What do you mean by pushing on the ground? Standing on it? Doing a push-up?

 

Say i actually push down on the ground. I'm exerting force on it, right? That force goes somewhere and has to continually progress until it somehow magically stops? Say I push on a table that table exerts more force on the floor then on the joists to the foundation to the earth? OR does the force eventually just stop at say the steal beam? If I put a weight on the center of a string it bends in the middle, doesn't wood do the same? However doesn't steal remain perfectly straight until it reache the point of failure? Explain where this force goes and how if ever it stops?

 

Yes, you are pushing the earth in one direction and yourself in another. Since the earth is much bigger than you, it will be much less affected. Likewise, gravity pulls you towards the earth and pulls the earth towards you. But again, size matters in determining who does most of the moving.

 

The force has to be "reacted". This means that it has to be opposed by an opposite force, OR it means that an acceleration occurs.

If we treat the earth as a solid, rigid object (which it is for the purposes of this case), you will be accelerating yourself in one direction and the earth in the other. But gravity is constantly accelerating you both together. So if you only push hard enough to balance gravity, then nothing moves. If you push harder than that, you accelerate away from the earth (and you accelerate the earth away from you).

When you jump you move the whole earth. But you don't move the center of mass of you and the earth combined as a single system.

 

[QUOTE=exceltoexcel,Dec 8 2005, 07:20 PM] Say i actually push down on the ground. I'm exerting force on it, right?

 

[QUOTE=no_really,Dec 8 2005, 05:49 PM]ever heard of the sun?

 

Or you just didn't notice it.

 

An even more simple method is to pretend that the cabin is a sealed environment (it's not, but that doesn't factor in here). The entire mass of the cabin remains exactly the same, whether the bird is flying or not. (But the cg may move a tiny bit.) From the standpoint of the rest of the universe outside the airplane, it doesn't matter what is happening inside the airplane -- the airplane (including its contents) still has the same mass, and so therefore the airplane still has the same weight. And since lift equals weight in level flight, the lift required for the airplane does not change.

 

Pushing with 160lbs on a small sphere that's, say, resting against a building is a lot different from that force being exerted because of a gravitational attraction between two bodies.

 

Can someone please explain again how airplanes take off?