Class 9 Gravitation MCQ

Explanation: This question asks how the pressure exerted by a sharp needle compares to that of a blunt one when both are pressed against a surface. Pressure depends on how force is distributed over an area. A smaller area leads to a greater concentration of force. The formula for pressure is force divided by area, so when the contact area decreases while force remains constant, pressure increases significantly. A sharp needle has a very fine tip, meaning the area of contact is extremely small compared to a blunt needle. As a result, the same applied force becomes highly concentrated at that point. This is why sharp objects easily pierce surfaces. Think of wearing high heels versus flat shoes: the heel concentrates your weight on a tiny area, increasing pressure on the ground. In summary, reducing the contact area increases pressure, making sharp objects more effective at penetrating surfaces.

Explanation: This question explores the fundamental force responsible for attraction between objects in the universe. Every object with Mass exerts a force on every other object with Mass, regardless of size or distance. This interaction is described by a universal principle that applies from tiny particles to massive celestial bodies. The strength of this force depends on the masses involved and the distance separating them, decreasing as distance increases. This force is responsible for keeping planets in orbit, causing objects to fall toward Earth, and holding galaxies together. For example, when you drop a ball, it accelerates downward due to this attractive interaction with Earth. Even though this force exists between all objects, it becomes noticeable only when at least one object has significant Mass. In summary, this is a universal attractive force acting between all masses, governing motion on both terrestrial and cosmic scales.

Explanation: This question focuses on understanding the direction of the force exerted by a Fluid on an object immersed in it. When an object is placed in a Fluid, it experiences pressure from all sides due to the Fluid particles. However, pressure increases with depth, meaning the force exerted at the bottom of the object is greater than at the top. This imbalance results in a NET force. This phenomenon is explained by Archimedes’ principle, which states that a Fluid exerts an upward force equal to the weight of the displaced Fluid. For example, when you try to push a ball underwater, you feel a force pushing it back toward the surface. In summary, the variation of pressure with depth creates a NET force that acts opposite to gravity on submerged objects.

Explanation: This question deals with the principles governing the gravitational interaction between objects. According to Newton’s law of Gravitation, every object in the universe attracts every other object. The strength of this attraction depends on two main factors: the masses of the objects and the distance between them. Specifically, the force increases with the product of the masses and decreases with the square of the distance between their centers. This relationship is expressed mathematically using an inverse square law. For instance, planets remain in orbit around the Sun due to this precise balance of gravitational attraction. In summary, gravitational force depends directly on Mass and inversely on the square of the distance separating objects.

Explanation: This question examines the reason behind the circular motion of the Moon around the Earth. For any object to move in a circular path, there must be a continuous inward force acting toward the center of the circle. This force keeps the object from moving in a straight line due to inertia. In the case of the Moon, Earth provides this inward pull through gravitational attraction. This force constantly changes the direction of the Moon’s motion, keeping it in orbit. Without this inward force, the Moon would drift away into space. A common analogy is a stone tied to a string being whirled in a circle—the tension in the string keeps it moving in that path. In summary, an inward-directed force is essential for maintaining orbital motion.

Explanation: This question explores how weight behaves at the center of the Earth. Weight is defined as the gravitational force acting on an object due to Earth. Inside the Earth, the gravitational pull experienced by an object depends only on the Mass beneath its position. As one moves toward the center, the surrounding Mass begins to cancel out the gravitational effects from different directions. At the exact center, gravitational forces from all sides balance each other completely. This results in no NET gravitational force acting on the object. For example, if you imagine being pulled equally from all directions, the forces cancel out, leaving no movement. In summary, symmetry of gravitational forces at the center leads to zero effective pull.

Explanation: This question aims to clarify the physical meaning of weight. Weight is not the same as Mass; rather, it is a force resulting from gravitational attraction. When an object is placed in a gravitational field, such as near Earth, it experiences a force pulling it toward the center of the Earth. This force depends on both the object’s Mass and the acceleration due to gravity. Mathematically, it is expressed as W = mg, where m is mass and g is gravitational acceleration. For example, the same object would have different weight on the Moon due to lower gravity. In summary, weight represents the force exerted on an object due to gravitational attraction.

Explanation: This question highlights a key concept in motion under gravity. In a vacuum, there is no air resistance to oppose motion, so all objects fall solely under the influence of gravity. According to fundamental Physics principles, the acceleration due to gravity is constant for all objects near Earth’s surface, regardless of their mass. This means heavy and Light objects fall at the same rate in the absence of air resistance. For example, a feather and a coin dropped in a vacuum chamber reach the ground simultaneously. This was famously demonstrated in experiments on the Moon. In summary, the absence of air resistance reveals that gravitational acceleration is uniform for all objects.

