Question

A ball moving on a table top eventually stops. What is the reason?
A) Due to gravity
B) Due to friction between the ball and the table
C) Due to other external forces
D) None of these

Answer 1

Net unbalanced external forces that act on a body can change the state of uniform motion or rest of a body. To change the state of motion in a single direction, the unbalanced external force must have a non zero component parallel to or anti parallel to that direction.

Complete step by step answer:
A ball moving on a table faces various different forces like air resistance, frictional force from the table, normal force from the table, and the gravitational force.
However, only unbalanced external forces that are parallel to or anti parallel to the direction of motion of the ball on the table can change the state of motion of the ball and bring it to rest.
The normal forces and the force of gravity act perpendicular to the surface of the table and hence, perpendicular to the motion of the ball and they anyway balance each other. So they cannot provide a net unbalanced external force on the ball.
However, the major unbalanced external force on the ball is the frictional force between it and the table. This frictional force tries to resist the motion of the ball and eventually brings it to a stop.
Even though air resistance also does the same thing, it is negligible in magnitude and its effect is negligible on the state of motion of the ball.
Therefore, the ball eventually stops due to the friction between the ball and the table.

Therefore, the correct option is B) Due to friction between the ball and the table.

Note: Students must note that in air the effect of air resistance is negligible. However if the whole setup is kept in a much thicker and viscous medium such as honey, then the resistive viscous force offered by the honey will be much greater than the friction offered by the table, especially for a light ball (since for a heavier ball, frictional force would be more). Then the major reason why the ball would eventually stop would be the viscous force between the ball and the medium (honey).