MCQ of Gravitation Class 9

Explanation: Imagine Earth as a perfectly uniform sphere where gravity depends mainly on how far an object is from the planet’s center. In this situation, one scientist moves downward into a mine below the surface while another scientist moves upward in a hot-air balloon. The goal is to understand how gravitational field strength changes when the position relative to Earth’s center changes.

The gravitational field outside Earth follows the inverse-square relationship, meaning it becomes weaker as distance from the center increases. Inside Earth, however, the situation is different. Only the Mass located at a smaller radius than the observer contributes to the gravitational pull. The outer layers of Earth do not contribute to the NET gravitational force at that depth.

When the scientist rises in the balloon, the distance from Earth’s center becomes slightly larger, so the gravitational field changes according to the inverse-square relation g ∝ 1/R². For the scientist descending into the mine, the distance from the center decreases slightly, but the amount of Earth’s Mass effectively contributing to the gravitational pull also decreases. This change in enclosed Mass alters the gravitational field gradually as depth increases.

A useful comparison is standing in the middle of a crowd where people pull toward the center. If some of those people step away, the total pulling effect becomes weaker even though you might move slightly inward.

Thus, both movements involve changes in gravitational field strength, but the reasons are connected to distance from Earth’s center and the distribution of Mass inside the planet.

Explanation: measurements of gravitational acceleration show small variations at different places on Earth’s surface. When moving from the equator toward the poles at sea level, the value of gravitational acceleration gradually becomes larger. Understanding this requires considering Earth’s shape and its rotation.

Earth is not a perfect sphere; it is slightly flattened at the poles and bulged at the equator, giving it an oblate shape. Because of this, the equatorial radius is larger than the polar radius. Since gravitational strength depends on distance from Earth’s center according to g ∝ 1/R², locations that are farther from the center experience a slightly weaker gravitational pull.

Another factor comes from Earth’s rotation. As the planet spins, objects on its surface experience a centrifugal effect directed outward from the axis of rotation. This effect is strongest at the equator and decreases as one moves toward the poles. At the poles, the rotational effect is essentially zero because these points lie along the rotation axis.

Due to the combination of slightly greater distance from Earth’s center and the outward rotational effect at the equator, the effective gravitational acceleration there becomes slightly smaller compared with regions closer to the poles.

A simple analogy is spinning a ball with objects attached to it. Objects near the middle feel a stronger outward effect than those near the ends of the spinning axis.

These geometric and rotational factors together explain the gradual variation in gravitational acceleration across Earth’s surface.

Explanation: Gravitational force is one of the four fundamental forces of nature and plays a central role in the structure and motion of the universe. The question asks which statement about gravity does not correctly describe its behavior or characteristics.

According to Newton’s law of Gravitation, any two objects with Mass attract each other. The magnitude of this attraction depends on the masses involved and the distance between them. The mathematical relationship is commonly expressed as F = G(Mm/R²), showing that gravitational force decreases rapidly as the separation between objects increases.

Gravity is the dominant force acting on astronomical scales. It governs the motion of planets, stars, galaxies, and satellites. Even though gravity acts between all objects with Mass, it is extremely weak compared with electromagnetic or nuclear forces when dealing with atoms and molecules. For this reason, its effects are usually negligible at microscopic scales.

Another important idea is that gravitational interactions depend on the specific masses and distances involved. Because different pairs of objects have different masses and separations, the gravitational force between them cannot be identical in every situation.

A simple way to imagine this is comparing the pull between two small magnets versus two massive iron spheres. The strength of attraction changes depending on their properties and separation.

Understanding these principles helps determine which statement correctly represents gravitational behavior and which one does not align with established physical laws.

Explanation: Artificial satellites move around Earth while remaining in orbit for long periods. At first glance, it may seem puzzling why these satellites do not simply fall straight down toward the planet under the influence of gravity.

Gravity constantly pulls the satellite toward Earth. However, the satellite also possesses a large horizontal velocity when it is launched into orbit. This sideways motion causes the satellite to continually move forward while gravity pulls it inward.

In circular motion, a body moving along a curved path requires a centripetal force directed toward the center of the circle. In the case of satellites, Earth’s gravitational attraction provides exactly this inward force. The balance between the satellite’s forward motion and the inward pull of gravity produces orbital motion.

Mathematically, this relationship is expressed by equating gravitational force with the centripetal requirement, often written as GMm/R² = mv²/R. This shows how orbital speed depends on the gravitational influence and distance from Earth’s center.