Explanation: This question explores the force responsible for planetary motion. For a planet to move in a circular or elliptical orbit, it must experience a continuous inward force directed toward the center of motion. This force prevents the planet from moving in a straight line due to inertia. In the Solar system, this inward force is provided by the gravitational attraction between the Sun and the planet. The balance between the planet’s forward motion and this inward pull results in a stable orbit. For example, Earth remains in orbit because its motion is constantly redirected by this force. In summary, orbital motion requires a central force that continuously alters the direction of motion.

Explanation: This question deals with how gravitational acceleration varies across Earth. The value of g is not constant everywhere; it depends on factors such as altitude, latitude, and Earth’s shape. Earth is slightly flattened at the poles and bulged at the equator, which affects gravitational strength. Additionally, rotation causes a slight reduction in effective gravity at the equator. At higher altitudes, such as mountain peaks, the distance from Earth’s center increases, reducing gravitational pull. Conversely, regions closer to the poles experience slightly stronger gravity due to reduced centrifugal effects and shorter distance from the center. In summary, gravitational acceleration varies with location due to Earth’s shape and rotation.

Explanation: This question examines the direction of gravitational acceleration during upward motion. Regardless of whether an object is moving upward or downward, the acceleration due to gravity always acts toward the center of the Earth. When a ball is thrown upward, its velocity is initially in the upward direction, but gravity continuously acts in the opposite direction, slowing it down. Eventually, the velocity becomes zero at the highest point, after which the ball begins to fall back down. For example, when you toss a ball upward, it slows down until it stops briefly before descending. In summary, gravitational acceleration consistently acts downward, independent of the object’s direction of motion.

Explanation: This question relates to the nature of forces involved in circular motion. For an object to move in a circular path, it must experience a force that constantly changes the direction of its velocity. This force is directed toward the center of the circle and is known as centripetal force. Without this inward force, the object would move in a straight line due to inertia. The magnitude of this force depends on mass, velocity, and radius, often expressed as F = mv²/r. A common example is a car turning on a curved road, where friction provides the necessary inward force. In summary, circular motion requires a continuous inward force that maintains the curved path.

Explanation: This question examines how objects behave when falling under gravity. In the absence of air resistance, all objects are influenced solely by gravitational force. Although heavier objects experience a greater gravitational force, they also possess proportionally greater inertia, resulting in the same rate of acceleration. This means that the motion of falling objects is independent of their mass when only gravity acts. For instance, in a vacuum chamber, a feather and a metal ball fall together and reach the ground at the same time. This demonstrates that gravity imparts a uniform acceleration to all bodies. In summary, gravitational influence results in identical acceleration for all freely falling objects when external resistive forces are absent.

Explanation: This question explores the reason behind the sensation of weightlessness in orbit. Astronauts in orbit are still under the influence of Earth’s gravity, but they are in a state of continuous free fall toward the Earth. At the same time, they possess sufficient horizontal velocity to keep missing the Earth, resulting in an orbit. Because both the spacecraft and the astronauts are accelerating downward at the same rate, there is no normal reaction force acting on them. This absence of a supporting force creates the feeling of weightlessness. For example, it is similar to being in a falling elevator where everything inside appears to float. In summary, weightlessness arises from free fall where no contact force is experienced.

Explanation: This question focuses on the direction in which gravity acts on an object in free fall. Gravitational force always acts toward the center of the Earth, regardless of the object’s motion. Even if an object is moving upward initially, the gravitational pull continues to act downward. This constant direction ensures that objects eventually slow down, stop, and reverse direction if thrown upward. For example, when a ball is dropped or thrown up, it always returns to the ground due to this downward force. The direction does not change based on motion but remains fixed toward Earth’s center. In summary, gravitational force consistently acts downward toward the Earth’s center.

Explanation: This question investigates the factors influencing gravitational attraction. According to the universal law of Gravitation, the force between two objects depends on the product of their masses and the square of the distance between them. Additionally, a constant known as the gravitational constant plays a role in determining the magnitude of the force. However, the formula does not involve the simple addition of masses as a determining factor. For example, doubling both masses increases the force significantly, while increasing distance reduces it sharply. The structure of the equation highlights which quantities directly influence the force. In summary, only specific mathematical relationships between mass and distance determine gravitational strength.

