How much work can a 5 kg mass do if it is 6 m above the ground?

Answer 1

Its gravitational potential energy is (mass) x (gravity) x (height) = (5 x 9.8 x 6) = 294 joules.
That's the amount of work that gravity will do to that mass in pulling it to the ground, and
if you couple it to something else by some suitable mechanical arrangement, you can re-direct
a large part of that gravitational energy to accomplish some useful task for you.

(I say "a large part of that ... energy" rather than all of it, because no mechanical
arrangement is going to be 100% efficient, and some of the energy is going to be
lost in the levers and gears.)

Before we leave the subject, it's worth considering where that energy came from ...
how it got packed into the 5kg mass in the first place. It turns out that the 294 joules
is the work that YOU had to do, against gravity, to raise that 5kg up 6 m off the ground.
So if the task you need done involves some time flexibility, you might just as well use
your own work and energy to do the task directly and get it over with, because you're
going to lose a good bit of it in the process of lifting the 5kg and then using the fall of
the 5kg to do the task. In our real world, moving energy from one place to another,
or changing it from one form to another for storage, ALWAYS incurs some loss.

Answer 2

Use the formula for gravitational potential energy: GPE = mgh (i.e., mass x gravity x height). The gravity is ca. 9.8 newton/kilogram.