Work done concept 一問 10分

WD = energy
energy can change the state of motion of a body
eg. increase/decrease the velocity and bring it to rest

(I) consider the upward motion
gravity "takes away" the K.E. of the body
acutally you are right
the total mechanical energy of the body is consered
KE + PE= constant
W.D. by the gravity is actually the gain in PE in this case

(II) consider the downward motion
the body is momentarily at rest at its highest postion
v=0. KE=0
the gain in PE can be explained in 2 ways:
1 PE is converted into KE
2 the gain in KE is actually the WD by the gravity


ombining the two cases, WD by the gravity change the velocity of the body from v to -v.

note that gravity is a FORCE, not energy, it is not WD
WD is considered to be ENERGY.
you may say WD by the gravity, but not gravity is the WD.