Class 9 Science Chapter 9 Question Answers - Gravitation

Q1: What do you mean by free fall?
Ans: It is the object falling towards earth under the influence of attraction force of earth or gravity.

Q2: What do you mean by acceleration due to gravity?
Ans: During free fall any object that has mass experiences force towards centre of earth and hence an acceleration works as well. “acceleration experienced by an object in its freefall is called acceleration due to gravity.” It is denoted by g.

Q3: Why is it difficult to hold a schoolbag having a strap made of a thin and strong string?
Ans: It is difficult to hold a schoolbag having a strap made of a thin and strong string because a bag of that kind will make its weight fall over a small area of the shoulder and produce a greater pressure that makes holding the bag difficult and painful.

Q4: What do you mean by buoyancy?
Ans: It is the upward force experienced by an object when it is immersed into a fluid.

Q5: You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
Ans: Mass will be slightly more than 42 kg.

Q6: You have a bag of cotton and anir on bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?
Ans: The bag of cotton is heavier since volume of cotton bag is greater than iron bar, so the up thrust is larger in case of cotton hence real mass of cotton bag is more and it is heavier.

Q7: How does the force of gravitation between two objects change when the distance between them is reduced to half?
Ans: The force of gravitation between two objects is inversely proportional to the square of the distance between them therefore the gravity will become four times if distance between them is reduced to half.

Q8: Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than alight object?
Ans: In free fall of objects the acceleration in velocity due to gravity is independent of mass of those objects hence a heavy object does not fall faster than alight object.

Q9: What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? (Mass of the earth is 6 x 1024 kg and radius of the earth is 6.4 x 106 m.)
Ans: 

Q10: The earth and the moon are attracted to each other by gravitational force. Does the  earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why?
Ans: The earth and the moon are attracted to each other by same gravitational force because for both of them formula to calculate force of attraction is the same 

d is also same for both.

Q11: If the moon attracts the earth, why does the earth not move towards the moon?
Ans: Earth does not move towards moon because mass of moon is very small as compared to that of earth.

Q12: Amit buys few grams of gold at the poles as per the instruction of one of his friends. He hands over the same when he meets him at the equator. Will the friend agree with the weight of gold bought? If not, why? [Hint: The value of g is greater at the poles than at the equator.]
Ans: Since  W = m x g and given in the question that value of g is greater at the poles than at the equator, hence weight of same amount of gold will be lesser at equator than it was on the poles. Therefore, the friend will not agree with the weight of gold bought.

Q13: Why will a sheet of paper fall slower than one that is crumpled into a ball?
Ans: A greater surface area offers greater resistance and buoyancy same is true in the case of a sheet of paper that has larger surface area as compared to paper crumpled into a ball. So sheet of paper falls slower.

Q14: Gravitational force on the surface of the moon is only 1/6 as strong as gravitational force on the earth. What is the weight in newton’s of a 10 kg object on the moon and on the earth?
Ans: value of gravity on earth = 9.8 m/s²
value of gravity on moon = l/6^th of earth = 9.8/6 = 1.63 ms2
weight of object on moon = m x 1 .63 = 10 x 1.63 = 16.3 N
weight of object on earth = m x. 9.S = 10 x 9.8 = 98 N

Q15: A ball is thrown vertically upwards with a velocity of 49 m/s. Calculate
(i) the maximum height to which it rises,
(ii) the total time it takes to return to the surface of the earth.
Ans: (i) v = u + gt
0 = 49 + (-9.8) x t
9.8t = 49
t = 49/9.8 = 5 s

= 49 x 5 + 1/2 x 9.8 x 25
= 245 - 122.5
= 122.5
(ii) total time taken to return = 5 + 5 = 10 s

Q16: Why does a block of plastic released under water come up to the surface of water?
Ans: Since of plastic has density very less as compared to water i.e. weight of plastic is less than the buoyant force experienced by it therefore a block of plastic released under water come up to the surface of water/floats.

Q17:  The volume of 50 g of a substance is 20 cm³ . If the density of water is 1g /cm³ , will the substance float or sink?
Ans: Density of that substance (d) = mass/volume = 50/20 = 2.5 g /cm³
since the density of substance (2.5) is greater than the density of water (1) therefore it will sink.

Q18: State the Universal law of Gravitation?
Ans. According to Universal law of Gravitation every particle in the universe attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Let M₁, M₂ = Masses of two bodies
r = Distance of separation
f = force acting b/w them


Q19: If heavier bodies are attracted more strongly by the earth, why do they not fall faster to the ground?
Ans: Heavier bodies do not fall fast on the ground even though they are attracted by the earth strongly because of their larger mass, the acceleration produced in them by the force of earth will be less as
F = m
F/m = a
m-mass, F = force, a = Acceleration
so if Mass is more, Acceleration will be less

Q20: How does acceleration due to gravity change with the shape of earth?
Ans: Since earth is not a perfect sphere, it is flattened from the top and bulges at the centre and acceleration due to gravity (g) is inversely proportional to the radius of earth so, g is more at poles because of lesser radius and less at equator because of greater radius.

Q21: What do you understand by the gravitational force of earth and weight?
Ans: Gravitational force of earth is the force by which earth exerts on any object towards itself.
Weight is the force which the object exerts on the earth.

Q22: A man of mass 60 Kg is standing on the floor holding a stone weighing 40 N. What is the force with which the floor is pushing him up?
Ans: The gravitational pull on the man = Mg
= 60 x 10 = 600.V
The weight he is carrying = 40 N
The total downward force on the floor = 40 N + 600 N
= 640 N
The Gravitational force and upward force of the floor is an action - reaction pair.
The force with which the floor pushes the man = 640 N.

Q23: What is acceleration due to gravity and how is it different from acceleration?
Ans: Acceleration due to gravity is the acceleration produced in the object when it falls freely under the effect of gravitational force of earth only. Acceleration is produced when any external force applied on the body makes it to move.

Q24: What is the importance of the universal law of Gravitation?
Ans: The importance of universal law of gravitation:

• The force that binds us to the earth.
• The motion of the moon around the earth.
• The motion of the planet around the sun
• The tides due to the moon and the sun

Q25: Define Pressure? How is thrust different from Pressure?
Ans: The pressure due to a force is defined as the force acting or unit area.

A unit of Pressure is N / m²
Thrust is also the pressure but it is the force acting on a surface normal to its area.

Q26: What are fluids? What are the factors on which the upward pressure at a point on a  fluid depends?
Ans: Fluids are that which flow and it includes both liquids and gases.
Factors on which the upward pressure at a point of the fluid depends are:

• the depth of the point from the surface of the liquid.
• the density of the liquid
• the acceleration due to gravity.

Q27: State the universal law of gravitation.
Ans: According to Newton’s universal law of gravitation:
Every mass in this universe attracts every other mass with a force which is directly proportional to the product of two masses and inversely proportional to the square of the distance between them.

Q28: Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.
Ans: The formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth is given below:

F = magnitude of gravitational force
G = Universal gravitation constant
M = mass of earth
m = mass of object
d = distance of object from the centre of earth

Q29: What are the differences between the mass of an object and its weight?
Ans:

Q30: Why is the weight of an object on the moon 1/6th its weight on the earth?
Ans. since we know FF = mx g Mass of object remains the same whether on earth or moon but the value of acceleration on moon is 1/6th of the value of acceleration on earth. Because of this weight of an object on moon is 1/6th its weight on the earth.