Final answer:
The key difference lies in the spring constant; a stiff spring has a higher spring constant, meaning it's harder to deform, while a more flexible spring has a lower spring constant and deforms more easily.
Step-by-step explanation:
The difference between a flexible and stiff spring concerning its spring constant is a measure of the spring's stiffness. A stiff spring has a higher spring constant, indicating it requires more force to compress or extend it by a certain amount. In contrast, a flexible spring has a lower spring constant, meaning it extends or compresses more easily under force. Hooke's Law states that the force required to compress or extend a spring is directly proportional to its displacement, described by the formula F = kx, where F is the force applied, x is the displacement, and k is the spring constant. The higher the value of k, the stiffer the spring and the greater the restoring force it exerts. Thus, when comparing two springs with different spring constants, the one with the higher constant will be stiffer.