question about friction (AL)

Johnny Lam Cheuk Yin

2010-12-08 10:24 pm

何謂relative motion?? 如果一車輛moving without skidding, 推動它的係static friction , 而這friction 係冇 energy loss , 因為車輪跟地下冇 relative motion, 而主要嘅energy loss 係車軸跟車身which come from kinetic friction , 因為它們才有relative motion. 我這說法啱唔啱??
(既係話在行車(without skidding)時, 車輪跟本冇磨到地下, so , 冇relative motion between wheels and the floor)

那麼在braking(without skidding) 時, 地下跟車輪都係冇磨到so 冇relative motion in between, so in this case, braking 係由energy loss due to kinetic friction between車軸跟車身 and 持相反方向嘅static friction 而做出來的效果
而all energy loss 都係去在 heat and sound energy between車軸跟車身, static friction 只不過係exteral force, 完全冇任何contribution on energy loss.我這說法啱唔啱??

那麼在braking(with skidding) 時, 地下跟車輪有磨到, so , 有realtive motion, 有kinetic friction , 有 contribution on energy loss so as the kinetic friction between 車軸跟車身 . 我這說法啱唔啱??

那麼在車轉向時, (with skidding or without skidding) 係唔係都持以上相同concept

回答 (1)

天同

2010-12-08 11:02 pm

✔ 最佳答案

In an ideal situation, you could regard that when the wheels of a car are rotating withoit slipping, there is no slidding motion of the wheels on the road. Hence, there is no energy loss by friction.
During braking, assume that that the wheels are not locked, the kinetic energy of the car mostly is lost on the brake pads (or sometimes called brake shoes) that pressed hardly against the brake disk. This the reason why the brake pads would get hot in frequent braking.

However, if the wheels are locked in hard braking, the wheels silde on the road surface. Thus producing a large amount of heat by friction. The kinetic energy of the car is lost because of this retarding frictional force.

Do not mix the frictional force acting on the wheels, when the car is turning a corner, with the frictional force described above. The centripetal force that enables the car to turn a corner comes from the lateral frictional force, which is in direction perpendicular to the direction of the motion of the car.

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