the universe attract every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

###### Explain:

Consider two bodies of mass m1 and m2. The distance between the centre of masses is d.

According to law of gravitation, the gravitational force of attraction F with which the two masses m1 and m2 separated by a distance d attract each other is given by.

F∞m1 m2

F∞1/d²

F∞m1 m2/d²

F=Gm1m2/d²

##### Q.9 How the mass of earth can be determined?

Ans. F= GmMe/R²

F=mg

mg=GmMe/R²

g=GMe/R²

Me=R²g/G

Me=(6.4×10⁶m)²×10ms-2/6.673×10-11Nm²kg-2

= 6.0×10²⁴kg

Thus, mass of the earth is 6×10²⁴kg.

##### Q.10 Can you determine the mass of our moon? If yes, then what you need to know?

Ans. Yes we can determine the mass of the moon by same method used to measure the mass of the earth with the help of law of gravitation. The formula is;

Mm=gR²/G

From the about relation it shows that we require,

g= gravitational acceleration on the surface of moon= 1.62ms-2

R= Radius of moon=1.74×10⁶m

G=Gravitational constant=6.673×10-11Nm²kg-2

##### Q.11 Why does the value of g vary from place to place?

Ans. The formula g=GM/R² it shows that value of ‘g’ depends upon the radius of earth and its value is reciprocal to radius of earth so as we go up value of radius of earth increase value of “g” is decrease with change in place.

##### Q.12 Explain how the value of g varies with altitude?

Ans. Altitude is the height of an object or place above sea level.

Consider a body of mass m at an altitude h. The distance of the body from the center of the earth become R+h. Therefore, using equation we get

gh= GMe/ (R+h)².

According to the above equation, we come to know that at a height equal to one Earth radius above the surface of the earth, g become one fourth of its value on the earth. Similarly at a distance of two Earth radius above the Earth’s surface, the value of g become one ninth of its value on the Earth.

##### Q.13 What is artificial satellite?

Scientists have sent many object into space. Some of these revolve around the Earth. These are called artificial satellite. Large numbers of artificial satellite have been launched in different orbit around the Earth. They take different time to complete their one revolution around the Earth depending upon their distance h from the earth.

##### Q.14 How Newton’s law of gravitation help in understanding the motion of satellite?

The motion of satellites is due to force of gravitation and this gravitational force is provided by earth. As we know that Newton gave the law of gravitation. So, we can say that Newton help us in understanding the motion of satellites.

##### Q.15 On what factors the orbit speed of satellite depends?

Ans. Formula for orbit speed of satellite is

Vo=√gh(R+h)

So, we can say that orbit speed depends upon the gravitational acceleration and distance between the center of earth and the satellite.

##### Q.16 Why communication satellite are stationed at geostationary orbits?

Ans. Communication satellite take 24 hours to complete their one revolution around the Earth. As Earth also completes its one rotation about it axis in 24 hours, hence, these communication satellite appear to be stationary with respect to Earth. It is due to this reason that the orbit of such a satellite is called geostationary.