[單元]
功與能
[來源]
96 中山 (9)
[題目]
As a block slides a distance d down an incline, the incline
exerts a constant frictional force of magnitude f on the block.
The quantify fd gives the magnitude of:
(A) The work done by the frictional force
(B) The change in the internal energy of the block
(C) The change in the internal energy of the block-incline system
(D) The change in the mechanical energy of the block-earth system
(E) The change in the kinetic energy of the block
[想法]
手邊一個答案是說無解, 另外一個答案給(C)
前者的話, 我的想法是題目給的 fd 相乘為正值
然而不論是根據定力作功 W = F dot d,
或是根據功能原理 W_nc = (K2 + U2) - (K1 + U1)
其作功值, 總力學能變化值皆為負, 故不合。
然而後者若是不考慮正負, 只考慮其量值,
是否(A)也可以選?
考慮定力作功, W = -fd, 則 |-fd| = fd