Correct Answer to Question 10

As the sphere falls, it experiences two forces: its weight Mg downward and a force F upward due to air resistance. The latter force increases as the sphere's speed increases. At terminal velocity for the sphere, the two forces are equal and the acceleration is zero.

Hence, statement A is correct: there is no net force.

Statement B is also true: since the net force is zero, no net work is done on the sphere and its kinetic energy remains constant.

Statement C is true: the sphere is losing gravitational potential energy but there is no corresponding gain in kinetic energy. Some of the energy goes into thermal energy in the sphere and its temperature rises.

Statement D is not true: when terminal velocity is reached, the resultant force is zero.

Formula Sheet

Question 10

Question 11

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