The minimum number of forces that act on a book resting on a table is
a. 4.
b. 2.
c. 3.
d. none of the above
e. 1.
The correct answer and explanation is:
Correct Answer: b. 2
A book resting on a table experiences at least two forces acting on it at all times when it is in equilibrium. The first force is the gravitational force, or weight, which pulls the book downward toward the center of the Earth. This force is due to the mass of the book and the acceleration due to gravity. The weight acts vertically downward through the center of mass of the book.
The second force is the normal force, which is an equal and opposite force exerted by the surface of the table. The normal force acts perpendicular to the surface of the table and pushes upward on the book. This upward push balances the downward pull of gravity.
According to Newton’s first law of motion, when an object is at rest and remains at rest, the net force acting on it must be zero. Therefore, the downward force due to gravity must be balanced by an upward force. If the normal force did not exist, the book would accelerate downward due to gravity and would fall through the table.
No other forces are necessarily acting on the book if air resistance is ignored and no one is pushing or pulling the book. Friction could be present if there is a horizontal force trying to move the book, but when the book is simply resting and not moving, static friction does not play a role in balancing any horizontal forces because there are none.
So the minimum number of forces required for the book to remain at rest on the table is two: gravity pulling it down and the normal force pushing it up. These two forces are equal in magnitude but opposite in direction, resulting in no net force and no movement. This balance illustrates the concept of static equilibrium in physics.