Work done on an object is equal to $$FD\cos(\mathrm{angle}).$$
So, naturally, if you lift a book from the floor on top of the table you do work on it since you are applying a force through a distance.
However, I often see the example of carrying a book through a horizontal distance is not work. The reasoning given is this: The force you apply is in the vertical distance, countering gravity and thus not in the direction of motion.
But surely you must be applying a force (and thus work) in the horizontal direction as the book would stop due to air friction if not for your fingers?
Is applying a force through a distance only work if causes an acceleration? That wouldn't make sense in my mind. If you are dragging a sled through snow, you are still doing work on it, since the force is in the direction of motion. This goes even if velocity is constant due to friction,
Best Answer
Yes, when accounting for factors such as air resistance and biological processes, there is clearly energy used, and work being done somewhere.
If you're only considering a book moving at a constant velocity in the horizontal direction, you could say there is no net work done on the book.
If we analyze it further, you will find that if it moves with a constant velocity, there must be some other force acting on it to counteract the drag force. This is the force supplied by you when moving the book.
If this book is moving with constant velocity, this must mean that the net force acting on the book in the horizontal direction is $0$. If that is the case, the net work in the horizontal direction required to keep it moving at the horizontal velocity is zero. The work done by the person on the book is exactly countered by the work done by the air resistance on the book, and thus no net work.
Work is required for a person to move a book through the air at a constant velocity, because the work done by air resistance needs to be countered, but the net work on the book during that movement is none.
It's also often the case that it's assumed you can neglect air resistance. In that case, it would not require work to keep the book moving at a constant velocity. I suspect they may be making that assumption when they are discussing work.
You could also consider a situation where you are starting from a standstill, and ending at a standstill while covering some distance. You can see there is work done to accelerate and decelerate the book. From an energy/work perspective, the work required to slow down the book is exactly opposite of the work required to get it up to the moving speed. This also means that no net work is done on the book during the entire travel (though both the person and the air will have had net work done on them).