Gravity Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. If the earth stops rotating, the value of g at the equator

1. No effect
2. Decreases
3. Increases
4. First increases and then decreases

Ans. The correct answer is c. Increases

Explanation: At the equator, the value of acceleration due to gravity is given by

g_e = g - Rω²

If the earth stops rotating, then angular acceleration becomes zero i.e. ω = 0

⇒ g_e = g

Hence the value of g increases.

Ques. A satellite revolves very near to the earth's surface. Its speed should be around

1. 2 km/s
2. 5 km/s
3. 11 km/s
4. 8 km/s

Ans. The correct answer is d. 8 km/s

Explanation: Near the surface of the earth, the orbital speed is given by

v₀ = √gR

Where R is the radius of the earth i.e. R = 6400 km

On substituting the values of g and R, we get

v₀ = 8 km/s

Ques. The mass of an object can never be zero, but the weight of an object can be zero.

1. True
2. False

Ans. The correct answer is a. True

Explanation: The weight of an object becomes zero when the net gravitational force acting on it is zero. However, because mass is a characteristic of matter, an object's mass can never be zero.

Ques. If the gravitational constant decreases with time, what will remain unchanged in the case of a satellite orbiting around the Earth?

1. Areal velocity
2. Orbiting radius
3. Time period
4. Angular velocity

Ans. The correct answer is a. Areal velocity

Explanation: The central gravitational force decreases, but no torque is produced. As a result, the angular momentum and, hence the areal velocity remain constant.

Ques. An astronaut, while revolving in a circular orbit happens to throw a spoon outside. What will happen to the spoon?

1. It will remain stationary
2. It will reach the Earth's surface
3. It will continue to move in a circular orbit
4. It will escape the universe

Ans. The correct answer is c. It will continue to move in a circular orbit

Explanation: If a spoon is thrown outside while the astronaut is in a circular orbit, the spoon will continue to revolve in the same circular orbit and chase the astronaut. It will never reach the earth's surface.

Ques. Why do the astronauts experience weightlessness in the international space station?

1. Their spacesuits are specially designed for this purpose
2. Acceleration due to gravity is zero at that height
3. At that height, the gravity of the moon and earth cancel out each other
4. Because they are falling towards the earth

Ans. The correct answer is d. Because they are falling towards the earth

Explanation: The astronauts on the International Space Station experience weightlessness as a result of this freefall. Because the parabolic trajectory of the ISS's free fall is so fast that it circles the Earth.

Ques. A satellite doesn’t need any fuel to rotate around the Earth.

1. True
2. False

Ans. The correct answer is a. True

Explanation: The centripetal force is provided by the gravitational attraction between the earth and the satellite, which causes the satellite to move in a circular orbit.

Ques. In the case of the centre of gravity, which of the following statements is true?

1. The centre of gravity lies inside the body
2. The centre of gravity may lie inside or outside the body
3. The centre of gravity only lies outside the body
4. Irrespective of the shape, the centre of gravity lies at the centre of the body

Ans. The correct answer is b. The centre of gravity may lie inside or outside the body

Explanation: Because it is a point in the body where the resultant force acts, the centre of gravity does not have to be inside the body; it can be inside or outside the body.

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Short Answers Questions [2 Marks Questions]

Ques. What is gravity?

Ans. Gravity is a universal force of attraction that attracts every object in the world around us. It is a basic force that attracts two masses towards one another.

Ques. What is the relation between gravity and weight?

Ans. The weight of the body is the force of gravity acting on it. The relation between gravity and weight is given by

F_g = mg

Where

• Fg is the force of gravity
• m is the mass of the body
• g is the acceleration due to gravity

Ques. What is the formula of gravity?

Ans. The formula of gravity is given by the

\(F_g = G \frac{m_1m_2}{R^2}\)

Where

• Fg is the force of gravity between masses m₁ and m₂
• G is the gravitational constant
• R is the distance between the two masses

Ques. What is the Universal Law of Gravitation?

Ans. In 1680, Sir Isaac Newton proposed the Universal Law of Gravitation. According to Newton's Universal Law of Gravitation, the force of attraction between any two particles in the universe is exactly proportional to the product of their masses and inversely proportional to the square of their distance.

Ques. Explain why the tennis ball bounces higher on the hills than on the plains.

Ans. On the hills, the value of 'g' is less than on the plains. As a result, the weight (mg) of the tennis ball will be smaller on the hills than on the plains. In other words, the gravitational force exerted by the earth on the ball will be smaller on the hills than on the plains.

Ques. Define acceleration due to gravity.

Ans. Acceleration due to gravity refers to the acceleration of freely falling bodies caused by the force of attraction of another body. It is a constant quantity for a particular attracting body in a given location. The average value of acceleration due to gravity for Earth on or near its surface is 9.8 m/s²

Ques. Earth is continuously pulling the moon towards its centre. Why does the moon not fall on the earth?

