The universal gravitational constant, G, is the fundamental constant (about 6.674 × 10⁻¹¹ N·m²/kg²) that sets the strength of the gravitational force between any two masses in Newton's law of universal gravitation, F = Gm₁m₂/r². It is the same everywhere in the universe.
The universal gravitational constant, written as G, is the number that converts masses and distance into an actual gravitational force. It lives inside Newton's law of universal gravitation, F = Gm₁m₂/r², and its value is roughly 6.674 × 10⁻¹¹ N·m²/kg². That tiny exponent (10⁻¹¹) is why you don't feel a gravitational pull toward your desk. Gravity is incredibly weak unless at least one of the masses is planet-sized.
The word universal is doing real work here. G has the same value on Earth, on Mars, and in a galaxy a billion light-years away. That's what separates it from g, the local acceleration due to gravity, which changes from planet to planet. When you derive the surface gravity of a planet (g = GM/r²), G is the universal ingredient and the planet's mass and radius are the local ingredients. Historically, Henry Cavendish measured G in 1798 using a torsion balance, which is why his experiment is sometimes called "weighing the Earth." Once you know G, you can solve g = GM/r² for Earth's mass.
G anchors Topic 3.4 (Gravitational Field/Acceleration Due to Gravity on Different Planets) in Unit 3: Work, Energy, and Power. The topic's learning objectives (AP Physics 1 Revised 3.4.A, 3.4.B, and 3.4.C) are about identifying the energies in a system and applying conservation of mechanical energy, and gravity is the classic conservative force those objectives lean on. Gravitational potential energy only exists because two masses interact through the force that G quantifies. On the exam, G shows up whenever a problem leaves Earth's surface: finding g on another planet, comparing weights across planets, or working with orbits. The good news is that G is printed on the AP equation sheet, so you never memorize the digits. You memorize what it means and where it goes.
Newton's Law of Universal Gravitation (Unit 3)
G is the proportionality constant in F = Gm₁m₂/r². Without G, the law would only tell you that force grows with mass and shrinks with distance squared. G turns that relationship into a number in newtons.
Acceleration Due to Gravity (Unit 3)
Set Newton's gravitation law equal to mg and the small mass cancels, leaving g = GM/r². This is the single most-tested move involving G. It explains why g is about 9.8 m/s² on Earth but different on every other planet.
Cavendish Experiment (Unit 3)
Cavendish measured G in 1798 with a torsion balance sensitive enough to detect the pull between lead spheres in a lab. Knowing G let physicists calculate the mass of the Earth for the first time.
Orbital Speed (Unit 3)
For a satellite in circular orbit, gravity supplies the centripetal force, which gives v = √(GM/r). Notice the satellite's own mass drops out. Only G, the central body's mass, and the orbital radius matter.
You will almost never be asked "what is the value of G" directly, because it's given on the equation sheet. Instead, multiple-choice questions test whether you can use it correctly in proportional reasoning. A classic stem gives you a planet with twice Earth's mass and half its radius and asks for the new surface gravity (answer: 8 times Earth's g, since g = GM/r²). Another favorite is canceling G entirely by setting up a ratio between two planets. No released FRQ has centered on G by name, but free-response questions about gravitational fields, orbits, or energy conservation in gravitational systems expect you to pull G into equations like g = GM/r² or v = √(GM/r) without being told to. The trap to avoid is writing g where the problem needs G, especially off Earth's surface where 9.8 m/s² no longer applies.
Capital G and lowercase g are completely different quantities, and mixing them up is the most common gravity mistake in AP Physics 1. G is a universal constant (6.674 × 10⁻¹¹ N·m²/kg²) that never changes anywhere in the universe. Lowercase g is a local value (9.8 m/s² near Earth's surface) that depends on which planet you're on and how far you are from its center. They're linked by g = GM/r². Think of G as the recipe's universal ingredient and g as the dish that comes out differently in every kitchen.
G is the universal gravitational constant, approximately 6.674 × 10⁻¹¹ N·m²/kg², and it has the same value everywhere in the universe.
G appears in Newton's law of universal gravitation, F = Gm₁m₂/r², where it converts masses and separation distance into a force in newtons.
Capital G is a universal constant, while lowercase g is the local acceleration due to gravity, and they connect through g = GM/r².
G's tiny size (10⁻¹¹) explains why gravity between everyday objects is unnoticeable and only matters when one mass is planet-sized.
Henry Cavendish measured G in 1798 with a torsion balance, which made it possible to calculate the mass of the Earth.
G is printed on the AP Physics 1 equation sheet, so the exam tests whether you can use it in equations like g = GM/r² and v = √(GM/r), not whether you've memorized it.
It's G, the constant in Newton's law of universal gravitation (F = Gm₁m₂/r²), with a value of about 6.674 × 10⁻¹¹ N·m²/kg². It sets how strong gravity is between any two masses and supports Topic 3.4 on gravitational fields and surface gravity on different planets.
No. Capital G is a universal constant that's identical everywhere, while lowercase g is the local free-fall acceleration that changes from planet to planet. They're related by g = GM/r², so Earth's g of 9.8 m/s² comes from plugging Earth's mass and radius into that formula with G.
No. G is provided on the AP Physics 1 equation sheet. What you do need is to recognize when an equation calls for G versus g, and how to use it in g = GM/r² or orbital speed problems.
Because it has the same value for every pair of masses anywhere in the universe, whether that's you and the Earth or two distant galaxies. The masses and distances change from situation to situation, but G never does.
Henry Cavendish measured G in 1798 using a torsion balance that detected the tiny attraction between lead spheres. His result is often called 'weighing the Earth' because once G was known, g = GM/r² could be solved for Earth's mass.
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