Answer
(d) a contact force perpendicular to the contact surfaces between two solid objects.
Work Step by Step
The correct answer is: (d) a contact force perpendicular to the contact surfaces between two solid objects.
If a box sits on a table, the contact force exerted on the box by the table is directed straight up. This contact force is perpendicular to the contact surfaces between the box and the table, and therefore it is called a normal force.
