Understanding magnetic flux change during the falling of magnet

[RED3RED3]

TL;DR

What's the factor which induce EMF in case of falling magnet into the coil

If I drop the magnet into the electromagnetic coil, is magnetic flux density changed or cross sectional area changed during its falling? Actually I dropped a neodymium magnet into the coil and there was the voltage induced but I don't know what's the factor that induce the EMF in this case. Also if there is change of magnetic flux density or cross sectional area, how can I calculate those value without using experimental result I mean the induced EMF value.

 

[Charles Link]

See https://www.physicsforums.com/threads/calculating-magnetic-field-strength-of-a-magnet.1005917/page-4 especially posts 120-122 and thereabouts. You might find the physics a little advanced=perhaps you have had a college level physics course or two...