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Gravitational force is a fundamental interaction between two masses. It's the force by which objects with mass attract each other. The strength of this force is dictated by their masses and the distance separating them, as per Newton's Law of Universal Gravitation. This law can be expressed mathematically as \( F = \frac{G \, m_1 \, m_2}{r^2} \). In this equation, \( F \) is the gravitational force, \( G \) is the gravitational constant, \( m_1 \) and \( m_2 \) are the masses, and \( r \) is the distance between the centers of the two masses.
Through this understanding, we can see why gravitational force is crucial in exploring the interactions between planets, stars, and galaxies. But it also affects objects on Earth, like when you drop a book and watch it fall to the floor.

A hollow sphere is a three-dimensional shape that is empty inside, like a beach ball or a shell. According to the shell theorem, a hollow sphere exerts no gravitational force on an object located inside it. This happens because the gravitational forces from all parts of the shell cancel each other out.
Imagine standing inside a hollow sphere. Every small portion of the sphere's shell pulls you equally in different directions. Because of these equal pulls, the net effect is zero, and thus, you experience no gravitational force from the sphere itself. This is an interesting result of how geometry and gravity interact with empty spaces.

Net force refers to the total force acting on an object when all individual forces are considered. It’s like summing up all the pushes and pulls an object experiences. If two people are pushing a box from opposite sides with equal strength, the net force is zero, and the box stays put.
In our scenario with the marble inside the hollow sphere on Earth's surface, the gravitational force from the hollow sphere is zero because of the shell theorem. However, Earth’s gravity is another force impacting the marble. These forces must be combined to determine the net force on the marble. Here, the net force isn't zero because Earth's gravitational pull remains active. This is why the marble still experiences motion or a tendency to move when inside the hollow sphere.

Earth's gravity plays a significant role in the forces we experience daily. It pulls objects towards the center of the Earth and is responsible for what we commonly refer to as "weight." The force of gravity is approximately \( 9.81 \, \text{m/s}^2 \) near the Earth's surface. This means for every kilogram of mass, Earth pulls with a force of 9.81 Newtons.
In the given exercise, even when the marble is inside the hollow sphere, Earth's gravity continues to act on it. This gravitational force doesn't stop or change because the marble is within another object. It is Earth’s gravity that ensures the marble has a non-zero net force and is pulled towards the ground, irrespective of the hollow sphere’s presence.