Before I go into this topic, for the benefit of people who does not know the difference between weight and mass, I am going to talk about them. Mass and weight meant the same in daily life, but, in science they are different. Weight means how strong gravity pulls on that matter while mass means the amount of matter that item have. Also weight is measured in Newtons (N) while mass in measured in kilograms (kg), grams (g) etc. An example is that: Person A have a mass of 50 kilograms and weighs 491 newtons on the surface of the Earth; on the surface of the Moon, Person A still has a mass of 50 kilograms but weighs only 81.5 newtons. This is the difference of weight and mass.
Now I am going my topic. We will be lighter at the equator is because of the gravitational acceleration. The cause of the difference of gravitational acceleration is due to the planet's rotation around its axis. The gravitational acceleration is less at the equator than at the poles. At the equator, the true gravitational acceleration is 9.8144 m/s² . At the poles, the gravitational acceleration is 9.8322 m/s². The difference of 0.0178 m/s² between the gravitational acceleration at the poles and the true gravitational acceleration at the equator is because objects located on the equator are about 21 kilometers further away from the center of mass of the Earth than at the poles, which corresponds to a smaller gravitational acceleration. Thus we will weigh less at the equator than at the poles.
Another factor is that the earth is non-spherical and thus the shape of the earth will be like a little "squashed". This thus will affect the amount of gravitational force on the poles and at the equator as the further the place is from the center of gravity, the lesser the amount of gravity pulling the items on the place down.
In summary, there are two contributions to the fact that the effective gravitational acceleration is less strong at the equator than at the poles. About 70 percent of the difference is contributed by the fact that there is a difference in the gravitational acceleration, and about 30 percent is due to the non-spherical shape of the Earth.
I have come to the end of this topic. Thank you and good bye!
Aloysius Chan
Websites I went to get the infomation: http://en.wikipedia.org/wiki/Equatorial_bulge
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