# Centripetal Forces And Helicopters

LuGoBi

## Main Question or Discussion Point

I have a helicopter flying, but it is at rest in relation to the Earth, i.e., it's just hovering. That means the aerodynamical force equals minus the weight, so the forces cancel out, and the helicopter doesn't fall. But now, since the weight was cancelled, I can't say there's a centripetal force acting on the chopper and instead of rotating with the Earth, it should go off on a tangent. But that doesn't happen in reality. I wonder what's wrong with the reasoning.

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Just because the forces add up to be zero doesn't mean that gravity doesn't pull on the helicopter. There is always a radial force between the Earth and the helicopter.

If there was no gravity the helicopter would just continue it's ascent radially outward of the Earth. Meanwhile the Earth would be spinning below the helicopter. Which simply isn't the case.

If we go to some point in space and observed the helicopter, it would be moving in a circle at a decently fast speed, in according to the Earth's spin.

LuGoBi
Just because the forces add up to be zero doesn't mean that gravity doesn't pull on the helicopter. There is always a radial force between the Earth and the helicopter.
Yes, but the net force is zero, right? So how come it accelerates centripetally?

Imagine I have this helicopter over the Earth, and then I bring in another Earth, which is rotating in the opposite direction and put it in a way so the helicopter is equidistant to both "Earths". Then both planets would pull on the helicopter, the net force would be zero again, and what would happen? Would it rotate with one of the Earths or would it stand still? There's no reason why it would go along with one Earth and not the other, so it would probably stand still. And what's the difference between this case and the other one? The nature of the force, which in one case is an aerodynamical force and in the other is a gravitational force.

LuGoBi
Sorry, what I just said is wrong. I didn't take into account the inital sideway velocity of the helicopter. But I still have doubts about this.

rcgldr
Homework Helper
I have a helicopter flying, but it is at rest in relation to the Earth, i.e., it's just hovering. That means the aerodynamical force equals minus the weight ...
Technically, (aerodynamic force + centripetal force) + gravitational force = 0. The helipcopter moves with the air, and assuming the air is still relative to the earth's surface, then the helicopter moves in a very low circular path as the earth and air rotate.

You could make things even more complicated by including effects from the moon, the earths orbit around the sun, the sun's orbit around the Milky Way galaxy, ...