C.2 · IB Physics
Wave speed on a string
Flick pulses along a 3.00 m string, change its tension and mass per unit length, and watch how the speed and the reflection at a fixed or free end change.
v = √(T / μ)
travel time t = L / v
L = 3.00 m
fixed end → pulse inverted
free end → pulse upright
travel time t = L / v
L = 3.00 m
fixed end → pulse inverted
free end → pulse upright
Try this
- With T = 20 N and μ = 5 g m⁻¹, send one pulse and read its measured time to reach the far end. Compare it with L / v.
- Raise the tension from 20 N to 80 N. Does the speed double or quadruple? Now return to 20 N and raise μ from 5 to 20 g m⁻¹ instead.
- Switch between fixed and free ends and watch the returning pulse: which one comes back inverted, and what happens to the displacement of the free ring at the moment of reflection?
Common error: μ is often given in g m⁻¹, so convert to kg m⁻¹ before using v = √(T/μ), otherwise the speed comes out too small by a factor of about 31.6 (√1000).