P.E-time graph[最佳解答:40分]

Question

http://img329.imageshack.us/img329/7169/24510696yx5.jpg

A or B shows the relationship of PE. and time when a object is fallen from a building.
A or B shows the relationship of PE. and time when a object is thrown vertically upwards.

Explain.

Answer 1

A shows the relationship of PE. and time when a object is fallen from a building.
因為物體由高處跌下時它的位移
s = ut + 0.5gt2 (因初速為零)
s = 0.5gt2
它的勢能
PE = mg(h – s)
= mg(h – 0.5gt2)
變成一抛物線，最大值是開始時的勢能(PR =mgh)，因愈跌愈快，所以勢能減少得愈來愈快。

A and B not are the relationship of PE. and time when a object is thrown vertically upwards.
因為向上抛時，它的位移
s = ut – 0.5gt2
勢能便是
PE = mg(ut – 0.5gt2)
開始時 s = 0，PE = 0，而其後速度減少，勢能變化減少得較慢，而到最高點時勢能為最高。所以 B 圖並不是答案，但若將 B 圖上下倒轉才是答案。

圖片參考：http://hk.geocities.com/namsm4e/P9.jpg