Friction can be found anywhere objects come into contact with each other. The brakes on a car causes friction between the brake pads and the wheels of the car, allowing the car to come to a stop. A person running on a sidewalk may stop quickly because of friction caused between the bottom of the shoes and the asphalt or concrete.
物体与其接触面的摩擦力随处可见。汽车刹车是因为刹车踏板与汽车车轮之间的摩擦力让车停下来。在人行道上跑的人能快速停止是源于鞋底和沥青或混凝土之间的摩擦力。
However, other variables can lessen the effects of friction. For instance, if the sam e person running tried to stop on a water-covered sidewalk, friction would be less and the runner may not be able to stop as quickly, and in some cases may fall. This is similar to a car trying to stop on an ice-covered roadway. The friction is still there, but it is much less and may lead to accidents. Also, during rain, there is still friction between the brakes and the wheels, yet if the brakes are wet, the wheels would not be as much in contact with the ground. As a result, cars hydroplane when they go too fast on puddles of water.
然而,其他变量可以减少摩擦力的作用。例如,跑步者试图在积水的人行道上停下来,但是因为摩擦力减小他可能很难立刻停下来,而且有时会摔倒。同样,汽车试图在冰雪覆盖的道路上刹车。摩擦仍然存在,但要小得多,而且可能引起事故。又例如,下雨时,刹车踏板和车轮之间的摩擦力仍然存在,但如果离合器是湿的,车轮不会与地面接触一样多。其结果是,车子如果在水洼行驶速度过快如同水上飞机。
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