What should I know?

1. Newton said that all masses attract all other masses in the universe through the force of gravitation.

2. This force is directly proportional to the product of the masses and inversely proportional to the distance between them squared.

3. Gravitation is the weakest of the fundamental forces over short distances, but the strongest over long distances.

4. The size of the gravitational force is directly proportional to the product of the masses involved and inversely proportional to the square of the distance between them.

5. For objects that are large in extent or near a source of strong gravitational force, the effects of the differing gravitational pulls on different parts of the object -- i.e., tidal forces -- must be taken into account.

6. On the surface of the Earth, gravitational effects between smaller masses are completely overwhelmed by the much larger gravitational force applied by the Earth itself.

7. The Earth’s gravitational effect on the motion of objects can only affect the vertical component of that motion. This is a review of the concept of free fall.

8. The horizontal motion of a real-world projectile is only affected by air resistance. Under ideal conditions, the horizontal motion of the a projectile is constant.

9. Projectiles launched from the Earth’s surface follow a parabolic trajectory.

10. Orbiting objects are pulled toward the focus of an elliptical path by the gravity of the object being orbited -- in essence, they are constantly falling. They avoid colliding with the object being orbited because their horizontal motion is great enough to cause them to miss.

11. If the horizontal speed is increased past a certain amount called the escape velocity -- about 11 200 m/s on Earth -- the object will escape the gravitational domination of the planet it is on.

12. Orbiting objects follow an elliptical path, with the object being orbited at one focus. This is KEPLER'S FIRST LAW OF PLANETARY MOTION.

13. A line that is drawn between the orbiting object and the object being orbited sweeps out equal areas in equal times. This is KEPLER'S SECOND LAW OF PLANETARY MOTION.

14. The square of the orbital period is directly proportional to the cube of the orbital radius. This is KEPLER'S THIRD LAW OF PLANETARY MOTION.

15. The force of gravity determines the shape of large objects and helps to determine their motions with respect to the masses around them.

16. Stars exist in a state of hydrostatic equilibrium, a balance between gravitational forces pulling inward and pressure pushing outward. This explains both why they are spherical and why they produce the core temperatures and pressures required to initiate fusion of light elements.

17. When fusion ceases in the inner layers of a star, inward gravitational forces exceed the outward pressure, causing a collapse, which can lead to the formation of a white dwarf, a neutron star or black hole.

18. Einstein's relativity theory proposes dispensing with gravity as a force in favor of explaining the motions usually associated with gravitation with a curvature of spacetime itself.

19. Our current lack of understanding of the cause of gravitational forces or the cause of the curvature of spacetime is one of the main areas of focus of modern physics.

    • Quantum mechanics predicts that if gravity is a fundamental force, it should be mediated by a particle called the graviton. However, gravitons have never been detected, and a complete theory that successfully unifies quantum mechanics with gravity does not yet exist.

    • Dark matter is an unseen form of matter inferred from its gravitational effects on galaxies and large-scale structure in the universe. Although its presence is strongly supported by astronomical observations, its composition and particle nature remain unknown.

    • Dark energy is a form of energy proposed to explain the observed accelerated expansion of the universe. While measurements clearly show that cosmic expansion is speeding up, the underlying cause and physical nature of dark energy are still not understood.