circular motion

2010-03-17 2:13 am
see~~
please explain in detail
you may not need to draw the diagram~
I know how to resolve the forces


the answer is C
but i think statement is correct......


圖片參考:http://i256.photobucket.com/albums/hh182/zilu_photo/sshot-2010-03-16-18-11-53.png
更新1:

Is W.sin(theta) = m.[L sin(theta)].w^2 ? what if I get g=L w^2 sin(theta) and so the statement 1 is correct?

更新2:

If W.sin(theta) = m.[L sin(theta)].w^2 holds, w^2=W/(m L) and cos(theta)=mg/W same conclusion-> 1st statement is true

回答 (2)

2010-03-17 5:24 am
✔ 最佳答案
Statement (1) is wrong
Let T be the tension in the string
since T.cos(theta) = mg, where m is the mass of the bung and g is the acceleration due to gravity
But T = W at equilibrium
i.e. W.cos(theta) = mg ----------------- (1)
cos(theta) = mg/W
Thus, angle (theta) depends on m and W, and is independent of w (the angular speed)

Statement (2) is wrong.
since T.sin(theta) = mLw^2
i.e. W.sin(theta) = m.L.w^2
if angle (theta) is constant, L and w^2 are inversely proportional. L decreases with increase of w
Statement (3) is correct.

From (1), W = mg/cos(theta)
When angle (theta) increases, cos(theta) decreases, hence W increases.

2010-03-17 7:34 am
Resolving the force and you'll get 3 equations governing the whole motion,
Let T = tension of string,
T = W
Tcos(theta) = mg
Tsin(theta) = mw^2r
Here, you should note that there are four variables changing which will affect each other simutaneously, namely theta, w, T and r.
When w decreases, L must increases as T and theta must not be affected (because mg is constant)
(1) is wrong.
(2) is patently wrong with the third equation.
(3) is correct. When W increases, T increases, and as mg is constant,
cos (theta) must decreases, theta will hence increase.
Also, w and r may increase correspondingly.
參考: Myself


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