about friction-compensated runway

1.如果冇net force 施於一個物體,物體只係靜止或者均速移動,若果物體是靜止,代表物體沒有在一開始的時侯已經是靜止,若果物體是以均速移動,則代表物體本身被一net force推動而移動,但在物體均速移動的時侯,該net foce已經消失,因此只net force係0N.所以可以均速移動.


2.Friction compensated runway 係一定有friction的,只不過同mgsinx抵消左,所以net force係0N.如果一條friction compensated runway 係冇frcition,e條跟本唔係friction compensated runway,而且放係上面既物體仲會向下加速,而唔係均速移動,因為如果冇FRICTION,就會有NET FORCE,拫據F=MA,有NET FORCE,有MASS就有加速.
你所指既現實係咩意思?如果係日常生活既情況既話,其實要天然做到friction compensated runway都好少有,因為斜坡既角度要岩岩好配合到條斜坡既friction,角度細D,FRICTION大D都會令到物體AT REST,因為其實一條路上面既friction係有maxium的,你比佢幾多力物體friction就會向反方向比番幾多力你,令到物體唔可以移動,但係friction比番你既力係有最大值既,當你比既力大過佢既最大值,物體就會加速,當你比既力同佢既max值一樣,物體就可以at rest 或者uniform velocity,當你比既力細過佢既max值,物體就一定會at rest.

你老師果個實驗，本身個d野停左張table度，當老師把張table抬高，而個angle between the table and the horziotal 還未夠大既時候，mgsinx 因為太小所以被frcition 抵消左。由於無net force 作用果物件而且果物件本身stay at rest,物件唔會行既因為佢有inertia (即係tendency to stay at rest)
點解一個係at rest,一個係uniform velocity?
題目會講明究竟本身個object 係at rest 定係本身已經有一個velocity.例如:a block is placed on a inclined plane/a block is released at rest on a inclined plane,咁個object 本身就會當at rest.
如果題目話:a block is projected down/up the inlcined plane(類似呢D)就既係話 個object 有intial velocity,因為有人將個object 推一下，令到個object 有左change in momentum,所以個object 會有返個velocity
如果題目完全冇果明本身個object  at rest 定係有左個intial velocity(即係object被projected), 而直接問:Which of the following statement about a friction compensated runeway is correct? A.The object would move in uniform velocity on a inclined runeway B xxxx Cxxxx Dxxxx
A既答案可以當既，因為 A. 都可以解釋到object 既acceleration 係0,既係可以解釋到net force 0,即係話mg(sin x)同個friction compensate左，呢個正正就是frictioncompensated runway既特性

2008-12-13 15:25:38 補充：
果個人將件object 只是推左一下，所以當個object 既velocity 由零變做有左velocity 之後，推果個force就會消失，所以呢個force 唔駛考慮入free body diagram 度既

2008-12-13 15:42:06 補充：
你個同學完全唔，現實同埋實驗既runway 都係有friction 既.如果真係冇friction,呢個runway 已經唔係friction compensated,因為都冇frcition 同mgsin@ balance

2008-12-13 15:43:36 補充：
仲有move in uniform velocity 是因為object 無受到net force,這點已經當你知道左