For a one-dimensional conservative system with potential energy $U\left(x\right)$, which expression gives the force along the $x$-axis?

A particle moves in one dimension with potential energy $U\left(x\right)=(5x^4-2x^3+7)$ J, where $x$ is in meters. What is the force function $F\left(x\right)$?

At $x=x_0$, a smooth potential energy graph has a horizontal tangent, with negative slope immediately to the left of $x_0$ and positive slope immediately to the right of $x_0$. Which statement is correct?

A molecular interaction along a line is approximated by $U\left(x\right)=C{x}^5-\frac{D}{x}$, where $C$ and $D$ are positive constants. Determine the expression for the force.

For the spring potential $U\left(x\right)=\frac{1}{2}k{x}^2$ with $k=180$ N/m, what is the force at $x=-0.20$ m?