The reason, plain and simple, is that the Earth bulges slightly at the Equator and all places there are nearly 13 kilometers farther away from the planet’s center, i.e. center of its gravity. As you move away from it, the force of gravity is reduced in inverse proportion. A 75 kilogram person living in London would weigh 250 grams less in Singapore.
Human weight (Wikipedia)
- Do things weigh heavier or lighter when hot or cold?
- Why our height increases a little during night’s sleep?
- Why the composition of atmosphere is so specific and steady with 21% oxygen and 78% nitrogen?
- Why aren’t people at the equator thrown off into space by centrifugal force since the Earth is rotating at a high speed?