Gravitation
9th Class Physics Chapter 5 Exercise Short Questions with Answers
Q 5.2 : What is meant by the force of gravitation ?
Force of Gravitation :
” The force due to which every body of the universe attracts every other body is called Force of Gravitation .”
Formula :
Q 5.3 : Do you attract the Earth or the Earth attracts you ? Which one is attracting with a larger force ? You or Earth ?
According to Newton’s Third Law of Motion :
” To every action , there is always an equal but opposite reaction .”
So , Earth attracts us and in reaction , we also attracts the earth .
- Both of these forces are equal in magnitude .
Q 5.4 : What is field force ?
Field Force :
” The force acting on the body whether the body is in contact with it or not is called Field Force .”
Example : Gravitational force
Q 5.5 : Why earlier scientists could not guess about the gravitational force ?
Reason :
Because, they were unknown about the concept of “Gravity “.
- Concept of Gravity was given by Issacs Newton in 1665 .
Q 5.6 : How can you say that gravitational force is a field force ?
Gravitational force is a field force .
Reason : Because Gravitational force is a non-contact force .
For Example :
- The velocity of a Ball , thrown up , goes on decreasing and while on return its velocity goes on increasing .
- This is due to the gravitational pull of the earth acting on the body , whether the body is in contact with earth or not .
→ So , gravitational force is a field force .
Q 5.7 : Explain , what is meant by gravitational field strength ?
Gravitational Field Strength :
” In the gravitational field of the earth , the gravitational force per unit mass is called the Gravitational Field Strength of the Earth .”
Value : At any place , its value is 10 Nkg-1 .
Q 5.8 : Why is law of gravitation important to us ?
Law of Gravitation is important to us , because it help us to understand , What / why :
- Keeps the atmosphere close to Earth .
- Keeps the moon revolving around the Earth .
- Binds all terrestrial object to earth .
- It also helps us to understand Gravitational pull of Sun on planets keeps them revolving around Sun .
Q 5.9 : explain the law of gravitation .
Law of Gravitation :
” The force of attraction between two objects is directly proportional to the product of their masses and inversely proportional to the sqyuare of the distance between their centers .”
Formula :
Dependence : Law of gravitation depends upon :
- Masses of objects and
- Distance between them
→ Greater the masses of objects , greater will be force of gravitation .
→ Greater the distance between objects , less will be force of gravitation.
Q 5.10 : How can the mass of Earth be determined ?
Mass of earth can be determined with the help of ” Law of Gravitation “ .
R = 6.4 ×106 m
g = 10 ms-2
G = 6.673 × 10-11 Nm2kg-2
Thus , mass of Earth is 6.0 × 1024 kg .
Q 5.11 : Can you determine the mass of our mon ? If yes , then what do you need to know ?
Yes , we cannot determine / calculate the mass of our moon by following formula :
Need to know :
- Value of radius of moon .
- Gravitational acceleration on moon .
- Gravitational constant on moon .
Q 5.12 : Why does the value of g vary from place to place ?
→ The value of “g” is inversely proportional to the square of the radius of the earth .
→ But value of “g” does not remain constant . It decreases with altitude .
- That’s why , the value of “g: varies from place to place .
Q 5.13 : Explain how the value of g varies with altitude .
Value of Gravitational Acceleration is determined by following formula :
- So, g is inversely proportional to ( R + h )2 .
- It means that , with increasing altitude , value of g decreases .
Q 5.14 : What are artificial satellites ?
Artificial Satellites :
” Scientists have sent many objects into space . Some of these objects revolve around the Earth . These are called artificial satellites .”
Example : Geostationary Satellite .
Q 5.15 : How Newton’s law of gravitation helps in understanding the motion of satellites ?
Newton’s law of gravitation helps in understanding the motion of satellites .
Reason : Because Newton introduced centripetal force .
- A satellite requires centripetal force , that keeps it to move around the earth .
- The gravitational force of attraction between the satellite and the earth provides the necessary centripetal force .
Q 5.16 : On what factors the orbital speed of a satellite depends ?
Orbital speed of a satellite is given by :
Dependence : So , Orbital speed only depends upon :
- Height of satellite from the surface of Earth .
→ While , gh ( gravitational acceleration of Earth ) and R ( radius of Earth ) are constant .
Q 5.17 : Why communication satellites are stationed at geostationary orbits ?
Geostationary satellites appear to be stationary with respect to Earth .
Reason : Because Geostationary satellites take 24 hours to complete their one revolution about its axis . And Earth also complete its one rotation about its axis in 24 hours .
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