It's funnny this question should come up- I've been asked it by several people the past few months. Here's the answer I give them.
Let's not look at this situation with planetary motion and large scale numbers. Pretend you are on a train, travelling 100 km/hour down a track. The train is moving quite smoothly. To pass the time, you toss a ball between you and your friend. Now, when you toss the ball to your friend, sitting in front of you, the ball might be travelling 30 km/hour. When he throws it back to you, it travels 30 km/hour. The question is, does the fact that the train is travelling at 100 km/hour affect this? Simply, no. Since its "frame of motion" is based on the train and not the ground, it doesn't matter how fast the train is travelling. To the people, the air, and the objects in the train, the ball is moving 30km/hour relative to them.
What throws off a lot of people is the idea of perspective. To the people in the train, the ball is travelling 30km/hour. But to a person standing outside the train, the ball would actually be travelling 130km/hour, or 70km/hour, depending on the direction that the ball is thrown (forwards or backwards).
Let's tie this back to the aircraft. If a plane takes off from a runway, it is travelling 185kts based on ground speed. At this point, we will assume that the air is still and that there is no wind. Just like on the train, everything on the earth is moving at a constant speed. Therefore, we don't percieve motion. So even though the earth, me, and the plane are all travelling at 700mph in planetary terms, to me on the ground, everything is motionless, except the plane travelling at 185kts.
I'll try to give you one more example. When you are driving down the highway, and another car is in the lane next to you, passing you, it looks like it is slowly driving past you. But in reality, you are travelling at 100km/hour, and he is travelling at 110km/hour. To a person on the side of the road, you both zoom by. But to you in the car, he's only going 10 km/hour more than you are! Now imagine the earth spinning at 700mph. The plane is travelling 700+185kts in planetary terms, but to us on the earth, we can only percieve the 185kts.
Now to really bend your mind, consider this. If a train is travelling as fast as a speeding bullet (a bullet train, if you will
) and someone fires a gun off the back of the train, the bullet's speed and the trains speed would cancel each other out, leaving the bullet to fall to the ground harmlessly.
Hope this clarifies the whole situation!
Someday, we'll look back at this, laugh nervously, and then change the subject.