physcis 有關 work done 問題,,唔該幫幫手

There is work done when a force is applied. If a force is not needed, then there is no work done.
Tp push a box forward by 8m, in theory, needs no force at all, unless there is friction on the surface where the box is placed.
Therefore, only a force is needed when the box is pushd up by 6m, which is equal to the gain in potnetial energy of the box.

向前8m 個work done :
w=Fs
=500x8
=4000J



F=weight=500x10=5000N
向上 6m 個work done:
w=Fs
=5000x6
=30000J

2008-12-27 19:18:09 補充：
so: w=PE=mgh

8^2+6^2=a^2(畢x定理)
a=10

W=FS
W=5000*10
W=50000J

work done (horizontal)=0 (no change in potential)

work done (vertical)=mgh
=500x10x6
=30000J

so total work done=30000J