An analogy is swinging a stone tied to a string in a circular path. The tension in the string pulls the stone inward while the stone’s motion keeps it moving around the circle.

Thus, orbital motion results from the interaction between gravitational attraction and the satellite’s tangential velocity, allowing it to remain in continuous motion around Earth rather than falling directly to the surface.

Explanation: Consider a satellite moving around Earth in orbit at a certain altitude. Inside the satellite, objects appear weightless because both the satellite and everything within it are moving together under the influence of gravity. The question asks what happens if a ball is released from inside such a satellite.

In orbit, both the satellite and the ball already possess a large horizontal velocity. When the ball is released, no additional horizontal force suddenly changes its motion. Therefore, the ball initially retains the same horizontal velocity that the satellite had at that moment.

Gravity continues to act on both the satellite and the ball. Because they are moving with nearly identical velocities and are at almost the same distance from Earth’s center, they continue following very similar curved paths around Earth. This shared motion creates the sensation of weightlessness.

Orbital motion can be described using the balance between gravitational attraction and the requirement for centripetal motion, often written in the form GMm/R² = mv²/R. This relationship explains how objects in orbit maintain curved trajectories around Earth.

A helpful comparison is two stones thrown side by side with the same speed and direction. If no external disturbance acts differently on them, they continue moving together along similar paths.

Thus, the behavior of the released ball is determined by its existing velocity and the gravitational conditions of orbital motion.

Explanation: Inside a spacecraft orbiting Earth, astronauts and objects appear to float freely. This condition is often described as weightlessness, though gravity still acts on the spacecraft and everything inside it.

The spacecraft moves around Earth with a large horizontal velocity that allows it to remain in orbit. Gravity continuously pulls the spacecraft toward Earth, but because of its forward motion, it keeps falling around the planet rather than directly toward it.

When an apple is released inside the spacecraft, it already shares the same velocity as the spacecraft because it was moving with it before being released. Once released, no additional force suddenly changes that velocity. As a result, both the spacecraft and the apple continue to move together under the influence of Earth’s gravity.

Orbital motion arises from the balance between gravitational attraction and the required centripetal motion of a circular path. This balance is commonly expressed as GMm/R² = mv²/R, showing how orbital speed and gravitational force are related.

A familiar analogy is passengers inside a smoothly moving airplane. If an object is gently released, it initially continues moving along with the airplane rather than instantly falling behind.

Thus, the motion of objects inside an orbiting spacecraft is governed by shared velocity and the continuous gravitational pull acting on the entire system.

Explanation: The weight of an object on Earth is related to the gravitational pull exerted by the planet. However, the measured weight at the surface is also influenced by Earth’s rotation, which introduces an additional outward effect.

As Earth rotates, objects on its surface move in circular paths around the axis of rotation. This motion produces a centrifugal effect directed away from the axis. The magnitude of this outward tendency depends on the rotational speed and the distance from the axis.

The centrifugal effect is strongest at the equator because that region is farthest from the axis of rotation. It gradually decreases toward the poles and becomes essentially zero at the poles themselves. This outward effect reduces the effective weight of objects measured on Earth’s surface.

The relationship between rotational motion and outward effect can be expressed through circular motion principles where acceleration is proportional to v²/R or ω²R. Increasing rotational speed increases this outward influence.

A simple analogy is spinning a bucket of water tied to a rope. As the spinning speed increases, the water presses outward more strongly against the bucket’s walls.

Therefore, changes in Earth’s rotational speed influence the effective weight measured at different locations on the planet’s surface.

Explanation: The weight of an object depends on the gravitational acceleration at its location. Because gravity changes slightly with position relative to Earth’s center, measurements taken at different heights or depths can show small variations.

At higher altitudes such as mountain tops, the distance from Earth’s center increases slightly. Since gravitational acceleration follows a relationship similar to g ∝ 1/R², increasing distance causes a slight decrease in gravitational strength. As a result, objects tend to weigh a little less at higher elevations.

Inside Earth, the situation differs from the surface. When an object moves below the surface into a mine, the outer layers of Earth no longer contribute to the gravitational pull. Only the Mass enclosed within that smaller radius affects the gravitational attraction.

Because the enclosed Mass becomes smaller as depth increases, gravitational acceleration gradually changes when moving deeper below the surface.

A helpful comparison is standing at different distances from a large object pulling you inward. Moving slightly farther away weakens the pull, while moving inside the object changes how much of its Mass can influence you.

