為什麼太陽的引力不能將行星越拉越近???

If the earth is stationary, then gravitational attraction from the would surely pull the earth towards the sun.

But the earth is in motion. If there were no attraction fom the sun, the earth simply would move in a straight line, instead of a circular orbit. The gravitational pull from the sun makes the earth to circle around it.

In terms of force, it is because the sun's attraction provides a centripetal force for the earth to perform a circular orbit round the sun.

圖片參考：http://imgcld.yimg.com/8/n/HA00364049/o/701108240040913873448350.jpg

由於力係向量,可以分拆成幾個分量,行星ge慣性力就會分成F1同F2,所以同F1就同F3 Cancel左,就唔會跌落個SUN到喇~~

First I want to correct your concept of 離心力, this is just a common term. In physics, there is no such force, instead, it is the centripetal force (向心力).
As the Earth rotates, its motion is known as circular motion. Circular motion processes Centripetal force. By newton's law,
F=mv^2 / r where v is the velocity, m is mass and r is radius of motion

The sun gravitational force BALANCES the centripetal force of Earth's rotation. By newton's law of gravitation, gravitational force is:
F=GMm/ r^2 ,where G is gravitaional constant, M is the mass of Sun, m is the mass of Earth, r is the radius of circular motion.

Since the two forces are balanced, then
GMm/ r^2 = mv^2 / r

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只是地球有離心力
就如你拿繩索住石頭轉一樣
石頭不會飛出,又不會拉近!
當然不關反作用力事!