Hovercrafts, which can travel on water, ice and land, are perhaps the most versatile vehicles around. Their versatility comes from their unique method of propulsion. They travel on a thin film of air, making only slight contact with the ground. In today’s experiment we will take a close look at the forces that propel these amazing machines.
What you will need: an old CD, several different sized balloons, a pop-top cap from a drinking bottle, glue and a stopwatch.
(JOHN PHILLIPS, STAFF WRITER)
PHOTO: AFP
可行駛於水、冰和陸地上的氣墊船恐怕是功能最多的交通工具。氣墊船的多用途來自它們獨特的推進方式。它們行駛在一層空氣薄膜上，僅和地面有些微的接觸。今天的實驗中，我們要深入了解驅使這種神奇交通工具前進的動力。
實驗所需：一張舊光碟片、幾個大小不同的氣球、一個吸嘴瓶、膠水和計時碼表。
（翻譯：袁星塵）
METHODOLOGY 方法
Step 1: Get all your materials together and clear a space to prepare.
Step 2: Take the CD. It won’t work after this experiment, so make sure nobody needs it.
Step 3: Take the pop-top cap and glue it to the center of the CD. Push the cap down to the closed position.
Step 4: Blow up one of the balloons, then pinch the end so that no air escapes. Stretch the balloon over the cap.
Step 5: Open the pop-top cap and give your hovercraft a push. Start the stopwatch, then stop it when all the air has escaped.
步驟一：準備好所有的材料，並清出一個空間準備做實驗。
步驟二：拿出光碟片。實驗後，這張光碟片在就不能使用了，所以務必拿一張沒人要的光碟。
步驟三：拿出吸嘴瓶，把它黏在光碟片的正中央。按壓瓶蓋至關閉狀態。
步驟四：將其中的一個氣球吹氣，然後捏住吹氣口，不讓裡面的空氣逸出。把氣球套在瓶蓋上
步驟五：拉開吸嘴瓶蓋的開關，並輕推氣墊船。按下碼表開始計時，等氣球裡的空氣都跑光才停止計時。
HOW IT WORKS 實驗原理
The CD floats because of a thin cushion of air formed by the escaping air. When you push the CD, it moves easily because there is little or no friction between the CD and your work surface. Remember, friction is the force that resists movement between two objects.
In a real hovercraft, the lift is generated by large propellers on the top of the craft. The designers must get the amount of lift right. If there isn’t enough lift, the hovercraft won’t move, but if there’s too much it will tip over.
You will have noticed that your experimental hovercraft moved easily when you pushed it. Real hovercrafts are very efficient, and can propel themselves with relatively little effort.
氣球內逸出的氣體會形成一層薄薄的氣墊，可使這張光碟浮起來。當你輕推光碟片，光碟片可輕易被推動，這是因為此時工作平台和光碟間幾乎沒有摩擦力。記住，摩擦力是兩個物體作相對運動時互相施加抵抗的力。
真正氣墊船的浮力來自船上的大型推進器。工程設計人員一定要精準算出氣墊船所需的浮力。如果浮力不夠，氣墊船則無法前進；如果太多，它就會翻覆。
實驗中你會發現，你隨手一推，船就會前進。真正的氣墊船效能很好，只要些微作用力就可以向前驅進。
EXTENSION 延伸知識
Experiment with different sized balloons and time how long they hover for. Make sure you test each balloon three times, then record the average time. You can make better conclusions if you can calculate the volume of air in the balloon. The best way to calculate the volume is by inflating the balloon and using Archimedes’ principle of water displacement, which we covered last month.
用不同大小的氣球進行實驗，看看它們分別能懸浮多久。每個氣球務必各實驗三次，然後記下平均時間。若你能算出氣球中空氣的體積，就能得到更好的結論。計算空氣體積最好的方法就是使氣球充氣後，再用我們上個月介紹過的阿基米德定律之水置換法計算。
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