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Introduction > Kepler's Laws of Planetary Motion > Newton's Laws of Motion and Gravity > Spaceflight > Identifying Orbits > Lagrange Points > Orbital Elements > Orbit Terminology > Orbit Info and Resources

Understanding the Terminology used in Orbits

Acceleration means a change in motion, usually an increase in speed or a change in direction.

Action is the result of a force, when a cannon fires the cannonball flies. The movement of a cannonball is an action. And conversely, the firing of the cannonball causes the the cannon to recoil. That is called a reaction.


Force is a push or pull exerted on an object through interaction with another body. The interaction can stop a moving object, move an object from rest, or change the direction of its motion. The force has magnitude (or speed) and direction therefore it is a vector quantity.

Centripetal force causes a body to move in a curved path rather than continuing in a straight line. It is directed toward the center that the body is circling. This center seeking force is found in atoms as well as orbits. In orbits, gravity is the force that causes centripetal motion.

Centrifugal force is not a real force, it is caused by the object's own inertia. The body's inertia causes it to resist the centripetal force. Centrifugal force is only used in a rotating frame of reference.

The standard unit of force is the newton, that is the force required to accelerate a 1-kg mass 1 m/sec2 (one meter per second per second. A dyne is the force required to accelerate a 1-g mass 1 cm/s2.

Mass is a measurement of the amount of matter in an object. Mass is a combination of the total number of atoms, the density of the atoms, and the type of atoms in an object.

The base unit of mass is the kilogram. It was originally defined in 1795 as the mass of a liter of water, but that was hard to replicate. In 1879, a cylinder of platinum-iridium, the International Prototype of the Kilogram (IPK), became the standard of the unit of mass for the metric system. Even though the cylinder was kept in a controlled environment, evidence accumulated that the mass of the IPK had been slowly changing. Several competing efforts came about until finally it culminated with a definition in terms of a physical constant. The kilogram is now defined in terms of the Planck constant. The new definition was approved by the General Conference on Weights and Measures on November 16, 2018.

Momentum is the amount of resistance an object in motion has to changes in its speed or direction of motion. Momentum is the result of combining an object’s mass and its velocity, therefore momentum = mass x velocity. (Usually written p = m X v). There are two types of momentum, linear and angular.

Rest and motion as are used are both relative. In other words, an object is at rest or in motion relative to its surroundings. This means an astronaut sitting in a spacecraft is at rest.

Velocity is a vector, it has both magnitude (or speed) and direction.

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Last update: August 2023