The gravitational constant, denoted as G, is referred to as a universal constant because its value is constant throughout the universe and fundamental to the law of universal gravitation. This constant quantifies the strength of the gravitational force between two objects. It was first determined by Henry Cavendish in 1798 through an experiment using a torsion balance. Cavendish measured the tiny gravitational attraction between two lead spheres, which allowed him to calculate the value of G. This experiment was significant because it not only provided a way to measure Earth's mass and density but also confirmed that gravitational force follows an inverse square law. The precision in measuring G has improved over centuries, but the method remains rooted in observing gravitational attraction and applying Newton's law of universal gravitation. The universality of G underscores the fundamental nature of gravity in the cosmos, applicable from the smallest particles to the largest structures in the universe.

The distance between two objects significantly affects the gravitational force between them, following the inverse square law. According to this law, the gravitational force between two masses is inversely proportional to the square of the distance between their centers. This means that if the distance between the two objects is doubled, the gravitational force between them becomes one-fourth as strong. Conversely, if the distance is halved, the gravitational force becomes four times stronger. This relationship is crucial in understanding how the gravitational force operates over vast distances in space. For example, it explains why the gravitational pull of the Sun on Earth is strong enough to keep it in orbit, despite the vast distance between them. The inverse square law is fundamental in celestial mechanics and affects the orbits of planets, satellites, and even the behavior of comets and asteroids within solar systems.

Gravitational field strength is a measure of the gravitational force exerted per unit mass at a point in a gravitational field, typically measured in Newtons per kilogram (N/kg). It represents the force that a small test mass would experience at that point, providing a way to quantify the intensity of the gravitational field without specifying the mass of the object experiencing the force. Gravitational force, on the other hand, refers to the actual force exerted by a gravitational field on a given mass, measured in Newtons (N). It is calculated using the formula F = mg, where m is the mass of the object and g is the gravitational field strength at the location of the mass. While gravitational field strength is a property of the gravitational field itself, dependent only on the mass creating the field and the distance from it, gravitational force is specific to the interaction between the field and a particular mass within it. This distinction is important for understanding how different objects will experience gravity in the same point in space.

Gravitational field strength cannot be negative because it is defined as the magnitude of the force per unit mass that an object would experience in a gravitational field, and force magnitude is always a positive quantity. The concept of positive and negative values in physics often relates to direction rather than magnitude. In the case of gravitational fields, the direction of the force is always towards the mass creating the field, which is conventionally considered a positive direction. However, since gravitational field strength is concerned with magnitude and not direction, it remains positive. If we were to describe the vector field of gravity, we might use negative values to indicate the direction of the force towards the mass creating the field in a coordinate system, but the field strength itself, as a measure of intensity, remains positive. This positive value reflects the attractive nature of gravity, pulling objects together rather than pushing them apart.

The gravitational field strength at the surface of a planet is directly proportional to the planet's mass and inversely proportional to the square of its radius. This relationship is encapsulated in the formula g = G(m/r^2), where g is the gravitational field strength, G is the gravitational constant, m is the mass of the planet, and r is the radius of the planet. This means that larger or more massive planets will have a stronger gravitational field strength at their surface, assuming the mass increases more significantly than the radius. Conversely, if two planets have the same mass but different radii, the planet with the smaller radius will have a stronger gravitational field at its surface because the surface is closer to the center of mass, decreasing the r^2 term in the denominator. This relationship is crucial for understanding not only the gravitational pull experienced on different planets but also the implications for atmospheric retention, surface pressure, and the potential for liquid water, which are all influenced by gravitational field strength.