How to play Harbour Buoy
- Choose a test and read the marks on the post, the swell's size, and its period on the card.
- The amplitude is measured from the middle level, not from top to bottom.
- Type your prediction and press Start. The buoy bobs to scale beside the post.
- Watch the orange marks: your amplitude on the post, your period on the graph, or your distance, speed, or energy against the real one.
What is amplitude?
Anything that oscillates swings back and forth about a middle position. The amplitude A is the largest distance from that middle: for a buoy on the swell, how far it rises above the calm-water level, which is the same as how far it sinks below it. It is half the distance from top to bottom, which is often called peak to peak. The period T is the time for one full bob, and in simple harmonic motion it doesn't depend on the amplitude: a bigger swing has farther to go but moves faster, in the same proportion. The amplitude still matters a lot. Each period the buoy travels 4·A, and its top speed, at the middle, is 2π·A ÷ T. The energy of an oscillation grows with the square of the amplitude: a swell twice as big carries four times the energy, and one three times as big carries nine times as much. The same idea measures how loud a sound is and how strong an earthquake is.
A focused learning model
The swell is a smooth sine wave with a period of 8 s, and the buoy rides it exactly, rising and falling with the water at the post. Only the up-and-down motion counts; the wave passes without carrying the buoy along. Heights are to scale on the post, measured from the sea bed at 0 m. A swell's energy per square metre is ½·ρ·g·A², with sea water at 1025 kg/m³ and g = 9.8 m/s². Runs last one or two periods, or a minute for the distance, and play 2 to 6 times faster than real time. Answers within 2% are right.