Matric Notes Physics 9th Ch 5 Gravitation Short Questions

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Q1. If there is an attractive force between all objects, why don’t we feel ourselves gravitating towards nearby massive building?

Answer: The value of Universal constant of Gravitation ‘G’ is very small i.e. 6.67 x 10-11Nm2kg-2. Due to its small value, the gravitational force of attraction between the object and building is very small and so it can not be felt.

Q.2) Does the sun exert a larger force on the Earth than that exerted on the sun by the Earth? Explain.

Answer: No, because according to Newton’s law of Universal Gravitation the force of attraction between two objects is given by:

where ‘ms‘ is the mass of sun

and ‘mE‘ is the mass of earth

Since the gravitational force of attraction depends on the product of the masses of the sun and the earth. Hence the force exerted by the sun on earth would be equal in magnitude to the force exerted by earth on the sun.

Q.3) What is the importance of gravitational constant ‘G’? Why is it difficult to calculate?

Answer: The gravitational constant ‘G’ is very important because Newton’s law of gravitation can not be held true without it. According to Newton the force of attraction is directly proportional to the product of masses and inversely proportional to the square of the distance between their centers.

In order to hold it correct a proportionality constant is needed which is G.

Gravitational force is very difficult to measure because, its magnitude is very small for ordinary masses available in the laboratory at ordinary distances. Therefore it is very difficult to calculate the value of G.

Q.4) If Earth somehow expanded to a larger radius, with no change in mass, how would your weight be affected? How would it be affected if Earth instead shrunk?

Answer:

The value of g is given by;

If the radius of the earth expands to a larger radius, keeping everything else same, the value of ‘g‘ will be reduced. As w = mg, hence our weight will be reduced.

In the opposite scenario, if earth shrinks, it means the radius of the earth will be reduced. Or in other words, we will be even closer to the center of the earth due to which ‘g‘ increases and hence our weight will also increase.

Q.5) What would happen to your weight on earth if the mass of the earth doubled, but its radius stayed the same?

Answer:

The value of g is given by:

If the mass of the earth is doubled, the value of ‘g’ will be doubled.

As weight = w = mg

This means the weight would also get doubled.

Q.6) Why lighter and heavier objects fall at the same rate toward the earth?

Answer: The gravitational field strength is given by;

where ‘me‘is the mass of the earth

and ‘re‘ is the radius of the earth

This shows that ‘g‘ does not depend on the mass of the object. This means that light and heavy bodies should fall towards the centre of the earth with the same acceleration.

Q.7) The value of ‘g’ changes with location on earth, however, we take the same value of ‘g’ as 9.8 ms-2 for ordinary calculations. Why?

Answer: Although the value of ‘g’ varies with location on earth but this variation is small at different places on the surface so we neglect the variation to avoid complications and take 9.8 ms-2 as a standard.

It is given as;

For example; if we calculate the value of ‘g’ at different altitudes i.e. h = 0, 10 km, 20 km, it can be found that the variation is small;

Here G = 6.67 x 10(-11) Nm2kg-2

me = 6 x 10(24) kg

re = 6.4 x 10(6) m

From the above example, the argument that the variation is small is proved.

Q.8) Moon is attracted by the earth, why it does not fall on earth?

Answer: As we know that, moon is a natural satellite of the earth. It orbits around the earth with some tangential speed ‘v’. Though the earth attracts the moon towards its center, its tangential speed prevents it from falling towards the earth. The presence of gravitational force between earth and moon provides the necessary centripetal force to the moon due to which it revolves around the earth and does not fall on earth.

Q.9) Why for same height larger and smaller satellites must have same orbital speeds?

Answer: The orbital speed of satellite is given by the formula;

i.e. the orbital speed depends upon the mass of the earth ‘me‘ (the planet around which the satellite is revolving), the radius of the earth and the height from the surface of the earth. It does not depend on the mass or radius of the object orbiting around the planet. Hence the orbital speed would be same for larger and smaller satellites.

Q10. Moon is attracted by earth why it does not fall on ground?

Ans. Moon is attracted by earth toward its center due to its tangential speed, but it does not fall on ground. Because the gravitational forces of earth on moon in this case is acting as centripetal force.

Q11 Why the water does not fall out of a bucket when it is whirled in a vertical center?

Ans. When water whirled in a vertical circle in a bucket it does not fall down because of centripetal force.

Q12. Why is it not easy to whirl a hammer by a longer chain?

Ans. It is not easy to whirl a hammer by a longer chain because the moment of inertia is greater and the moment of inertia depends upon mass of the body and radius of the circle. That’s why we can’t whirl a hammer easily by using longer chain.

Q13. Explains if a stone held in our hands released it falls tower earth center?

Ans. We know that everybody is attracted by earth toward it’s center because of force of gravity so that’s why when a stone held in our hands is released it falls toward the earth center.

Q14. What is the value of “G” on moving?

Ans. The value of “G” is constant every where it does not depends upon the nature and size of the masses and nor it depends upon the nature of medium b/w bodies But it is same in all universe.

Q15. If distance b/w the objects is tripled what is decrease in gravitational force?

Ans. Gravitational force b/w bodies in 1st case is given by

F1 (G m1 m2)/r2.................(2)

When distance is tripled then r is replaced by 3r ie

F2 = (1/9) (Gm1m2/r2) 

=> F2 = 1/9 (F1)

Hence force decrease by 1/9

Q16. What is the difference b/w force of gravity and force of gravitation?

Ans. The force which exerts earth on bodies to attract them toward its center is called force of gravity. While the force of attraction b/w any two bodies in this universe is known as force of gravitation.

Q17. If mass of earth is taken as doubled but remain in same size, what will happen to the value of “g” and “G”

Ans. (1) value of G:-it is a universal constant it does not depend upon masses of bodies it will remain the same.

(2) Value of “g” we know that g1 = (GMe) Re2.............(i) when mass is doubled then g2 = G.2Me/Re2

=>g2 = 2. (GMe/Re2) => g2 = 2 (g1)

Hence value of g is doubled if mass of earth is doubled.

Q18. If mass in earth field is doubled what will happen to the force exerted?

Ans. When mass is single then, F1 = (GMem)/r2..............(1)

When mass is doubled then, F2 = (GMe 2m)/r2

 F2 = 2 (GMem/r2) 

=> F2 = 2F1

Hence force b/w them is doubled if mass is doubled.

Q19. What provides the force that produces centripetal acceleration in an orbit?

Ans. The gravitational force provides the force that produces the centripetal acceleration in an orbit because gravitational force acts as centripetal force on satellite to move it in a circular path.

Q20. A satellite is moving around the earth on which of the following does it depend (a) Mass of satellite (b) distance of satellite? (c) mass of earth?

Ans. We know that formula of speed of satellite is V = √GMe/r so this equation shows that speed of satellite depends upon mass of earth.