Potential Energy (PE) is found using the equation mgh, where
m = mass,
g = force of gravity,
h = height
Since we are using the metric system, our force due to gravity will be 9.8m/s^2. Plug in what we know, and we get
(1200kg)(45m)(9.8m/s^2)=PE
Multiply those three variables together, and you will get your potential energy, in Joules! Good luck!
