Acceleration due to gravity (g) is the acceleration of any object in free fall near a planet's surface, about 9.8 m/s² on Earth, directed toward the planet's center. It equals the gravitational field strength (N/kg) and is the same for every object regardless of mass.
Acceleration due to gravity, written as g, is how fast an object's velocity changes when gravity is the only force acting on it. Near Earth's surface, g ≈ 9.8 m/s², pointing straight down. Drop a bowling ball and a marble at the same time (no air resistance) and they hit the ground together, because g doesn't depend on the falling object's mass. The gravitational force on an object is mg, but when you apply Newton's second law, that mass cancels. Bigger mass means bigger force AND more inertia, so the acceleration comes out the same for everything.
Here's the deeper idea the revised CED pushes hard. The number 9.8 isn't really a property of falling objects at all. It's a property of Earth. It's the strength of Earth's gravitational field at the surface, measured in N/kg, which is the exact same unit as m/s². On a different planet, g is a different number, set by that planet's mass and radius. So g shows up in two costumes on the exam, as a field strength (force per kilogram) in dynamics problems and as a free-fall acceleration in kinematics problems. They're the same quantity.
g is one of the few quantities that touches almost every unit of AP Physics 1. In Unit 1 it's the constant acceleration you plug into the kinematics equations for free fall and projectiles (LO 1.2.B defines acceleration as Δv/Δt, and g is the cleanest real-world example of a constant one). In Unit 2, Topic 2.2 reframes g as the gravitational field, and your free-body diagrams (LO 2.2.B) almost always include the weight vector mg. In Unit 3, g lives inside gravitational potential energy, ΔUg = mgΔy, which powers every conservation-of-energy argument (LOs 3.4.B and 3.4.C). And in Unit 6, the period of a simple pendulum depends on g, which is exactly what the 2024 short FRQ tested. If you're shaky on g, you're shaky on a third of the course.
Keep studying AP Physics 1 Unit 2
Free Fall and Projectile Motion (Unit 1)
Free fall is just kinematics with a = g, and a projectile is free fall with sideways motion bolted on. The horizontal velocity never changes because g only acts vertically. Every projectile problem secretly splits into a constant-velocity problem and a free-fall problem sharing the same clock.
The Gravitational Field (Unit 2)
Topic 2.2 reveals what g actually is. It's Earth's gravitational field strength, the force per kilogram that Earth exerts on anything nearby. That's why weight is mg on a free-body diagram, and why g changes if you move to a different planet or far above the surface.
Gravitational Potential Energy (Unit 3)
Gravity is a conservative force, so it gets a potential energy, ΔUg = mgΔy. The g in that equation is the same 9.8 m/s². When an FRQ asks you to use conservation of mechanical energy for a dropped or launched object, g is doing the energy bookkeeping.
Period of Simple Harmonic Oscillators (Unit 6)
A simple pendulum's period depends on length and g, not on the mass of the bob. Stronger gravity means a faster swing. This is the classic way the exam tests whether you understand g as a planetary property; the same pendulum swings slower on the Moon.
Expect g everywhere, usually without being named. MCQs love conceptual traps, like asking whether a heavy and light object dropped together land at the same time, or comparing g on a planet with twice Earth's mass and twice its radius. You'll plug g into kinematics equations for free fall, draw mg on free-body diagrams, and use mgΔy in energy conservation. The 2024 Short FRQ Q4 gave a simple pendulum pulled to a small angle, where the period's dependence on g is the whole point. A favorite experimental-design question asks you to measure g using a pendulum or a dropped object, so know that plotting the right variables (like T² vs. L) lets you extract g from a slope. And remember the m/s² vs. N/kg equivalence; the exam expects you to treat field strength and free-fall acceleration as the same quantity.
Lowercase g (≈ 9.8 m/s²) is local. It's the gravitational field strength at a particular place, like Earth's surface, and it changes from planet to planet and with altitude. Uppercase G (6.67 × 10⁻¹¹ N·m²/kg²) is universal. It's the same constant everywhere in the universe and appears in Newton's law of gravitation. They're related, since g near a planet's surface comes from G times the planet's mass divided by its radius squared. Mixing them up in an equation is an instant lost point.
Acceleration due to gravity (g) is about 9.8 m/s² near Earth's surface, points toward Earth's center, and is the same for every object regardless of mass.
g is really the gravitational field strength in N/kg, which means it's a property of the planet, not of the falling object, and it's different on different planets.
All objects in free fall accelerate at g because the gravitational force mg and the inertia m cancel in Newton's second law.
g threads through the whole course, appearing in free-fall kinematics (Unit 1), weight mg on free-body diagrams (Unit 2), gravitational potential energy mgΔy (Unit 3), and the pendulum period (Unit 6).
Don't confuse lowercase g (local field strength, varies by location) with uppercase G (the universal gravitational constant, 6.67 × 10⁻¹¹, same everywhere).
It's the acceleration of any object in free fall near a planet's surface, approximately 9.8 m/s² on Earth, directed downward. The revised CED also defines it as the gravitational field strength in N/kg, which is the same quantity in different units.
No. Without air resistance, all objects fall at the same acceleration g, because the larger gravitational force on a heavier object is exactly canceled by its larger inertia. With air resistance things change (that's where terminal velocity comes in), but in ideal free fall, mass doesn't matter.
Lowercase g (≈ 9.8 m/s²) is the local gravitational field strength and changes depending on the planet and your distance from it. Uppercase G (6.67 × 10⁻¹¹ N·m²/kg²) is a universal constant that's the same everywhere. You calculate a planet's g using G, the planet's mass, and its radius.
g itself is a magnitude, about 9.8 m/s². Whether you write +9.8 or -9.8 in an equation depends on which direction you chose as positive in your coordinate system. Pick a convention at the start of a problem and stay consistent; that's where most sign errors come from.
No, and the exam loves testing this. g depends on the planet's mass and radius, so a planet with twice Earth's mass and twice its radius has g of only 4.9 m/s² (half of Earth's), since radius is squared in the denominator. A pendulum on that planet would swing with a longer period.