A box is pushed across a horizontal table at constant speed.Of the forces on it,which pair do we know are equal in magnitude because of Newton's third law?
A) Normal force on the box and the weight of the box.
B) Normal force on the box and the normal force on the table by the box.
C) The force of kinetic friction on the box and the force applied to the box by the one pushing it.
D) None of these are interaction pairs.

Question

Quizplus · Accepted Answer

According to Newton's third law, every action has an equal and opposite reaction. In this case, the normal force on the box by the table and the normal force on the table by the box are a pair of forces that are equal in magnitude but opposite in direction, satisfying Newton's third law. The weight of the box is the force due to gravity, which is acting on the box but not on the table, so it does not have an equal and opposite force. The force of kinetic friction on the box is in the opposite direction to the direction of motion and does not have an equal and opposite force, and the force applied to the box by the person pushing it is not necessarily equal in magnitude to any other force acting on the box. Therefore, the only option that satisfies the query is B.

A Box Is Pushed Across a Horizontal Table at Constant