Question

The potential energy $U$ between two molecules as a function of the distance $X$ between them has been shown in the figure. The two molecules are

• A. attracted when $x$ lies between $A$ and $B$ and are repelled when $X$ lies between $B$ and $C$
• B. attracted when $x$ lies between $B$ and $C$ and are repelled when $X$ lies between $A$ and $B$
• C. attracted when they reach $B$
• D. repelled when they reach $B$

Answer 1

Answer: (B)

$F=-\left(\frac{d U}{d x}\right)$.
In the region $B C$, slope of the graph is positive.
$\therefore F=$ negative, i.e., force is attractive in nature.
In the region $A B$, slope of the graph is negative.
$\therefore F=$ positive, i.e., force is repulsive in nature.