[单元]
功与能
[来源]
96 中山 (9)
[题目]
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]
手边一个答案是说无解, 另外一个答案给(C)
前者的话, 我的想法是题目给的 fd 相乘为正值
然而不论是根据定力作功 W = F dot d,
或是根据功能原理 W_nc = (K2 + U2) - (K1 + U1)
其作功值, 总力学能变化值皆为负, 故不合。
然而后者若是不考虑正负, 只考虑其量值,
是否(A)也可以选?
考虑定力作功, W = -fd, 则 |-fd| = fd