Created by falling objects. Depends on two factors: 

1. the greater the frontal area, the greater the air drag. 

2. the speed of the falling object. (The greater the speed, the greater the air drag because the amount of air molecules that the object has to plow through per second is greater. 

Newton's 1st Law of Motion

(Law of Inertia)

Every material object continues in its state of rest or of uniform motion in a straight line unless it is compelled to change its state by forces impressed upon it. 

Any object at rest will stay at rest. Objects in motion will stay in motion. 

1. No forces are acting on an object. 

2. Forces are equal and opposite and cancel each other out. 

This is part of Newton's First Law.  It only can occur when the net external forces equal zero. 

The acceleration of an object is directly proportional to the net force acting on the object in the direction of the net force and it is inversely proportional to the mass of an object. 

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. 

"To every action there is an equal and opposite reaction"--example is like when shooting a gun. Bullet is tiny and can travel far. The guns is bigger, so doesn't go (back) as far. 

Every mass attracts every other mass with a force that, for any two masses, is directly proportional to the product of the masses involved and inversely proportional to the square of the distance separating them. 

Force~mass 1 x mass 2

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Distance ²