Question

A stack of books is sitting on a table. Which force is pulling downward on it?

Answer 1

Gravitational force

Answer 2

Assuming the books are stacked in a neat, vertical pile on a table that it is itself horizontal to the earth and at an altitude comparable to sea level (I.e. not on a ship at sea, or on an airplane) then the force that is acting on the stack of books is described by the formula:
\[F = ma\]("force" = mass * acceleration)

Assuming the mass is in kilograms, the acceleration due to gravity on the Earth is \[-9.80665 m /s^{2}\] and the resultant force is in Newtons.

The magnitude of the acceleration is negative because it is "pulling" the stack of books toward the center of the Earth.

A cool online force calculator is available: “ http://www.ajdesigner.com/fl_force/force.php ”

I hope this helps,
David