-- When the 15-N book is placed on the table, the table
provides 15N of upward 'support' force.
-- When the hand pushes down on the book with a force
of 20N ... making the book appear to weigh 35N ... the table
then provides 35N of upward support force.
-- When a rope or a helium balloon lifts up on the book with
a force of 10N ... making the book appear to weigh 5N ... the
table then provides 5N of upward support force.
Many ways to explain why. To me, the easiest explanation
goes like this:
Newton's 2nd law of motion: Net force = (mass) x (acceleration) .
If there were any net vertical force acting on the book, it would
take off ... either jumping up from the table, or plunging down
through it. Nothing like that happens. The book's acceleration
in each case is zero ... it just continues to lay there on the table,
motionless.
The net vertical force on it must be zero. The upward support
force provided by the table must exactly balance the downward
force of gravity on the book, plus the hand, minus the helium
balloon, whatever.
