A boy has a mass of 55 kg.
He climbs 12 m up a tree.
What is his gain in GPE?

The formula is:
Change in GPE = mass × gravity × change in height

Entering our numbers:
Change in GPE = 55 × 10 × 12
= 6600 J
= 6·6 kJ

A kJ (kilo-joule) is 1000 J.
It's sensible to round off your answers in this way.

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