A gravitational field is the model physicists use to describe how a mass influences the space around it, defined as the gravitational force per unit mass (g = Fg/m) at a point. Near Earth's surface it points downward with magnitude about 10 N/kg, which is why objects accelerate at g in free fall.
A gravitational field is how AP Physics 1 explains action at a distance. Earth doesn't have to touch a ball to pull on it. Instead, Earth's mass creates a field everywhere in the space around it, and any other mass placed in that field feels a force. The field strength at a point is defined as force per unit mass, g = Fg/m, with units of N/kg. Drop a bowling ball or a marble at the same spot and the forces differ, but the field is the same. The field belongs to the planet, not to the object you put in it.
This matters because the field is a long-range force, unlike contact forces such as normal force or friction, which are macroscopic effects of interatomic electric interactions (2.2.A). The field's value depends on the source mass and your distance from it, so g is not a universal constant. It's roughly 10 N/kg at Earth's surface, weaker at high altitude, and completely different on the Moon or Mars. Numerically, the field strength in N/kg equals the free-fall acceleration in m/s², which is why the same symbol g does double duty.
The gravitational field anchors Topic 2.2 in Unit 2 (Force and Translational Dynamics), where 2.2.A asks you to describe forces as interactions between objects and 2.2.B has you draw the gravitational force on every free-body diagram you'll ever make. It carries straight into Unit 3, where Topic 3.4 covers how g changes on different planets and Topic 3.3 ties the field to gravitational potential energy. Per 3.3.A, a two-object system interacting through gravity (a conservative force) stores potential energy, and the familiar ΔU = mgΔh only works because g is approximately constant near a planet's surface. The field also hides inside oscillation problems, since a pendulum's period depends on g. Almost every mechanics problem on the exam starts with mg on a diagram, so this concept is load-bearing for the whole course.
Keep studying AP Physics 1 Unit 2
Force of Gravity (Unit 2)
The field and the force are two sides of one equation, Fg = mg. The field g describes the planet's influence at a point, and the force is what an object of mass m actually feels when it sits there. On a free-body diagram (2.2.B), you draw the force mg, not the field itself.
Gravitational Potential Energy (Unit 3)
Gravity is a conservative force, so a system of an object plus Earth stores potential energy that depends on position in the field (3.3.A). When you use conservation of mechanical energy (3.4.B), the field is what converts height into stored energy and back into kinetic energy.
Period of Simple Harmonic Oscillators (Topic 6.1)
A pendulum's period depends on the local value of g. Take the same pendulum to the Moon, where the field is weaker, and it swings more slowly. This is a classic exam move that connects a Unit 2 idea to oscillation problems.
Gravitational Mass vs. Inertial Mass (Unit 2)
Gravitational mass measures how strongly the field pulls on an object, while inertial mass measures how much it resists acceleration. Experimentally they're equal, and that's exactly why all objects in free fall accelerate at the same rate g regardless of their mass.
Multiple-choice questions love testing whether you treat g as a variable instead of a magic number. Expect stems like "a planet has twice Earth's mass and twice its radius, what is g at its surface?" along with free-body diagram questions where the gravitational force mg must appear correctly. On FRQs, the field usually works behind the scenes. The 2024 Short FRQ Q4 gave a simple pendulum pulled to a small angle, and reasoning about its motion and period requires understanding that the gravitational field provides the restoring force. You also need g whenever you set up energy conservation with mgΔh or justify why an object's free-fall acceleration doesn't depend on its mass.
These share the symbol g and the same numerical value, but they describe different things. The gravitational field (N/kg) is force per unit mass and exists at a point whether or not anything is there to feel it. The acceleration due to gravity (m/s²) is what a freely falling object actually does in that field. An object sitting on a table is in Earth's field and feels the force mg, but its acceleration is zero because the normal force balances gravity. Field is always present; free-fall acceleration only shows up when gravity is the net force.
The gravitational field is the force per unit mass at a point in space, g = Fg/m, measured in N/kg, and it describes a planet's influence independent of any object placed in it.
The field belongs to the source mass, so g depends on the planet's mass and your distance from its center, which is why g differs on the Moon, Mars, and at high altitudes (Topic 3.4).
Field strength in N/kg and free-fall acceleration in m/s² are numerically equal, but the field exists everywhere while free-fall acceleration only describes objects whose net force is gravity.
Gravity is a conservative force, so position in a gravitational field gives a system potential energy, and near a surface that's the ΔU = mgΔh you use in energy conservation (3.3.A, 3.4.B).
On every free-body diagram, gravity appears as a single force mg drawn from the center of mass pointing toward the planet's center (2.2.B).
A pendulum's period depends on the local field strength g, which links this Unit 2 concept directly to simple harmonic motion problems.
It's the model for how a mass influences the space around it, defined as gravitational force per unit mass (g = Fg/m) with units of N/kg. Near Earth's surface it points downward with magnitude about 10 N/kg, and it explains why Earth can pull on objects without touching them.
No. The field g exists at a point in space whether or not an object is there, while the force Fg = mg only exists when an object of mass m sits in that field. A 2 kg rock and a 10 kg rock at the same spot feel different forces but experience the same field.
No. Astronauts in orbit float because they're in continuous free fall, not because the field vanishes. Earth's gravitational field at orbital altitudes is still close to its surface value; it just weakens gradually with distance from Earth's center.
They have the same numerical value but different meanings. The field (N/kg) describes force per unit mass at a point, while acceleration due to gravity (m/s²) describes the motion of an object in free fall. A book resting on a table is in Earth's field but has zero acceleration.
Field strength depends on the planet's mass and radius. A more massive planet creates a stronger field, while a larger radius puts you farther from the center and weakens it. Topic 3.4 tests this directly with problems comparing g across planets.