Class 9 – Science Extra Questions – Ch. 9 – Gravitation

Ans: D

Explanation: When a steel ship floats on water, it is because the buoyant force exerted by the water on the ship is equal to the weight of the ship. The buoyant force is an upward force that opposes the weight of an object immersed in a fluid. This phenomenon is described by Archimedes’ principle, which is the fundamental principle behind buoyancy. Gravity is the force pulling the ship down, but buoyancy is the force pushing it up, allowing it to float. Friction and inertia are not directly responsible for a ship floating.
Correct Option: D

Ans: D

Explanation: In projectile motion launched vertically upwards, the time taken to reach the peak height is equal to the time taken to fall back to the initial launch point (assuming no air resistance and the launch and landing heights are the same). Since the projectile reaches its peak height in 3 seconds, it will take another 3 seconds to fall back down. Therefore, the total time of flight is the sum of the time to go up and the time to come down.
Correct Option: D

Ans: C

Explanation: The acceleration due to gravity is determined by Newton’s Law of Universal Gravitation, which states that the force of gravity between two objects is inversely proportional to the square of the distance between their centers. For an object near the Earth, the acceleration due to gravity is given by g = GM/r^2, where G is the gravitational constant, M is the mass of the Earth, and r is the distance from the center of the Earth to the object. As the altitude increases, the distance r from the center of the Earth also increases. Since r is in the denominator and squared, an increase in r leads to a decrease in the value of g. Therefore, the acceleration due to gravity decreases as altitude increases.
Correct Option: C

Ans: A

Explanation: Nanomaterials have a significantly larger surface area to volume ratio compared to their bulk counterparts. This increased surface area exposes more atoms to the surrounding environment, leading to enhanced reactivity. Additionally, the unique quantum mechanical effects that become prominent at the nanoscale often result in improved mechanical properties like strength and hardness. Option B is incorrect as nanoparticles often *increase* electrical conductivity and thermal stability. Option C is incorrect because the surface area to volume ratio is significantly *larger*, not smaller. Option D is incorrect as nanoparticles exhibit different properties due to quantum effects and increased surface area.
Correct Option: A

Ans: A

Explanation: Pressure is defined as force per unit area. In this case, the force is the weight of the wooden block, which remains constant regardless of its orientation. Therefore, to exert the least pressure, the block must rest on the face with the largest area. The areas of the different faces are:
Face 1: 10 cm x 5 cm = 50 cm²
Face 2: 10 cm x 2 cm = 20 cm²
Face 3: 5 cm x 2 cm = 10 cm²
The largest area is 50 cm², which corresponds to the 10 cm x 5 cm face. Thus, resting on this face will exert the least pressure.
Correct Option: A

Ans: D

Explanation: Newton’s third law of motion states that for every action, there is an equal and opposite reaction. The gravitational force exerted by the Earth on the book (F1) and the gravitational force exerted by the book on the Earth (F2) are an action-reaction pair. Therefore, their magnitudes must be equal, regardless of the masses of the objects involved.
Correct Option: D

Ans: A

Explanation: The gravitational force between two objects is directly proportional to the product of their masses. This is described by Newton’s Law of Universal Gravitation, which states that F = G * (m1 * m2) / r^2, where F is the gravitational force, G is the gravitational constant, m1 and m2 are the masses of the two objects, and r is the distance between their centers. If the mass of the Moon (m2) were tripled, and assuming the Earth’s mass (m1) and the distance (r) remain constant, the new force would be F’ = G * (m1 * 3*m2) / r^2. This can be rewritten as F’ = 3 * [G * (m1 * m2) / r^2], which means the new force F’ is three times the original force F.
Correct Option: A

Ans: B

Explanation: Newton’s law of universal gravitation is expressed by the formula F = G * (m1 * m2) / r^2, where F is the gravitational force, G is the gravitational constant, m1 and m2 are the masses of the two objects, and r is the distance between their centers. The gravitational constant, G, is a universal constant, meaning its value is the same everywhere in the universe, regardless of the masses of the objects, the distance between them, or the density of the objects. It is a proportionality constant that, when multiplied by the product of the masses and divided by the square of the distance, gives the magnitude of the gravitational force. Option A is incorrect because G’s value is constant and does not change with masses. Option C is incorrect because while a stronger attraction implies a larger force, G itself is not what changes to cause this; the masses and distance change. Option D is incorrect because G is a fundamental constant and is not dependent on the density of objects.
Correct Option: B

Ans: A

Explanation: The acceleration due to gravity (g) on the surface of a planet is related to the universal gravitational constant (G), the mass of the planet (M), and the radius of the planet (R) by the formula: g = GM/R^2. To find the mass of the planet (M), we can rearrange this formula. Multiplying both sides by R^2 gives: gR^2 = GM. Then, dividing both sides by G gives: M = gR^2/G.
Correct Option: A

Ans: C

Explanation: In a vacuum, there is no air resistance. All objects, regardless of their mass or composition, accelerate towards the Earth at the same rate due to gravity. This phenomenon is a direct consequence of the principle of equivalence, which states that gravitational mass and inertial mass are equivalent, meaning that the gravitational force experienced by an object is proportional to its inertial mass, leading to the same acceleration for all objects in free fall. The strong nuclear force and the weak nuclear force are fundamental forces that operate at the subatomic level and are not relevant to macroscopic objects falling under gravity. The electromagnetic force is also not the primary reason for this observation.
Correct Option: C