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Very Short Answers - Gravitation, Science, Class 9 Notes | Study Class 9 Science by VP Classes - Class 9

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Q1. Give the S.I. unit of G and its value.
 Ans.
S.I. unit of G and its value = 6.67 × 10−11 Nm2/kg


Q2. Earth attracts apple from the tree and it falls but the earth does not appear to move towards the apple?
 Ans. 
For a force of attraction, acceleration is inversely proportional to the mass of the object. Hence the mass of an apple is negligible as compared to that of the earth. So, we do not see the earth moving towards the apple.
 

Q3. g = GM/R2, what does these symbols denote?
 Ans. 
g = acceleration due to gravity
G = Gravitational constant
M = Mass of the earth
R = Radius of the earth 


Q4. Is uniform circular motion taking place at a constant speed or constant velocity? Why?
 Ans. 
At constant speed but variable velocity because direction of motion is changing.
 

Q5. Which force is responsible for stability of our universe?
 Ans.
The force of gravitation
 

Q6. Which force is required to maintain a body in uniform circular motion?
 Ans. 
The centripetal force.
 

Q7. What is the direction of centripetal force?
 Ans. 
Towards the centre of circle.
 

Q8. Moon is revolving round the earth in a circular orbit and needs a centripetal force. From where does the force come?
 Ans. 
Centripetal force for circular motion of moon is provided by the force of gravitational attraction due to Earth.
 

Q9. What is gravity?
 Ans. 
Force of gravitational attraction on an object due to Earth.
 

Q10. An object is let free to fall from the top of a tower. What is its acceleration and what is its sign?
 Ans. 
Acceleration is positive having a value g = 9.8 m/s2.
 

Q11. How is ‘g’ on the surface of Earth related to ‘G’?
 Ans. g= GM/R2

2 , where M = mass of earth, R = radius of earth


Q12. A ball is projected vertically upwards with an initial velocity ‘u’ goes to a maximum height ‘h’ before coming to ground. What is the value of ‘h’?
 Ans.
  h = u2/2g
 

Q13. How is weight of an object related to its mass?
 Ans. 
Weight (W) = mass (m) × acceleration due to gravity (g).
 

Q14. State the S.I. unit of weight.
 Ans. 
S.I. unit of weight is Newton (1N).
 

Q15. Is the value of ‘g’ same at all places on earth?
 Ans.
No, it varies from place to place.
 

Q16. What is the S.I. unit of thrust?
 Ans.
Newton.
 

Q17. What is the S.I. unit of pressure?
 Ans. 
The S.I. unit of pressure = N/m2 = Pascal.
 

Q18. Define thrust.
 Ans. 
The net force exerted by a body in a particular direction is called thrust.
 

Q19. Define pressure.
 Ans. 
The force exerted per unit area is called pressure.
 

Q20. Why is it easier to swim in sea water than in river water?
 Ans. 
The density of sea water is more due to dissolved salts in it as compared to the density of river water. Hence the buoyant force exerted on the swimmer by the sea water is more which helps in floating and makes swimming easier.
 

Q21. Why a truck or a motorbike has much wider tyres?
 Ans. 
The pressure exerted by it can be distributed to more area, and avoid the wear and tear of tyres.
 

Q22. Why are knives sharp?
 Ans. 
To increase the pressure, area is reduced, As pressure ∝ (1/Area) hence the pressure or force exerted on a body increases.
 

Q23. Why is the wall of dam reservoir thicker at the bottom?
 Ans.
The pressure of water in dams at the bottom is more, to withstand this pressure the dams have wider walls at the bottom, to increase the area.
 

Q24. Why do nails have pointed tips?
 Ans. 
The force exerted when acts on a smaller area, it exerts larger pressure. So the nails have pointed tips.
 

Q25. While swimming why do we feel light?
 Ans. 
The swimmer is exerted by an upward force by water, this phenomenon is called buoyancy and it makes the swimmer feel light.
 

Q26. Define density and give its unit.
 Ans. 
The density of a substance is defined as mass per unit volume. Its unit is kg/m3.
 

Q27. What is relative density?
 Ans. 
The relative density of a substance is the ratio of its density to that of water.
Relative density  = density of a substance/density of water

 

Q28. If a planet and a moon both experience the same force, explain why the moon’s acceleration is noticeable while the planet’s is not.
 Ans. 
The moon has a much smaller mass, and acceleration is inversely proportional to mass, as shown by a (F/m).

 

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