The energy of an object held at a height of h is equal to its gravitational potential energy, given by
[tex]U=mgh[/tex]
where m is the mass of the object, g is the gravitational acceleration and h is the height.
If we re-arrange the formula and we substitute the data of the problem, we can find the weight of the object, mg:
[tex]mg= \frac{U}{h}= \frac{300 J}{4.5 m}=66.7 N [/tex]
