Question

A book falls from a desk. Which graph BEST represents the book’s change in potential energy?

Answer 1

Answer:

The graph C.

Explanation:

The potential energy of the book is maximum when it is on the table; therefore, the graph we are looking for must have maximum potential energy at $t=0.$

Also, as the book falls down, its potential energy is decreasing; therefore, the slope of the graph must be negative.

Finally, as the book lands on the ground, its potential energy becomes zero; therefore, the graph must show potential energy of zero at some time, i.e it must contain a point $(t_a, 0)$

Now, looking at the four graphs given, we see that the graph C satisfies all our conditions, and therefore it is the correct choice.

For the sake of completeness, let us look at other choices.

Graph A shows potential energy non linearly increasing—not true for a falling book.

Graph B shows potential energy increasing linearly—not true for a falling book because its potential energy is decreasing.

Graph D shows that potential energy of the book does not change—cannot be true since the book is falling.