Explanation: This question addresses the variation of gravitational acceleration on Earth. The value of gravitational acceleration is not perfectly uniform across the planet due to differences in Earth’s shape, rotation, and altitude. Earth is slightly flattened at the poles and bulged at the equator, affecting the distance from the center. Additionally, rotational motion introduces a centrifugal effect that slightly reduces effective gravity near the equator. At higher altitudes, such as mountains, gravity decreases because the distance from Earth’s center increases. For example, a person at sea level experiences slightly stronger gravity than someone at a high altitude. In summary, gravitational acceleration varies slightly depending on location and physical conditions.

Explanation: This question explores the scope of a fundamental physical law. Newton’s law of Gravitation states that every object with mass attracts every other object with mass. This principle is universal and applies not only to objects on Earth but also to celestial bodies such as planets, stars, and galaxies. It explains a wide range of phenomena, from falling objects to orbital motion. For example, the same law that causes an apple to fall also governs the motion of planets around the Sun. Its universality makes it one of the most important laws in Physics. In summary, this law applies broadly across all scales in the universe wherever mass is present.

Explanation: This question examines the relationship between density and buoyancy. An object floats in a Fluid if its density is less than that of the Fluid and sinks if its density is greater. When a substance floats in water, it indicates that its density is lower than water. However, if it sinks in another liquid like coconut oil, this implies that its density is higher than that liquid. This comparison allows us to determine the relative density range of the substance. For example, wood may float in water but sink in a denser liquid depending on its composition. In summary, floating and sinking behavior reveals how an object’s density compares with different fluids.

Explanation: This question focuses on the mathematical representation of the gravitational constant. The gravitational constant appears in Newton’s law of Gravitation, which relates force, masses, and distance. By rearranging the formula F = G(Mm / r²), one can express the constant in terms of measurable quantities like force, mass, and separation distance. However, not all expressions involving gravitational terms correctly define this constant. Some equations may represent related concepts such as gravitational acceleration rather than the constant itself. For example, expressions involving g and radius describe different relationships. In summary, only specific rearrangements of the universal law correctly define the gravitational constant.

Explanation: This question explores how gravitational force changes with variation in mass. According to the formula F = G(Mm / r²), the force is directly proportional to the product of the two masses. If both masses are reduced, their product changes accordingly, affecting the overall force. Since the distance remains constant, only the change in mass influences the result. For example, if each mass becomes half, their product becomes a fraction of the original, reducing the force significantly. This demonstrates the sensitivity of gravitational force to changes in mass. In summary, gravitational force varies proportionally with the product of the interacting masses.

Explanation: This question relates to motion in circular paths and the role of forces. When an object moves in a circle, a centripetal force continuously pulls it toward the center. If this force suddenly disappears, the object no longer follows a curved path. Due to inertia, it continues moving in the direction it was traveling at that instant. This direction is along the tangent to the circular path. For example, if a stone tied to a string is released, it flies off in a straight line rather than continuing in a circle. This behavior illustrates Newton’s first law of motion. In summary, removal of the inward force causes motion along a straight-line tangent.

Explanation: This question explores the concept of buoyancy and how objects behave in fluids based on density and forces. When an object is placed in water, it experiences two main forces: its weight acting downward and the buoyant force acting upward. The buoyant force depends on the amount of water displaced by the object. If the downward force is greater than the upward force, the object will sink. Materials like iron have a higher density compared to water, meaning they contain more mass in a given volume. As a result, the weight becomes dominant over the buoyant force. For example, a small iron nail sinks, while a large ship made of iron floats due to its design and overall density. In summary, sinking occurs when the downward force exceeds the upward buoyant support provided by the Fluid.

Explanation: This question examines how changes in Fluid properties affect buoyancy. When Salt is added to water, it dissolves and increases the density of the liquid. A denser fluid can exert a greater buoyant force on an object for the same volume displaced. If an object is already floating, it means the forces acting on it are balanced. Increasing the density of the fluid enhances the upward force, disturbing this balance. As a result, the object adjusts its position to maintain equilibrium with less volume submerged. For example, it is easier to float in seawater than in freshwater due to higher density. In summary, increasing fluid density increases buoyant force, causing the object’s submerged portion to reduce.

Explanation: This question focuses on the principles governing planetary motion as described by Kepler. These laws explain how planets move around the Sun in predictable patterns. The first law states that planetary orbits are elliptical, not circular, with the Sun at one focus. The second law explains that planets sweep out equal areas in equal intervals of time, indicating varying speeds along the orbit. The third law relates the time taken for a planet to orbit the Sun to its average distance, showing a mathematical relationship between them. For example, planets farther from the Sun take longer to complete one revolution. In summary, these laws collectively describe the shape, speed, and timing of planetary motion in the Solar system.