拿一包15kg金象米走75m要用多少功?

The work done is zero.

The weight acts vertically downward but not horizontally. You walked 75 m in a horizontal direction.
Hence, work done = (15g) x (75).cos(90) J = 0 J
----------------------------
Your question: 走完覺得辛苦只是因為HEAT?

Answer: It is because of increased metabolism in your body. You need to move your body along with the packet of rice.

E=Fs
E=15*9.81*75
E=11036.25J