In experiments about springs and stretching, which of the following is a variable?

• A. Number of coils
• B. Length of spring
• C. Diameter/thickness of spring or wire
• D. Selection of loads

Answer: (C)

The main idea is how stiff a spring is and how that stiffness determines how far it stretches under a given force. The diameter or thickness of the wire (or the overall spring diameter) directly affects stiffness: a thicker wire or larger diameter makes the spring stiffer, so for the same applied load it will extend less. Since this dimension is something you can vary by choosing different wires or springs, it acts as the variable you can change to study its effect on extension. In many experiments with springs, the number of coils and the natural length come from the particular spring you’re using and are kept fixed for a single test, while the loads you apply are used to produce different forces. But the dimension you can deliberately alter to see a different stretching behavior is the diameter/thickness.