1. 钟摆
    The pendulum of a clock is a rod with a weight at the end which swings from side to side in order to make the clock work.

pendulum什么意思

2. （局势意见等的）摇摆不定，变化不定
    You can use the idea of a pendulum and the way it swings regularly as a way of talking about regular changes in a situation or in people's opinions.

    e.g. The pendulum has swung back and the American car companies have made dramatic advances in safety...
           三十年河东，三十年河西，美国汽车公司在安全方面已经取得了巨大进步。
    e.g. The political pendulum has swung in favour of the liberals.
           政治形势转而对自由主义者有利。