f.4 phys,, about inertia

This does not violate Newton's first law of motion.

When the bicycle is in motion, air resistance and frictional force on the ground will exert on the bicycle to oppose the motion of it.

Hence, the rider has to pedal the bicycle in order to apply a force which is in opposite direction of the opposing forces stated above.

So, the resultant force acting on the bicycle will be zero. And by Newton's First law of motion, the bicycle moves with uniform velocity.



2008-10-18 16:02:40 補充：
The boy pedals the bicycle, and hence exerts a force on the bicycle.
As the bicycle is in motion, there exist air resistance and frictional force which oppose the motion of the bicycle.

2008-10-18 16:02:44 補充：
If the exerting force and the opposing force are equal, then the resultant force acting on the bicycle is zero. Hence, by Newton's 1st law of motion, the bicycle moves with uniform velocity.