Newton's First Law 牛頓第一定律，慣性定律: --Also called the "
law of inertia,"
--States that body at rest remains at rest, and a body in motion continues to move at a constant velocity 均勻速率
unless acted upon by an external force 外力.
[如果作用於物體上的淨力等於零，物體的運動狀態將會保持不變。本來靜止的永遠靜止，本來運動的就會在一直線上作均速運動。]
Newton's Second Law 牛頓第二定律，運動定律: --A force
F acting on a body gives it an acceleration
a 加速度 which is in the direction of the force and --has magnitude inversely proportional to the mass
m of the body:
F=ma
[這個定律描述物體所受的淨力、質量和加速度的關係。物體受淨力時會加速，加速度 a 與作用力 F 成正比，而與物體的質量 m 成反比，即
。在公制的單位，我們有F=ma 。]
力的 SI 單位是牛頓，牛頓 = 千克•米/秒2 (1N = 1kgms-2)。 公式： (1) F = d(mv)/dt or F= m(v-u)/t and
(2) F = ma
Newton's Third Law 牛頓第三定律，作用與反作用力定律: ---Whenever a body exerts a force on another body, the latter exerts a force of equal magnitude and opposite direction on the former
---known as the weak law of action and reaction.
[假如一物體 A 向另一物體 B 施加一個作用力，那麼物體 B 便會向物體 A 施加一個反作用力。作用力與反作用力大小相等，但方向相反，而且施加於不同的物體]
