點解萬有引力勢能公式有個負係前面?

It is simply because in calculating gravitational potential, we have adopted infinity as a reference for zero potential. This is understandable, places at infinity can be regarded as those that are completely free from any gravitational attraction, and so there is no gravitational potential.

Gravity is an attractive force. That is to say, when one goes up from the earth surface (or in general surface of any object), work needs to be done and so there is an increase of gravitational potential. Likewise, when one goes down towards the earth surface from a certain height, there is a decrease of gravitational potential.

With this idea in mind, just imagine that if we go from infinity towards the earth surface, we should have a decrease in gravitational potential. Since we have already taken the potential at infinity to be zero as a reference, a decrease from zero means the value of potential should be less than zero, i.e. a negative value.

This is the reason why there is a negative sign in front of gravitational potential. In fact, any attractive force field would produce negative potential values.

因為
勢能 ＝ m g h
而g是-9.8ms^-2
m是正數
h都是正數
正數*正數*負數一定等於負數,所以勢能系負數