Astronomy Chapter 4: Gravity Flashcards

Flashcards covering Newton's Universal Law of Gravitation, the physics of orbits, escape velocity, and the mechanics of tidal forces as presented in Chapter 4.

Mass

The measure of the amount of material in an object, which never changes.

Weight

The gravitational force acting on an object attracted by a planet, which changes depending on which planet the object is on.

Gravity

The mutually attractive force between objects that have mass.

Gravitational acceleration ($g$)

The constant rate at which all objects fall near the surface of Earth, approximately equal to $9.8\,m/s^2$.

Universal Law of Gravitation

A law stating that gravity is an attractive force proportional to the product of the masses ($m_1 \times m_2$) and inversely proportional to the square of the distance ($r^2$) between them.

Universal Gravitational Constant ($G$)

The constant that determines the strength of gravity between all pairs of objects, valued at $6.67 \times 10^{-11}\,m^3/(kg\,s^2)$.

Inverse Square Law

A principle stating that the force of gravity diminishes with the square of the distance between two objects.

Self-gravity

The mutual gravitational attraction that occurs among all parts of the same object, which pulls everything toward the center of mass.

Orbit

The path of one object that freely falls around another object.

Weightlessness

The state of being in free fall, where gravity still acts on the person but the ground is no longer pushing up on them.

Center of Mass

The common point around which two objects in a system orbit each other.

Universality

The principle that the laws of physics, including gravitational acceleration, are the same everywhere and at all times.

Uniform Circular Motion

Movement along a circular path at a constant speed.

Circular Velocity ($v_{circ}$)

The particular speed required for a satellite to maintain a perfect circular orbit, mathematically expressed as $v_{circ} = \sqrt{\frac{GM}{r}}$.

Sputnik 1

The first artificial object launched by the Soviet Union in 1957 to orbit the Earth.

Escape Velocity ($v_{esc}$)

The velocity an object must achieve to leave a planet's surface and enter an unbound orbit; for Earth, this is approximately $11.2\,km/s$.

Bound Orbit

An orbital path, such as a circle or an ellipse, where an object remains gravitationally trapped by another body.

Unbound Orbit

An orbital path, such as a parabola or hyperbola, where an object has enough speed to escape a system.

Tidal Force

The difference between the actual gravitational force at a specific point and the average force felt by the entire body, causing the object to stretch.

Spring Tides

Extreme tides that occur when the Sun and Moon are aligned with Earth.

Neap Tides

Lower than normal high tides that occur when the Moon, Earth, and Sun are at right angles (quarter phases).

Tidal Locking

A synchronization process where an object's rotation period becomes equal to its orbital period due to tidal friction.

Spin-orbit Resonance

A relationship between an object's rotation and its orbit, such as the Moon's $1:1$ resonance or Mercury's $3:2$ resonance.

Roche Limit

The distance within which tidal forces from a massive object can destabilize or break up another object.