driftrider

In the vertical plane while the bullet is moving upward away from the center of the Earth, yes, technically.

But a better way of thinking about gravity is that it is a constant force that acts to change the direction of a projectile in flight by accellerating it toward the ground. Drag, on the other hand, varies based on a number of variables, and effects a projectile no regardless of the projectiles relative velocity to the ground.

This means that if a fighter jet were to fire its cannon straight down toward the ground, gravity would provide an additional accellerative force that would tend to make the bullet fall faster. However, the force of drag created as the bullet passes through the air is of a much greater magnitude that the accellerative force of gravity and would cause the bullet to slow down considerably before the bullet hit the ground. Now, if we were to assume that the bullet was fired from an extremely high altitude, and that the density of the atmosphere was constant (which it's not, but this is for simplification and visualization purposes only), the bullet would eventually slow down to the point were the accelerative force of gravity exactly equaled the force of aerodnamic drag, and the bullet would settle into an equilibrium, or terminal, velocity.

rwilson

Wow! OK, That means that gravity's affect diminishes over time until it becomes negligible and air drag becomes the controlling factor?

driftrider

No, Gravity's effect is (almost) constant. It accellerates any body near the Earth toward the planet at 9.8m/s^2, all the time no matter what.

The air resistance (drag) is the variable force, which provided the shape and orientation of the object doesn't change relative to the airflow, increases exponentially with the velocity of the projectile. In other words, the faster the bullet goes, the more resistance the bullet encounters. When the bullet is going faster than terminal velocity, the drag force the bullet is encountering is greater than the force of gravity, so the NET FORCE, causes the bullet to slow down even though it's falling toward the ground (think: parachute). However, as the bullet slows down, the drag force it encounters decreases as well, until eventually the force of gravity and the force of drag exactly match each other in equilibrium. At that point the net force on the bullet is ZERO, and the bullet continues to fall at a constant (terminal) velocity. Stated differently, in equilibrium (terminal velocity) the force of gravity and the force of drag exactly cancel each other out. Since an object in motions tends to remain in straight line motion at a constant velocity unless acted on by a net force not equal to zero, the bullet, in the absence of a net force, continues to fall at its constant terminal rate unless there is a change in the system that causes the net force to be something other than zero.

Mike