I could explain with formulas but I dont think thats what your after. Energy can neither be created nor destroyed, only transfered. This is considerably more complicated than just change of motion. On any <a href="http://eyesonff.com" style="color: rgb(255, 255, 255) ! important; text-decoration: none ! important;cursor:default ! important;">energy transfer</a> there is transfer of heat for example - but its not something somebody thinks about. I am not really sure how to explain it without mathematics. Consider throwing a baseball - your merely transferring energy to the ball giving it an <a href="http://eyesonff.com" style="color: rgb(255, 255, 255) ! important; text-decoration:none ! important;cursor:default ! important;">acceleration</a> and velocity while some energy is lost in the act of throwing due to friction as it is released from the hand etc - thus creating heat loss and inefficiency. No energy transfer is 100% efficient.