# Gravitation Overview

> AP Physics C: Mechanics · Gravitation
> Source: https://www.owlsprep.com/study/ap-physics-c-mech-u7-overview/
> Weight: 4-6% of total AP exam score

This unit explores gravitational interactions between masses, from fundamental force laws to orbital motion of planets and satellites, a core tested topic on AP Physics C: Mechanics.

**Prerequisites:** [Newton's laws of motion](https://www.owlsprep.com/study/ap-physics-c-mech-u2-newtons-laws-overview/); [Uniform circular motion](https://www.owlsprep.com/study/ap-physics-c-mech-u3-circular-motion-overview/); Conservation of energy

## Learning objectives

- Apply Newton's law of universal gravitation to calculate gravitational force, fields, and potential energy between two masses
- Calculate escape velocity for objects leaving a large gravitational body
- Apply Kepler's three laws to describe and solve problems involving planetary motion
- Derive and calculate orbital speed, period, and total energy for circular orbits

## Unit at a Glance

This unit builds on your existing knowledge of forces, energy, and circular motion to explain gravitational phenomena that govern motion across the solar system. We start with the fundamental law of universal gravitation, then extend those core concepts to describe how planets and satellites move in stable orbits around larger bodies.

The two sub-topics below follow a sequential learning path: gravitational force and potential energy form the foundation you will use to analyze orbital motion, escape velocity, and orbital energy. We recommend completing them in the order listed below.

Core sub-topics in this unit:
- [AP Physics C: Mechanics Gravitational forces](https://www.owlsprep.com/study/ap-physics-c-mech-u7-gravitational-forces/) — Covers Newton's law of universal gravitation, gravitational fields, potential energy, and escape velocity calculations.
- [AP Physics C: Mechanics Orbits of planets and satellites](https://www.owlsprep.com/study/ap-physics-c-mech-u7-orbits-of-planets-and-satellites/) — Covers Kepler's laws, circular orbital parameters, orbital energy, and basics of elliptical orbits.

## Common pitfalls

- **Wrong:** Confusing gravitational force and gravitational potential energy scaling with radius.
  - Why it fails: Gravitational force follows an inverse-square relationship, while potential energy follows an inverse relationship, which is easy to mix up.
  - Correct: Remember $F = -dU/dr$, so if $U \propto 1/r$, $F \propto 1/r^2$.
- **Wrong:** Assuming orbital speed increases with increasing orbital radius.
  - Why it fails: Many students confuse tangential speed with angular speed, leading to incorrect conclusions.
  - Correct: For circular orbits, $v = \sqrt{GM/r}$, so speed decreases as orbital radius increases.

## Cheatsheet

| Concept | Formula |
| --- | --- |
| Newton's Law of Universal Gravitation | $F_g = G \frac{m_1 m_2}{r^2}$ |
| Gravitational Potential Energy | $U_g = - G \frac{M m}{r}$ |
| Escape Velocity | $v_{esc} = \sqrt{\frac{2GM}{R}}$ |
| Circular Orbital Speed | $v_{orbit} = \sqrt{\frac{GM}{r}}$ |
| Kepler's Third Law | $T^2 = \left( \frac{4 \pi^2}{GM} \right) a^3$ |
| Total Orbital Energy (circular) | $E_{total} = - \frac{GMm}{2r}$ |

## What's next

Begin your sequential study of this unit with the first sub-topic covering fundamental gravitational forces and potential energy. Once you complete both sub-topics in this unit, you can move on to the next unit covering rotational kinematics, torque, and angular momentum, the final major topic of AP Physics C: Mechanics.

- [Gravitational forces (first sub-topic of this unit)](https://www.owlsprep.com/study/ap-physics-c-mech-u7-gravitational-forces/)
- [Orbits of planets and satellites](https://www.owlsprep.com/study/ap-physics-c-mech-u7-orbits-of-planets-and-satellites/)

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ap-physics-c-mech-u7-overview/