His forerunner, Galileo Galilei, had contended that falling bodies and planetary motions had the same cause. But Newton was not the first to suspect that the same force caused both our weight and the motion of planets. Sir Isaac Newton was the first scientist to precisely define the gravitational force, and to show that it could explain both falling bodies and astronomical motions (see Figure 6.20). It is a force that acts at a distance, without physical contact, and is expressed by a formula that is valid everywhere in the universe, for masses and distances that vary from the tiny to the immense. It is the weakest of the four basic forces found in nature, and in some ways the least understood. Gravity is another example of underlying simplicity in nature. In fact, the same force causes planets to orbit the Sun, stars to orbit the center of the galaxy, and galaxies to cluster together. And the Moon orbits Earth because gravity is able to supply the necessary centripetal force at a distance of hundreds of millions of meters. An apple falls from a tree because of the same force acting a few meters above Earth's surface. Our feet are strained by supporting our weight-the force of Earth's gravity on us. What do aching feet, a falling apple, and the orbit of the Moon have in common? Each is caused by the gravitational force. The student is able to make claims about the force on an object due to the presence of other objects with the same property: mass and electric charge. 2.B.2.2 The student is able to approximate a numerical value of the gravitational field ( g) near the surface of an object from its radius and mass relative to those of Earth or other reference objects.To calculate the gravitational field due to an object with mass M, where the field is a vector directed toward the center of the object of mass M. The information presented in this section supports the following AP® learning objectives and science practices: Describe the gravitational effect of the Moon on Earth.By the end of this section, you will be able to do the following:
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