Thus, weight measurements at different vertical positions depend on distance from Earth’s center and the distribution of Mass contributing to gravitational attraction.

Explanation: Satellites remain in orbit because gravity continuously pulls them toward Earth while their forward motion carries them sideways around the planet. This combination produces a curved path known as an orbit.

In orbital motion, gravity acts as the centripetal force required to keep the satellite moving in a circular or elliptical path around Earth. The relationship between gravitational force and circular motion is often represented by GMm/R² = mv²/R.

If the gravitational interaction between Earth and the satellite were suddenly removed, the inward force responsible for maintaining the curved path would disappear. Without this centripetal force, the satellite would no longer follow the curved trajectory around Earth.

According to Newton’s first law of motion, an object in motion continues moving with constant velocity in a straight line unless acted upon by a NET external force. In orbit, gravity is the force that constantly changes the satellite’s direction.

A familiar example is a stone tied to a string and swung in a circle. If the string suddenly breaks, the stone flies off along a straight path tangent to the circle.

Therefore, understanding the role of gravity in providing the inward force explains how the motion of a satellite would change if that force were suddenly removed.

Explanation: Satellites orbit Earth while remaining at large distances above the surface for extended periods. This situation raises the question of why these objects do not simply fall directly toward Earth under the influence of gravity.

Gravity always acts on the satellite, pulling it toward the center of Earth. However, the satellite also possesses a significant horizontal velocity that was given to it during launch. This sideways motion plays a crucial role in maintaining orbital motion.

As the satellite moves forward, gravity continuously pulls it inward, causing its path to curve rather than remain straight. The inward pull provides the centripetal force needed for circular motion. This balance between gravitational attraction and forward velocity produces a stable orbit.

The mathematical relationship describing this balance is often written as GMm/R² = mv²/R, linking gravitational force with the required centripetal motion for orbiting bodies.

A simple analogy is throwing a stone horizontally from a high mountain. If thrown fast enough, the ground curves away beneath it as it falls, allowing it to keep moving around Earth.

Thus, orbital motion results from the interplay between gravitational attraction and the satellite’s horizontal velocity rather than the absence of gravity.

Explanation: Modern Communication systems rely heavily on satellites placed high above Earth to transmit television, radio, and internet signals across vast distances. A relay satellite acts as an intermediary station in space that receives signals from one location and retransmits them to another region on Earth.

For uninterrupted broadcasting, the satellite must remain positioned above the same region of Earth so that ground antennas can continuously point toward it. If the satellite constantly changed its position relative to Earth’s surface, Communication signals would frequently be lost or require constant repositioning of receiving equipment.

To achieve this stable positioning, satellites are placed in a special orbit where their motion around Earth synchronizes with the planet’s own rotation. When the orbital period matches the time Earth takes to rotate once about its axis, the satellite appears stationary relative to a point on the surface.

The motion of satellites in orbit depends on the balance between gravitational attraction and centripetal motion. This relationship is commonly represented by GMm/R² = mv²/R, which determines the orbital speed required at a given altitude.

A useful comparison is walking on a rotating carousel while maintaining exactly the same speed as the platform’s rotation. From the perspective of someone standing on the carousel, you would appear to remain at the same position.

Such synchronized motion allows Communication satellites to maintain constant alignment with ground stations, enabling reliable long-distance broadcasting across continents.

Explanation: Satellites placed in orbit around Earth can move in many different paths depending on their altitude, speed, and direction of motion. Among these orbits, one special type is designed specifically for Communication, weather monitoring, and broadcasting applications.

A geostationary satellite is arranged so that its motion around Earth keeps it fixed relative to a particular point on the planet’s surface. From the viewpoint of an observer on Earth, the satellite appears to remain motionless in the sky rather than moving across it like most satellites.

For this to happen, several physical conditions must be satisfied. The satellite must move in a circular orbit directly above Earth’s equator and travel in the same direction as Earth’s rotation. In addition, its orbital period must match the time Earth takes to rotate once about its axis.

The motion of the satellite is governed by the balance between gravitational attraction and centripetal motion, often expressed as GMm/R² = mv²/R. This balance determines the altitude and speed required for such synchronized motion.

An easy way to picture this is imagining a point on a rotating wheel moving at exactly the same angular speed as the wheel itself. Relative to the wheel’s surface, that point appears stationary.

These physical requirements allow the satellite to maintain a constant position relative to Earth, which is why this type of orbit is widely used for Communication systems.