braking 時的 friction

小Ｇ

2009-11-28 10:40 pm

「In normal braking, the friction between the tyres and the ground is static. This static friction accounts for deceleration. On the other hand, there is kinetic friction between thw wheel and the brake pads. This kinetic friction accounts for energy change.」

當車 moving 的時候，static friction 是向前的。但為甚麼 braking 的時候， friction 就向後的？
甚麼因素令到 friction 轉方向呢？
_____ -> friction 轉方向 -> decelerate

而為甚麼會有 static friction 和 kinetic friction 之分的？ kinetic friction 為甚麼只 for energy change ，而非 deceleration？

回答 (1)

天同

2009-11-29 12:43 am

✔ 最佳答案

".... .. On the other hand, there is kinetic friction between the wheel and the brake pads. This kinetic friction accounts for energy change."

Notice that the sentence refers to friction between the wheel and the brake pad, not between the tyres and road.

It is the sliding action of the brake pad with the wheel that produces heat, hence energy loss.

In normal circumstance, when a car is moving, static friction holds the tyres onto the road. The rotation of the wheels thus propel the car to move forward. Even in braking, as long as the tyres do not slip, static friction still holds the tyres on the road. The slowing down of the turning wheels, brcause of fiction between the brake pad and the wheel, forces the car to stop.

However, if the tyres slip during braking, then kinetic friction (between the typres and road) comes to play. The heat produced by friction between the tyres and road leads to the decrease of energy of the car.

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