The gravitational force is one of which we are probably most familiar with in our daily life ; it is the force that keeps our feet firmly planted on the ground and the force which keeps the earth in orbit around the sun 黑洞的理论建基于万有引力定律。万有引力是我们最为熟悉的一种作用力，这种力使我们紧贴在地上，同时使地球环绕太阳运行。
