when a feather is placed on a book, why do they fall at the same time?

Because there's no air under the feather to resist it.

When the feather and book fall separately, the air gets caught under the feather because of its shape, causing it to fall slower. When you place the feather on top of the book, the book stops the air getting caught under the feather, so they fall at the same speed.

In independent fall both object experiance two forces the first being Gravity which pulls them downwards and other is drag form air which try to prevent the motion through air.Drag act in direction opposite the direction of motion i.e. upwards.Magnitude of drag depends on surface area of the object and net force acting on body is differance of the forces.Mass to surface area ratio of feather is very less than that of book.When feather is made to fall with book under it feather does not experiance the drag because of nonexposure of air and hence the both fall togather

It's all to do with wind resistance. Everything accellerates to the ground at 9.8 metres per second per second if there was no such thing as wind resistance. The feather normally floats down at a steady speed because it catches the air underneath it so well, but on top of a book, there's turbulent air behind it sucking the feather down with it.

bcoz both the feather n book fall with an accleration g .its acceleration does not depend upon their masses

If it is not glued strong enogh,they will be separated and the book hits the ground first.

1. put fether in book & its even more shuer

2. did yu repeet the test 10 times tu chek if it du?