Worked examples on potential energy
Two problems with the same three quantities. The second one gives the mass in grams, which is the only real difficulty in either.
Example 1
The problem
Find the gravitational potential energy of an object of mass 7.5 kg at a height of 4 m.
gravitational potential energy = m g h
= 7.5 kg × 10 m s-2 × 4 m
= 300 J
Example 2
The problem
A fruit on a tree has a mass of 200 g. The vertical height from the ground to that fruit is 4 m. Find the gravitational potential energy of the fruit on the tree.
Figure to be added
mass of the fruit = 200 g = 0.2 kg
gravitational potential energy = m g h
= 0.2 kg × 10 m s-2 × 4 m
= 8 J
Grams have to become kilograms first
200 g is 0.2 kg — divide by 1000. Put 200 straight into the formula and you will get 8000 J, a thousand times too large, and nothing in the answer will look obviously wrong.
The height is measured from where it would land
h is the height above the level the object would fall to — the ground, in both of these. Potential energy is always stated relative to some level, and naming that level is part of stating the answer.