- Why doesn't the bottom of the Slinky fall immediately when I drop it?
- The bottom mass is initially in equilibrium, held up by tension from the spring above it. When the top is released, the tension in the top spring vanishes, but this change must travel down as a wave. Until that wave of reduced tension reaches the bottom, the spring directly above the bottom mass still exerts its original upward force, so the bottom mass remains momentarily suspended. This demonstrates that forces and information do not act instantaneously through a material.
- Is this a realistic model of a real Slinky?
- It captures the essential physics but makes simplifications. A real Slinky has mass distributed along its coils, experiences air resistance, and can twist and bend. Our model simplifies it to point masses and massless springs in one dimension, which isolates and clarifies the core mechanism of wave propagation without complicating factors. The qualitative behavior—the bottom lag—is accurately represented.
- What determines how fast the wave travels down the chain?
- The wave speed depends on the stiffness (spring constant, k) and the inertia (mass per segment, m) of the chain. For a discrete spring-mass system, the speed is proportional to sqrt(k/m). A stiffer spring (larger k) transmits forces faster, while a heavier mass (larger m) responds more sluggishly, slowing the wave. In a continuous, uniform spring, the speed is sqrt(kL/m_total), where L is the length.
- Does gravity affect the wave speed in this simulation?
- No, gravity does not affect the speed of the stress wave itself. Gravity provides a constant background force on each mass, setting the initial stretched equilibrium. The wave speed is determined by the elastic properties (k) and inertia (m) of the system. However, gravity is crucial for creating the initial tension and for the overall falling motion after the wave passes.