Responding to flat-earthers: How can a butterfly fly while oceans don't fly off?

Many flat earth believers will often say and write things like: "You are telling me that the same gravity that keeps oceans in place can't stop a butterfly from flying?" or something similar (oceans and butterflies seem very common, but any other objects of vastly different masses are often mentioned.)

The simple answer to this is: The force caused by gravity between two objects is directly proportional to the masses of both objects. In other words, the heavier the object, the stronger the gravitational force that it experiences.

This is not as asinine or "made-up" as it might sound at first. Think of it like this:

Push a 10-pound object with enough force that it reaches a particular speed in a given amount of time.

Now push a 20-pound object with enough force for it to reach that same speed in the same time.

You needed significantly more force for the 20-pound object than for the 10-pound object, didn't you? This only makes sense: It requires a stronger force to move a heavier object.

Gravity works a similar way: The force exerted on the objects depends on their masses: The larger the mass, the stronger the gravitational force.

And conversely, and crucially, the lighter the mass, the less force between them.

That's why gravity affects the ocean so much more strongly than the butterfly: Because the ocean has an enormously bigger mass than the butterfly.

This is also the reason why objects of different weights drop at the same rate: Drop a 10-pound rock and a 20-pound rock from the same height at the same time. The 20-pound rock will not accelerate twice as fast as the 10-pound one. They will accelerate at the same rate (with any tiny differences being caused by air resistance.)

This is because the force of gravity is stronger for the 20-pound object than for the 10-pound object. (And the proportion is linear, which is why they experience the same acceleration regardless of the mass.)

(And by the way, the two rocks will drop at the same rate even if their densities are different. Density has no effect on this.) 

This is also the reason why it's so much harder to make a 20-pound object fly than it is to make a half-ounce object fly, because the heavier object needs much stronger propulsion to overcome the stronger gravitational force. The heavier the object, the stronger the gravitational force, and thus the stronger the counteracting force needs to be to make it go up.

That is why a butterly can fly in the same gravitational field that keeps oceans in place.