The force of gravity is the universal attraction between any two objects possessing mass, keeping our feet firmly planted on the ground. While we experience its full strength on the Earth’s surface, the question of what happens to gravity as one travels deep inside the planet, toward its heart, is a fascinating scientific puzzle. The answer requires understanding how the distribution of mass affects the total gravitational pull.
Gravity on the Earth’s Surface
The gravity we experience is the result of the entire planet’s mass pulling us toward its center. At the Earth’s surface, the acceleration due to gravity is approximately \(9.8\) meters per second squared, commonly referred to as one \(g\). For calculations at this distance, scientists treat the entire mass of the Earth as if it were concentrated into a single point at the planet’s geometric center.
The strength of this force depends on the total mass of the Earth and the distance of an object from the planet’s central point. Because gravity weakens rapidly with distance, an object just outside the atmosphere experiences a reduced pull compared to one on the surface. However, once an object begins to penetrate the surface and travel downward, the physics undergo a profound change, requiring the simple relationship between mass, distance, and force to be re-evaluated.
The Principle of Effective Mass
When an object descends below the Earth’s surface, the total mass of the planet is distributed both below and above the object. This introduces the concept of effective mass, which explains how the gravitational force changes inside a massive sphere. The gravitational pull on the object is only determined by the mass that is closer to the Earth’s center than the object’s current position.
The mass of the Earth that exists in a hollow spherical shell outside the object’s position exerts no net gravitational force upon it. This counterintuitive effect occurs because the gravitational pulls from all directions of the outer shell cancel each other out perfectly. The mass directly above pulls it upward, while the mass on the opposite side pulls it downward, resulting in a perfect vector cancellation. Consequently, as the object moves deeper into the Earth, the total amount of mass effectively contributing to the gravitational pull steadily decreases.
The Journey Downward
As an object travels from the surface toward the core, the decrease in effective mass dictates the steady reduction of gravitational force. With every kilometer traveled inward, a new layer of mass is left behind in the non-contributing outer shell. This means the gravitational acceleration is only dependent on the radius of the sphere of mass currently beneath the object.
In a theoretical scenario where the Earth had a uniform density, the gravitational force would decrease linearly with depth, dropping to half its surface value halfway to the center. However, the real Earth is not uniform; its core is dramatically denser than its outer layers. Due to the concentration of dense material in the core, the gravitational acceleration actually increases slightly through the crust and mantle before beginning its significant decline. Regardless of these variations, the overall trend is a steady weakening of the net gravitational force as the mass pulling the object becomes progressively smaller.
Zero Gravity at the Center
The journey inward culminates at the Earth’s exact center, where the gravitational force reaches its absolute minimum: zero. At this point, the entire mass of the planet surrounds the object equally in all directions. Since there is no mass beneath the object, the entire planet acts as the non-contributing outer shell.
The immense gravitational pulls from all surrounding rock and metal are perfectly balanced, resulting in a net gravitational acceleration of zero. An object placed at this precise location would float in a state of weightlessness. This state occurs because the forces are exerted symmetrically, similar to the experience of being in deep space, even though the object is surrounded by the entire mass of the Earth.