phy friction report /......急 thx

I am sorry to point out that I don't think the first guy is right and completely explain the fact. To answer this question, you must imagine that there is a large and heavy box laying on the ground with ”limiting friction” value says 200N. Please follow the flowing scheme.流向圖 Please pay more attention to the frictional force ,f, acting on the box as well.
When you apply a force says 100N and the following things happen to the box.

(1)the box doesn't move. ------>　（due to f = 100N acting on the box also)
(2)then you try 120 N ------>（due to f = 120N acting on the box also)

(3)140N --------> doesn't move-------> （due to f = 140N acting on the box also)

(4)160N ------> doesn't move -----> (（due to f = 160N acting on the box also)

(5)180N-------->doesn't move ------> (（due to f = 180N also)

NOT UNTIL THE USE OF 200.0000000001N DOES THE BOX MOVE.

Once the box is moved, the limiting friction acting on the box will drop lower than 200N, says 198N

When you want to pull or push a heavy object, what you do is just同這件重物鬥力，只有在你作用在這物件的力大過作用在這物件的limiting friction時，這件物件才會給你推動.由於當物被推動時，其limiting friction會下跌少少,所以用的力也會少了一點的. Apply 2nd law Fapp - f = ma ; 200.0000000001 -198 = 20a, you can find out the acceleration if g = 10ms-2

Hence this explains why in pushing a car, it is more difficult to start the motion, but once the car is moving, it is relatively easier to maintain its motion. Besides, using the law of interia ,Newton 1st law ,can also explain this phenomenon. An object stays at rest unless a force is acted on it. The inertia makes the object difficult to be moved, pushed or pulled.
This graph, plotting frictional force against applied force can also be found in the past paper (MC type question). However , I can't remember which year.


In the design of the braking system in bicycles, what parameter(s) should be maximized to achieve a larger frictional force. 只有增加車軚紋的數目及增加車輪和地面的接觸面積才可以加大 frictional force between wheel and ground.原理和用光頭車輪為何危險一樣

I hope this information can help you.

2007-01-12 18:24:03 補充：
(1)for Fapp = 200N, the heavy object don't move even Fapp = friction force.(2)The friction changing with the applied force is called static friction while the friction becoming smaller than the static friction is called kinetic friction.

2007-01-12 18:24:31 補充：
(3)For kinetic friction, its value depends on the speed of the object (it only be counted when the speed is fast) and its value is independent of the contact surface area between the ground and the heavy object

2007-01-12 18:24:44 補充：
while static friction does depends on the contact surface area .between the ground and the heavy object .(4)The both friction coffieicents depend on the nature of matter.

1.friction can be classified as two kinds which are static friction and kinetic friction~
(friction = uR)...where u is a constant depending on the nature of the surface & R is the contact reaction~
[ kinetic friction & static friction are also = uR...but the value of u in static friction is greater then kinetic one]
before the motion start...static friction which is greater force then kinetic friction act on the car to oppose the motion~...once it is moving (sliding motion),,,kinetic friction act on the car instead of static friction...therefore...it is more difficult to start the motion, but once the car is moving, it is relatively easier to maintain its motion!!!

2.sorry...i don't know the structure of the braking system in bicycles.....but you can find the ways in this equation ( friction = u R).....try to increase the value of u to achieve large frictional force~