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

The correct option is B attracted when $X$ lies between $B$ and $C$ and are repelled when $X$ lies between $A$ and $B$

Force on the molecule is given by $F=-\left(\frac{dU}{dx}\right)$
In the region $BC$, the slope of the graph is positive, therefore $F$ is negative i.e, force is attractive in nature.
In the region $AB$, the slope of the graph is negative which means $F$ is positive i.e. force is repulsive in nature.