# Energy used to stop / slow an object

I'm trying to workout how much energy (if any), I use (imagining me as an efficient machine rather than a complex bio-mechanical human) when I lower or catch a weight. I understand that when I push it up, I have used energy to do work as I have the force of the object (it's mass and gravity) and distance. So I've got two similar questions...

If I am to stop a free-falling object, how much energy do I use? If I look at it from energy change I believe I need to use the same amount as the kinetic energy of the object at the moment before I catch?

If I slowly lower an object how much energy do I use? In an ideal world would the total be the same as in the free falling situation, assuming the height at which the object fell was the same.

• You can't answer your question by assuming an efficient machine, because the effect you're asking about is specifically a property of complex biomechanical systems. An efficient machine would actually gain energy by catching and lowering a falling weight. The fact that such an act is tiring for a human is exactly that we are not built like efficient machines. Sep 19, 2012 at 2:18

• Yes. If the object of mass $m$ rises a distance $h$ then the work you've done on the object is equal to the increase in it's potential energy i.e. $mgh$. The energy you expend to do this is a lot greater than $mgh$ because human muscles aren't particularly efficient. I think muscle efficiency is around 15-20%. Sep 19, 2012 at 14:33