[AL phy]點解gravitational PE一定係負數?

點解gravitational PE一定係負數?

The reason is because of the choice of reference level for zero potential energy.

It is conventionally adopted to use infinity as a reference level for zero potential energy. Since graviatational force is an attractive force, when an object falls from infinty (zero potential energy) towards the attracting object, e.g. the earth, its potental energy decreases, i.e. its potnetial energy value should be lower than zero. In mathematics, any value less than zero is negative. In other words, the potential energy becomes more and more negative with respect to that at infinity when the falling object is nearer and nearer to the earth.

點解F=負dU/dx???

The physical significance for the negative sign is that the direction of the attractive force is to [decrease] the potential energy of the falling object.

In mathematical term, the attractive force has a negative potential gradient. The force acts in a direction to decrease (i.e. -dU) the potential energy. This is, in fact, a property of attractive force.

但點解atom之間o既PE 又可以正又可以負呢????

Intermolecular force is attractive in nature. Thus the potential energy is neagtive. That is, the potential energy decreases with decrease of distance between the atoms.
However, for very close distance between two atoms when the electrons of these atoms are overlapping one another, the force becomes repulsive and the potential energy then increases with decrease of distance.

energy係sacle,咁咩情況係負,咩情況係正數呢???

Notice that the sign affixed to energy indicates its magnitude (unlike a vector quantity where the sign indicates its direction), and which shows whether the force involved is attractive (negative potential energy) or repulsive (positive potential energy).

負數 代表 attractive
正數 代表repulsive

gravitational PE一定係負數
因為 一定係attractive ...
被地球吸...

atom之間 ,
可以係attrative or repulsive
so,正又可以負....

energy係sacle,
但係佢前面個正 or 負
唔係指direction..
係指attractive ,,repulsive

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