We've already seen the first one, which expresses Newton's first and second laws of motion. Again, this says that the force on a body equals its mass times the acceleration that will be experienced due to that force. The other expresses Newton's law of universal gravitation, the law that states that every body in the universe attracts every other body with a force that depends on the masses of the bodies, and on the distance between them. Where F G is the force of the earth on the body due to gravity, m E is the mass of the earth, G is the universal constant of gravitation, and r is the distance between the two bodies. This property of physical objects, which we have termed mass , determines both the acceleration of an object when we push it, and also determines the force of gravity.
Physical bodies have other properties, such as electric charge.
Electric charge gives rise to another force, called the electromagnetic force. As we all know, opposite charges attract and like charges repel each other. Again, I expect that if you're at all like me, you'll be deeply unimpressed by the preceding discussion. When I first heard of this unexplained connection between the mass of acceleration and the mass of gravity, it took me quite a while before I could fully appreciate that there even was an unexplained mystery. That's because my common sense continued to get in the way.