# Definition of Energy “Capacity to do work”

My school textbook states if a ball falls due to free fall, its weight will do work, which is transferred to Kinetic Energy when dropped from a certain height.

Hence, work done = energy transferred.

But then it says this is as 'energy is defined as the capacity to do work'.

What does the term 'capacity to do work' mean?

I am very confused! How does this relate to the subject of work done = force x distance moved in direction of the force?

Also, it says that if a 40N force is applied on a box that is moving on a rough surface at constant speed, the work done on the box is "transferred into thermal energy between the box and surface below" - but isn't some of the work done transferred into kinetic energy to keep the box moving?

• "Energy is the capacity to do work " is an extremely bad definition of energy – Lapmid Mar 2 '17 at 17:19
• @Sherlock That's not the best phrasing but it is essentially the definition that was used from the time the concept was settled until Emmy Noether provided a better one. And it is still the only definition available at the introductory level. – dmckee Mar 2 '17 at 17:57
• "How does this relate to the subject of work done = force x distance moved in direction of the force?" That is the reason the idea is not as trivial as it sounds. There is, however, nothing built into the two statements that tells you that these ideas together form the basis of a self-consistent framework for useful physical analysis: you have to do the math. Start with Newton's laws and kinematics, define kinetic energy, derive the work energy-theorem, think about conservative forces and use that to bootstrap new energies from the work energy theorem, and posit conservation. Then test. – dmckee Mar 2 '17 at 18:01