✔ 最佳答案
Q1: 我想問呢幅FREE-BODY DIAGRAM（ｎｏ ｆｒｉｃｔｉｏｎ）岩唔岩?
如果岩,它唔係應該掉下去?點解可以仲做CIRCULAR ＭＯＴＩＯＮ?
A: The diagram is right.
I suppose the object is in motion. If not, it really would fall down vertically under the action of gravity.
When the object is in motion, there is a velocity pointing at some angle upward. The weight would only then slow down the upward motion of the object. This is similar to the case of a stone being thrown upward. In its upward journey, the force of gravity slows down the speed of the stone. But it (gravity) would not make the stone to fall down suddenly.
Because the object is bound by the track (i.e. the circle that you have drawn), this gives rise to a normal reaction R that causes the object to move in a circular path instead of going vertically upward.
Q2: 以上2個BALL都係IDENTICAL,我想問邊條STRING 的TENSION大D?
A: Again, you did not give the state of motion of the two balls that they are stationary or in continuous motion.
Suppose the ball on the left side is at rest. The string tension is clearly equal to the weight of the ball.
For the ball on the right hand side, I presume it is mementarily at rest, then it is obvious that the tension should be greater than the weight of the body. It is because the vertical component of the tension must be equal to the ball's weight in order to keep the ball at momentary rest.
2013-02-11 11:03:02 補充：
Sorry, I made a mistake on the last para. of my answer. The tension in the string is smaller than the ball's weight.
2013-02-11 11:04:44 補充：
It is because the tension is equal to the weight component of the ball along the direction of the tension.
