Gravitational Field Equations to Know for AP Physics 1 (2025)

Gravitational field equations explain how masses attract each other and how this affects motion in space. Understanding these concepts is crucial for solving problems in AP Physics 1 and AP Physics C: Mechanics, especially regarding orbits and energy.

1. Newton's Law of Universal Gravitation: F = G(m1m2)/r^2

   • Describes the attractive force between two masses.
   • G is the gravitational constant (6.674 × 10^-11 N(m/kg)^2).
   • The force is inversely proportional to the square of the distance between the centers of the two masses.

2. Gravitational Field Strength: g = GM/r^2

   • Defines the gravitational force per unit mass at a distance r from a mass M.
   • Indicates how strong the gravitational pull is at a specific point in space.
   • The field strength decreases with the square of the distance from the mass.

3. Gravitational Potential Energy: U = -GMm/r

   • Represents the work done to bring a mass m from infinity to a distance r from mass M.
   • The negative sign indicates that energy is released when masses come together.
   • Potential energy becomes less negative (increases) as the distance increases.

4. Escape Velocity: v_escape = √(2GM/r)

   • The minimum velocity needed for an object to break free from a gravitational field without further propulsion.
   • Depends on the mass of the celestial body and the distance from its center.
   • Higher mass or smaller radius results in a higher escape velocity.

5. Orbital Velocity: v_orbit = √(GM/r)

   • The velocity required for an object to maintain a stable orbit around a mass M.
   • Directly related to the mass of the central body and inversely related to the radius of the orbit.
   • Essential for understanding satellite motion and planetary orbits.

6. Kepler's Third Law: T^2 ∝ r^3

   • Relates the square of the orbital period (T) of a planet to the cube of the semi-major axis (r) of its orbit.
   • Indicates that more distant planets take longer to orbit the sun.
   • Provides a foundation for understanding planetary motion and distances in the solar system.

7. Gravitational Potential: V = -GM/r

   • Describes the potential energy per unit mass at a distance r from mass M.
   • Like gravitational potential energy, it is negative, indicating a bound system.
   • Useful for calculating energy changes in gravitational fields.

8. Acceleration due to gravity at Earth's surface: g ≈ 9.8 m/s^2

   • Represents the acceleration experienced by an object in free fall near the Earth's surface.
   • Varies slightly with altitude and geographical location.
   • Fundamental for solving problems involving motion under gravity.

9. Gravitational Field for a spherical shell: g = 0 (inside), g = GM/r^2 (outside)

   • Inside a uniform spherical shell, the gravitational field is zero.
   • Outside the shell, the gravitational field behaves as if all mass were concentrated at the center.
   • Important for understanding gravitational effects of planets and stars.

10. Superposition principle for gravitational fields

• States that the total gravitational field at a point is the vector sum of the fields due to individual masses.
• Allows for the analysis of complex systems with multiple masses.
• Essential for solving problems involving multiple gravitational sources.