action and reaction定義及gravitational force和accerlaration(20點)

-->我想問action and reaction既定義點解?
Action and reaction force:
根據Newton's third law of motion,當A物件施與一個力於B物件(作用力),B物件同時會施與力給A物件(反作用力)
其特性為
1)equal in magnitude 數值一樣
2)opposite in direction 方向相反
3)acting on diff.bodies 施與在不同物件上

-->gravitational force係指咩??有無example?
Gravitational force
CE Level:簡單黎講 就係當一件物件係地球上 就會受到W=mg既力 呢個W就係等於重力,就係gravitational force。而g就係重力加速度,地球上重力加速度假設為9.8ms-2或10-2
A Level:其實 每樣物體之間都會有力.叫做gravitational force
根據F=GMm/r^2.就可以計到gravitational force.
(F為grivitational force, G為universal constant M,m係兩個物體既mass,對象物體既半徑)
EXAMPLE係地球同人

-->當1個物體垂直掉下，accerlaration有無increase?
由於F=GMm/r^2 力其實隨住r改變,所以acceleration都有改變
但係 呢個改變實在太細.所以通常我地都當冇
因此,於地球既acceleration 假設係不變,其數值等於9.8/10ms-2

-->momentum點conserved?
根據動量守恆定律(law of conservation of energy)
一個系統既物件係撞擊後既總動量會同撞擊前一樣。
簡單黎講即係mv+m'v'-mu-m'u'=0 m,m'係兩件物件既mass, v,v'係final velocity, u,u'係initial velocity
呢個暫時以我所知係一個現象..."

-->net force點解?
net force即係等於resultant force,指既就係針對個別物件既總作用力(唔知係唔係咁叫.."),就係互相抵消或相加之後既力
而net force係一個vector quantity,有數值and方向既

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action&reaction pair可以參照Newton's third law of motion.....主要係話when a force is acting on an object, there's always an equal and opposite in magnitude force acting back to it.舉例:你用手掌推幅牆,愈用力,就愈feel到有個force頂番住你隻手,個個force係用你俾出去ge force 持相反方向而且一樣大,個個就係reaction force.....so,假設你打人,其實等於打番自己,自己一樣feel到痛
gravitational force係指weight(p.s.不同mass).......可以用F=ma計....係唔同星球,個gravitational force係不同的,因為不同星球地心的吸力不同....如有件物件係地球個weight係10N,咁件物件gegravitational force係月球就會係係地球ge1/6咁大la.....
當1個物體垂直掉下，accerlaration有無increase?無,increse了的只是velocity......計數時通常假設accerlaration due to gravity=10ms^-2
momentum is conserved if there's no external force act on it(e.g.friction will be a kind of external force)
咩係net force?假設你拉一件物件係個rough ge floor向前,件物件同地面有個friction向後......net force就係將你拉個力減左friction個力la......F=ma中的F是net force來的.....如果有net force,咁就有acceleration/deceleration........如果net force= 0,咁件object就係有一個uniform velocity la~