Question

A heavy ball is suspended as shown. A quick jerk on the lower string will break that string but a slow pull on the lower string will break the upper string. The first result occurs because :

• A. the force is too small to move the ball
• B. action and reaction is operating
• C. the ball has inertia
• D. air friction holds the ball back

Answer 1

The correct option is C the ball has inertia

$TheanswerisC$

The phenomena of the ‘inertia ball experiment’ can be explained in terms of Newton’s three laws of motion. The 3rd law of motion states that whenever object A exerts a force on another object B, object B will simultaneously exert a force on object A with the same magnitude in the opposite direction . Hence, when a force is applied to the lower string, the force is transferred along the string and exerts a force on the heavy ball. Assuming that the heavy ball is a stationary point, it will exert an equal and opposite force on the string.

These two forces, exerted by the physical tug and exerted by the heavy ball on the string causes tension in the string (as illustrated in the diagram). Similarly, the upper string experiences the tug at the lower string, but since the upper string has to carry the weight of the heavy ball, the tension in the upper string is greater than the lower string by the mass of the heavy ball multiplied by the acceleration of gravity. However, different rates of forces can influence the rate of tension in the upper and lower string. Newton’s second law of motion states that the acceleration of an object is proportional to the net forces applied and inversely proportional to the mass of the object. Therefore, because the piece of string has far less inertia than the heavy ball, the lower string accelerates. As a result, the upper string does not experience as much stretch as the lower string and the tension does not increase as much.