Why is the force of gravity not exactly the same everywhere on Earth's surface?

Because the force of gravity is stronger as it nears the center of the Earth and because the Earth is not completely flat, gravity is bound to change depending on your location. Ex:. Gravity is going to be less stronger on the peak of Mt. Everest than at the bottom.

Basic concepts of Geometry

Geometry

Geometry began with a practical need to measure shapes. The word geometry means to “measure the earth” and is the science of shape and size of things. 

Undefined Terms

An ‘undefined term’ is a term or word that doesn’t require further explanation or description.  It already exists in its most basic form.  These basic terms are used to define or explain more complicated terms or concepts.

Point

A point is the most important and basic concept of geometry. It is represented as a dot with a capital alphabet which is its name.

Line

 A line is a set of points and it extends in opposite directions on forever. It is represented by two points on the line and a single alphabet in the lower case.

Plane

A plane is a flat surface that extends in all directions on forever. A plane has no size and definitely no shape. However it is represented as a quadrangle and a single capital letter.

Collinear and Coplanar

Three or more points are said to be collinear if a single line contains all of them. Otherwise they are said to be non collinear.

Postulates

Postulates are statements the validity or truth of which are assumed without proof.

Theorems

In mathematics, a theorem is a statement that has been proven on the basis of previously established statements, such as other theorems, and previously accepted statements, such as axioms.

Angles

An angle is a figure formed by two rays with a common endpoint, and which are not on the same line. 

What would happen if we had no moon?

There would be no lunar tides. The earth could get hit by asteroids more often because the moon sometimes blocks their path. The earth would spin slower because the earth´s rotation is being slowed down through tidal interactions with the moon. It would´ve been awfully dark at night, obviously. There would be shorter days and a more extreme climate.