SME
Grade 8 Science · Problem type
Wave speed: frequency times wavelength
Use this for “A wave on a rope has a frequency of 2 Hz and a wavelength of 1.5 m. How fast does it travel?”
Diagram
Crest to crest is one wavelength, 1.5 m.
2 waves pass each second.
This is an example. Type your own values to start a new problem.
v (m/s)
=
f (Hz)
×
λ (m)
Formulas
v
=
f
×
λ
(wave
speed
=
frequency
×
wavelength)
3 = 2 × 1.5
Assumptions
•
Frequency is how many waves pass a point each second, in hertz (Hz). Wavelength is crest to crest.
•
Sound travels at about 340 m/s in air; light at 300,000,000 m/s.
•
A wave’s energy grows with its amplitude: twice the height, four times the energy. Amplitude does not change the speed.
Step-by-step
WE KNOW
f = 2 Hz (frequency), λ = 1.5 m (wavelength)
FIND
wave speed (v)
Step 1 · Find wave speed (v)
Use v = f × λ (wave speed = frequency × wavelength)
Each
second
f
waves
pass,
each
λ
long:
the
wave
front
moves
f
×
λ
meters.
v
=
2
×
1.5
v
=
3
m/s
Check
Put the numbers back in. Both sides should match.
3 = 2 × 1.5 ✓
Topic check
See what you know: 5 questions on this topic
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