Those that disputed the flat Earth theory countered the claim by observing that Christopher Columbus was able to sail around the Earth without falling off the edge and later, by presenting satellite imagery taken from space of a spherical Earth. This example demonstrates that

Large electric fields in cell membranes cause ions to move through the cell wall. the field strength in a typical membrane is 1.0 x 10 7 n/c. what is the magnitude of the force on a calcium ion with charge +e? what is its acceleration? is it possible for a particle with the same charge as as calcium to have a different acceleration if placed at this location? what would change the acceleration: mass of the particle? diameter of the particle?