## Newton's Laws of Motion and Gravity

A cartoon of the tale of how Isaac Newton discovered gravity.

Newton's most well-known work is his laws on motion and gravity. Yet, Newton’s work built upon Galileo’s previous work with vertical and horizontal motion. He synthesized Galileo’s ideas with his own to dictate the three laws of motion.

1. An object in motion stays in motion, and an object at rest stays at rest, unless acted upon by an outside force.

2. The acceleration of an object multiplied by the mass of the object is equal the force applied to it.

3. For every action there is an equal and opposite reaction.

Isaac Newton's Laws of Motion defined how objects move on Earth. But he didn’t stop there. The story goes that Newton was sitting under an apple tree and an apple hit him on the head. Then he suddenly formulated the law of gravity. This is sort of true, as Newton did propose that the forces acting on the apple must also be acting on him and everything on Earth. Why not the moon? he thought. This notion, along with his cannon on a mountain, laid the foundation for Newton to mathematically prove gravity. The cannon on a mountain was Newton’s visual representation for how the moon orbited the Earth. Basically, a cannonball projected at the right height and at the right speed would continue falling around the earth, due to the Earth's curvature, and thus fall into orbit around the Earth. Newton was prompted to continue this work by Edmond Halley. When Halley visited Newton and inquired about the mathematical proof for the orbit of the planets, Newton redid his calculations. He devoted the next three months of his life to writing the

1. An object in motion stays in motion, and an object at rest stays at rest, unless acted upon by an outside force.

2. The acceleration of an object multiplied by the mass of the object is equal the force applied to it.

3. For every action there is an equal and opposite reaction.

Isaac Newton's Laws of Motion defined how objects move on Earth. But he didn’t stop there. The story goes that Newton was sitting under an apple tree and an apple hit him on the head. Then he suddenly formulated the law of gravity. This is sort of true, as Newton did propose that the forces acting on the apple must also be acting on him and everything on Earth. Why not the moon? he thought. This notion, along with his cannon on a mountain, laid the foundation for Newton to mathematically prove gravity. The cannon on a mountain was Newton’s visual representation for how the moon orbited the Earth. Basically, a cannonball projected at the right height and at the right speed would continue falling around the earth, due to the Earth's curvature, and thus fall into orbit around the Earth. Newton was prompted to continue this work by Edmond Halley. When Halley visited Newton and inquired about the mathematical proof for the orbit of the planets, Newton redid his calculations. He devoted the next three months of his life to writing the

*Principia,*which completely changed man’s view of the universe. In it, Newton outlined his three laws of motion and the theory of gravitation."If the Earth is attracting the Moon gravitationally with a certain force holding it in its orbit, then the Moon is attracting the Earth with an equal force. So why isn’t the Earth going around the Moon? The answer is that the masses are so different. The Earth’s mass is more than one hundred times that of the Moon. Consequently, the Earth’s acceleration, “falling” towards the Moon, is very small. What actually happens is that they both circle around a balance point between them, which in fact lies within the Earth. This motion of the Earth is easily detectable with instruments, but tiny compared with the daily rotation. Of course, it also follows from the above considerations that since the Earth is attracting you downwards with a force equal to your weight, you are attracting the Earth upwards—towards you—with a force of exactly the same strength."

―Michael Fowler, Physics Dept., U.Va.

―Michael Fowler, Physics Dept., U.Va.

To see more of Michael Fowler's biography of Isaac Newton, click here.