F4 physic concept

最好用 Energy conservation,
P.E._1 + K.E._1 = P.E._2 + K.E,_2
(mgh_1) + 0 = 0 + mv_2^2 /2
m(10)(0.2) = mv_2^2 /2
2(10)(0.2) = v_2 ^2
4 = v_2 ^2
v_2 = 2 m/s
其實因為個s指displacement, 要1條straight line,
但如果你用v^2 - u^2 = 2as,的話，
咁個bob跌下來就成了垂直跌，而不是鐘擺了。
而v 就變成向下的速度，而不是向另一邊的速率了


2010-04-17 17:53:57 補充：
因為是鍾擺運動，
a 會因應 不同位置 而改變
因為有 tension向上 但如果不是正正於線下面, 向上的force就不是全向上，而是要隨角度改變而a改變~因為 NetForce keeps varying.
向下的force 一定為 ma (a = gravity), 但向上的force卻不斷改變,所以total Net Force 不斷改變.
根據 F=ma, m keeps the same, By the F varying, a is varying too~

2010-04-17 17:55:10 補充：
有唔同~
因為如果h=s, 這代表 bob 是垂直下降，而不是隨鐘擺的軌蹟移動

The forces acting on the thing is tension and gravity.
The direction of tension changes as it swings.
Therefore, the net force acting one the thing is not uniform in the process.

1/2mv2 =mgh同v^2 - u^2 = 2as個v都係velocity．佢地應該係同一樣野？

2010-04-17 16:44:42 補充：
我think h都係s,冇唔同

The crucial point is that the equation v^2 = u^2 + 2.a.s is one of the equations of motion for objects with uniform acceleration. In the swinging motion of a simple pendulum bob, the acceleration is not uniform.