F.6 physics question

2010-10-14 5:31 am

A student performinga centripetal force experiment whirls a rubber bung attached to one end of a string which passes through a glass tube with smooth openings ,and has a weight W hanging at its other end.The weight of the rubber bung is much samller than W.The rubber bung is set into a horizontal uniform circular motion wihth angular speed w whicle the length of the string beyond the upper opening of the glass tube is L and this portion of the string makes an angle threatre with the vertical as shown.Which of the following statements is/are correat?
(1) L is kept constant, threatre will decrease with w.]
Why is not (1) correct?Would you please explain briefly?

回答 (1)

天同

2010-10-14 11:31 pm

✔ 最佳答案

Let T be the tension in the string. Since the weight W is in equilibrium, we have,
T = W
Consider the horizontal component of tension T,
T.sin(theta) = m.(Lsin(theta)).w^2
where m is the mass of the rubber bung
i.e. T = mL.w^2

But T = W, hence
W = mL.w^2
i.e. w^2 = W/mL

If L is kept constant, any change of w doesn't affect (theta).

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