gravity
Summary
Gravity is the attractive force between objects with mass, proportional to the product of the masses and inversely proportional to the square of the distance between their centers. Near Earth's surface, freely falling objects share the same acceleration, g ≈ 9.8 m/s², when air resistance is negligible.
Detail
Meaning: Gravity is the attractive force between objects with mass. The two objects pull on each other with equal and opposite forces (Newton's third law). Near Earth's surface the gravitational force on an object is its weight, mg. Because the object's mass cancels in F = ma, freely falling objects share the same acceleration, g ≈ 9.8 m/s², when air resistance is negligible.
Equation: F = Gm₁m₂/r², where F is force (N), G = 6.67 × 10⁻¹¹ N·m²/kg², m₁ and m₂ are masses (kg), and r is the distance between centers (m). At a planet's surface, g = GM/r², where M is the planet's mass (kg) and g is in m/s².
On the MCAT: Expect inverse-square ratios: doubling r cuts F to one-fourth. In orbit passages, gravity supplies the centripetal force, so GMm/r² = mv²/r. It also appears in projectile motion and inclined planes, where mg sinθ acts along the slope.
Common confusion: Mass is not weight: mass stays constant, weight changes with g. Orbiting astronauts float because they are in free fall, not because gravity is absent.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- College Physics (Serway, Vuille)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.