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Astronomy Cheat Sheet (DRAFT) by

This is a draft cheat sheet. It is a work in progress and is not finished yet.

How do we describe motion?

the rate at which an object is moving
speed in a certain direction
a change in velocity, meaning a change in either speed or direction.
mass times velocity
can change an object’s momentum, causing it to accelerate

Mass vs. Weight

How is mass different from weight?
An object’s mass is the same no matter where it is located, but its weight varies with the strength of gravity or other forces acting on the object.
When is an object weightless?
An object is weightless when it is in free-fall even those the mass is unchanged

How did Newton change our view of the universe?

Newton showed that the same physical laws that operate on Earth also operate in the heavens, making it possible to learn about the universe by studying physical laws on Earth.

This went against Aristotle.

Newton's Laws of Motion

Newton's First Law of Motion
An object moves at a constant velocity if there is no net force acting upon it.
Newton's Second Law of Motion
Force = mass * velocity
Newton's Third Law of Motion
For any force, there is always an equal and opposite reaction force.

Why do objects move at constant velocity?

Conser­vation of momentum
an object’s momentum cannot change unless the object transfers momentum to or from other objects
When no force is present, no momentum can be transf­erred so an object must maintain its speed and direction.

What keeps a planet rotating and orbiting the Sun?

Conser­vation of angular momentum
a planet's rotation and orbit cannot change unless the planet transfers angular moment to another object
The planets in our solar system do not exchange substa­ntial angular momentum with each other or anything else, so their orbits and rotation rates remain steady.

Basic Energy

kinetic energy
energy of motion
radiative energy
energy carried by light
potential energy
stored energy
Energy is always conser­ved—it can be neither created nor destroyed. Objects received whatever energy they now have from exchanges of energy with other objects.

What determines the strength of gravity?

Universal law of gravit­ation
states that every object attracts every other object with a gravit­ational force that is propor­tional to the product of the objects’ masses and declines with the square of the distance between their centers

Newton’s and Kepler’s laws

1) Newton showed that any object going around another object will obey Kepler’s first two laws.
2) He showed that ellipses (or circles), which define bound orbits, are not the only possible orbital shape—­orbits can also be unbound and in the form of parabolas or hyperb­olas.
3) He showed that two objects actually orbit their common center of mass.
4) Newton’s version of Kepler’s third law allows us to calculate the masses of orbiting objects from their orbital periods and distances.