PHYSICS 三問(20分)

Q: 點解地球係圍住太陽轉,而唔係好似我地咁直接比地球吸住?
It is because the earth is performing a circular motion round the sun. The attractive force from the sun provides the necessary centripetal force, keeping the earth in its orbit.

Q:地球同太陽中間唔係有萬有引力,會有個FORCE令地球向太陽加速咩?
Yes, such acceleration is the "centripetal acceleration". This acceleration drives the earth to change its velocity direction continuously, thus moving in a circular orbit. Without such an acceleration, the earth would move in a straight line, thus going away from the sun.

Q: 仲有,地球的locUS明明係ellipse shape,點解D 書都話E個係
circular motion 的?

The trajectory of a "circular motion" is not necessarily a perfect circle. As long as the trjectory is a closed loop, the motion is a circular motion.

2011-02-07 20:49:54 補充：
Q:咁點解地球唔會直接衝向太陽而係圍住LOCUS轉的?
Because when the solar system was first formed, the earth had already got an initial velocity. Given the sun-earth distance, the force from the sun could only change the velocity direction of the earth...(cont'd)

2011-02-07 20:50:25 補充：
...and is NOT strong enough to attract the earth towards it in an inward spiral trajectory.
Should the earth had no initial velocity whatsoever when the solar system was formed, the earth would be attracted by the sun and crashed towards its surface.