We know that the earth is spherical like an orange. It may be a great surprise to think how people can stay on the spherical earth. Nepal and America are located just nearly opposite on the surface of the earth. A person living in Nepal will have their head directed towards the sky while standing on their feet on the surface. Likewise, a person living in America, just opposite to Nepal, will have their head faced towards the sky too while standing on their feet on the surface. The reason behind this effect is that the earth pulls both of them towards the centre of the earth with a force. This is called the force of the gravity of the earth.
On the basis of Newton's universal law of gravitation, the gravity of a planet or a satellite can be calculated.