F4 physics 力學問題

Q:當 air resistance>force of gravity，物體會唔會向上升？點解？

A: No.
Assume the object is moving vertically downward. When air resistance is greater than the force of gravity of the object (i.e. its weight), the net force is acting upward, and which would decelerate the velocity of the object.

But air resistance is velocity dependent, the lower the velocity, the smaller is the air resistance force. Hence, when the velocity of the object is reduced, the air resistance will decrease accordingly, until both forces are equal .

Q: 現實中會唔會發生air resistance>force of gravity既情況?

A: Yes, it could happen.

An example is a meteorite (隕石) entering the atmosphere from space. Because of the high speed of the meteroite, when it first touches the atmosphere, the air resistance that the meterorite encountered would be larger than the weight (i.e. force of gravity) of the meterorite. As the meterorite slows down, the air resistance will be reduced considerably.

A spacecraft performing re-entry from its earth orbit demonstrates a similar phenomenon.

如果你問一個物理學家
他可能會答你'
你可以說yes 也可以說no
說yes 是因為雖然air resistance>force of gravity不會(!!令!!)物件上升
但實際上宇宙膨漲會把所有的空間拉長
令到物件可能會遠離地下

答no 原因是因為宇宙膨漲的速度真的慢得可悲
F=10^(-123)x(Distance/ms^-1)

實際上air resistance不會大過引力 只會等於