Ans. The gravitational pull between the earth and the moon produces the centripetal force required for the moon to orbit the earth. If the moon was at rest, it would have collapsed to Earth.

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Long Answers Questions [3 Marks Questions]

Ques. What are the examples of gravity?

Ans. The following are the examples of gravity

• Gravity holds the gases in the Sun together.
• Because of gravity, we have an atmosphere that surrounds the earth's surface.
• The gravitational attraction between the earth and the moon generates tides in the ocean.
• Gravity causes the moon to orbit the Earth and the Earth to orbit the Sun.

Ques. What are the properties of the force of gravity?

Ans. The following are the properties of the force of gravity

• It is an attracting force that attracts two objects together rather than pushing them apart.
• It is a force that exists at a very long distance and is always present between two objects regardless of their medium. 
• Gravity is proportional to the masses of the object.

Ques. What are tidal forces and how do they cause tides on Earth?

Ans. Tidal forces are the gravitational forces that act on an object when it is in a non-uniform gravitational field.

• Tidal forces on Earth are caused by the gravitational pull of the sun and the moon. 
• The gravitational force of the moon and sun is strongest on the side of Earth closest to them and weakest on the side of Earth farthest away.
• Because of this difference in gravitational force, the ocean water bulges on the side of the Earth closest to the moon and sun and decreases on the side of the Earth farthest away. 
• This causes the tides.

Ques. How do black holes develop, and how does their gravitational pull impact the surrounding space?

Ans. When massive stars collapse at the end of their lives, black holes form. 

• When a star collapses, its gravity becomes so strong that nothing can escape, not even light.
• The gravity of a black hole is so strong that it bends spacetime around it. 
• This bending of spacetime causes the light from stars and galaxies behind the black hole to be distorted and magnified. 
• This effect is called gravitational lensing.

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Very Long Answers Questions [5 Marks Questions]

Ques. What are the properties of gravitation force?

Ans. The following are the properties of gravitational force

• The gravitational force is a long-distance force that occurs between two particles independent of the medium between them.
• The gravitational force is proportional to the product of the two bodies' masses. This indicates that a larger mass will produce a greater force.
• The gravitational force obeys the inverse square law. The force between them is inversely proportional to the square of their distance.
• The gravitational force produces a constant acceleration on the earth's surface, g = 9.8 m/s²
• Gravitational force is always an attractive force. It tends to bring the masses together, but it never pulls them away.
• It is unaffected by the medium in between.
• The weakest of the four fundamental forces is the gravitational force.
• It acts along the line joining the two objects' centers. Therefore, it is referred to as the central force.

Ques. A body weighs 36 kg on the surface of the earth. How much will it weigh on the surface of Mars whose mass is 1/9 and radius is 1/2 of that of the earth?

Ans. We know that the weight (W_e) of the body on the surface of the earth is equal to the force of attraction (F) between the earth and the body. i.e. 

W_e = mg = GM_em/R_e²

Where

• M_e = mass of the earth
• R_e = radius of the earth
• m = mass of the body
• g = acceleration due to gravity
• G = gravitational constant

Let M_m and R_m be the mass and radius of the Mars respectively.

Given

• Mass of the mars, M_m = 1/9 M_e
• The radius of the m_ars = 1/2 R_e

If Wm be the weight of the body of the surface of the mars, then

W_m = \(G\frac{M_mm}{R_m^2} = G \frac{(M_e/9)m}{(1/2R_e)^2} = \frac{4}{9} G \frac{M_em}{R_e^2}\)

⇒ W_m = (4/9) W_e

Given, the weight of the body on the surface of the earth, We = 36 kg

Then, the weight of the body on the surface of the mass will be

W_m = (4/9) x 36 = 16 kg

Ques. Two solid spheres of the same density and same radius r are placed in contact with each other. Show that the gravitational force between them is directly proportional to the fourth power of radius r.

Ans. Since the density and radius of both the spheres are same, their mass will be equal. Let the mass of each sphere be m.

Also, both the spheres are placed in contact with each other, therefore the distance between their center of mass will be 2r.

The gravitational force between the spheres will be

F = \(G\frac{m \times m}{(2r)^2} = G \frac{m^2}{4r^2}\)

But, the mass of each sphere, m = volume of the sphere (V) x density of sphere (ρ)

The volume of the sphere, V = 4/3 πr³

⇒ m = \(\frac{4}{3}\) πr³ρ

⇒ F = \(G \frac{(\frac{4}{3} \pi r^3 \rho)^2}{4r2} = \frac{4}{9} \pi ^2G \rho^2r^4\)

Since \(\frac{4}{9} \pi ^2G \rho^2\)= constant, therefore

F ∝ r⁴

Hence it is proved that the gravitational force between them is directly proportional to the fourth power of radius r.

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