Idea 4of 9Builds onHow long
How often
How many crests pass the post per second?
Today you'll discover
- a count that says how often a wave repeats
- the time one repeat takes
- the two names for the count and the time
Think firstCrests pass a post: two each second
Two crests pass the post each second. You shake twice as often. The count?
Decide first, then press Next. Count the marked bit's bobs against the clock.
A wave on a rope with one marked bit. After a prediction, the frames advance and the marked bit bobs once as each crest passes; then the rope is redrawn shaken twice as often and the bit bobs twice as often. The count per second and the time per crest are then named. The last step asks how long one wingbeat takes at fifty beats a second.
See the whole ideathe big idea and the key word, in one place
The big idea
Frequency: crests per second. Period: seconds per crest.
See the pattern2 Hz is 0.5 s per crest. 4 Hz is 0.25 s. 50 Hz is 0.02 s. Double the count, halve the time, every time.
Key words
- frequency
- how many waves pass a point each second, symbol f, in hertz (Hz)
- period
- the time for one whole wave to pass, T = 1 ÷ f, in seconds
About this modelThe pattern travels; the particle only bobs. One square of the drawing is 0.5 m.
The pattern travels; the particle only bobs. One square of the drawing is 0.5 m.
Ready to use it?
- Find the period from a frequency, and the frequency from a period.