不少人小时候都玩过秋千(图1)。人在秋千上做动作,秋千能从摆动幅度较小逐渐变得越来越大,只要你有足够的气力和适当熟练的动作,往往能把人荡得接近支架那么高。那末,秋千是怎么会越摆越高的呢? Many people played swings as a child (Figure 1). When a person moves on a swing, the swing can gradually become bigger and smaller from a small swing. As long as you have enough strength and appropriate skilled movements, you can often swing the person close to the bracket. Then, how can the swing get higher and higher?