The intent of this research is to evaluate the use of a formative assessment probe about balance and torque. The lesson was conducted with 52, 11th grade students studying in two physics classes at a state high school. A formative assessment probe was used at both the beginning and end of the study. After the students' prior knowledge about equilibrium and torque was determined, the demonstration experiment on this topic was applied to include the processes of Predict-Explain-Observe-Explain. At the beginning of the instruction most students believed that the weights of the objects on both sides were equal, though they in fact cannot be equal in the physical situation shown. After instruction, students showed significant increase on their conceptual understanding of the topic. Applications of this research include using formative assessment probes followed by experimentation in high school science classes as well as using such methods in teacher preparation programs.