A2L Item 147

• Description: Identify the points of a potential energy function for which the force is zero.
• Goal: Relate force to potential energy function
• Source: UMPERG-ctqpe66
• Keywords: Forces, Mechanics, Potentials

---

The question for students:

The potential energy function for a certain body is shown at right. For which points is F_x = 0?

1. Point 1 only
2. Points 2 and 4
3. Points 1 and 5
4. Points 3 and 5
5. Point 3 only
6. F_x not equal to 0 anywheree

---

Commentary for teachers:

Answer

(4) The force is given by the negative of the slope of the potential function. Points 3 and 5 are equilibrium points, but only 3 is a stable equilibrium point. Students frequently choose answer #2 thinking that the force is zero at these points.

---