【題組】

7. Which of the following is wrong?

(A) Divide a uniform spring of spring constant k into two identical halves. The springconstant of each half is 2k.
(B) Fix one endpoint of a rope on the edge of a cliff, and release the other endpoint fromequal height. At the moment when the free endpoint falls a distance h below the other endpoint, its velocity v is smallerthan

for any h ＞ 0. (See Fig. B above.)
(C) Newton's

law is necessary for the momentum conservation ofphysical systems.
(D) Given arbitrary 3-dimensional vectors A, B, and C, the following identity bolds: A X(BXC =B(AㆍC) - C(AㆍB)
(E) The angular momentum of a point mass in linear motion at constant velocity can be non-zero.