7. How much support force does a table exert on a book that weighs 15 N when the book is placed on the table? - What if a hand pushes down on the book with a force of 20 N? - What if a rope lifts up on the book with a force of 10 N? (The hand is no longer there.)

- - 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.