To today’s students, Newton’s Laws of Mo tion are familiar and may seem easy to understand. But many educated people in 1687 still had medieval notions. They believed that planets and stars moved under their own power. They had no concept of gravity. They did not understand that “what goes up must come down.” Nor could they grasp how objects in outer space follow the same laws that keep our feet planted firmly on Earth.Scientists today call Newton’s view of things the “mechanistic universe.” Descartes had also suggested that the universe operates like a giant machine, but Newton disagreed with Descartes’ claim that planets travel in whirlpools of vortices.
Newton wrote that gravity, not vortices, makes all objects- large and small, near and far-cooperate in machine-like perfection.Newton compared the universe to the mechanism of a huge clock. Clocks work in ways that can be measured and counted. Newton declared that gravity makes the universe work in the same manner. Gravity works far out in space in exactly the same way it works on Earth. Isaac Newton had de veloped the theory of universal gravitation.
Newton used the moon as the example for his claims about gravity. The moon was sim ple to observe, its orbit easy to track every 28 days. He applied his laws of motion to the moon’s orbit in this way:The moon is always falling down toward the earth (Law of Gravity) but The moon is always traveling in a straight line due to inertia (First Law of Motion)therefore The moon follows an orbit around the earth.Just as he had with optics, Newton used experiments to test his ideas about gravity. The Principia devoted much space to these explanations, but the book was nearly im possible to understand.
Except for a handful, even the most learned members of the Royal Society could not begin to grasp its ideas. Back in Cambridge, as Newton took a walk on the street one day, one student was heard to tell another, “There goes the man that has writt a book that neither he nor any body else understands.”moon’s orbit around Earth When Newton was an old man, he of fered a simpler story to a dinner guest about how he came upon the idea of gravity.
It all happened, Newton recalled, when he was a young man hiding from the plague at home in Woolsthorpe.Newton explained his discovery. Out- doors, he had watched an apple fall from a tree to the ground. As always, the apple fell down. Newton sensed that whatever pulled the apple downward also pulls the moon to- ward the earth. The apple was much closer, so the earth’s pull was much stronger on the apple than on the faraway moon. But apple or moon, the earth exerted a force on each.
Newton called this force “gravity.”Newton tested his thinking about gravity like this:1. He knew the apple bad fallen just 16 feet from the tree to the ground.2. He knew how far it is from the ground to the earth’s center.3. He also knew how far the moon is from the earth’s center-60 times farther than the apple tree was.But only one law must apply to both. And that, Newton declared, would be the inverse square law:Force (“d” stands for distance)Thus, Newton calculated, Earth’s gravita- tional pull on the apple is 60² 60 x 60- or 3,600 times stronger than its pull on the moon.
Newton well might have made up his tale about gravity to amuse his guest. Tall tale or not, the notion of an apple falling toward the center of the earth offered his guest a clear picture of how gravity works.In the Principia, Newton applied his ideas about gravity to other natural events such as the paths of comets. This notion excit ed astronomers like Flamsteed and Halley, who used Newton’s mathematics to predict the return of a giant comet that had swept through the skies in 1680. Newton also used his calculations to show how the moon’s gravity tugs at the earth to create the rise and fall of tides on oceans everywhere.
By the 1700s, during the Enlightenment, better instruments allowed natural philos ophers to make enormous progress in sci entific discoveries. They were able to test Newton’s ideas and confirm that Newton was correct.Then, in the early 1900s, an unknown pat ent clerk named Albert Einstein put forth his own ideas about the nature of the universe. Einstein asked about what happens to objects as they travel near the speed of light. Ein stein also asked about the motion of particles that are smaller than atoms and molecules.
Unlike the natural philosophers of Newton’s Time’s, physicists in the later 1900s had the benefit of instruments to test Einstein’s ideas for themselves.Like Newton’s ideas, Einstein’s theories shook the world. But even to this day, no one has found a way to fully explain gravity.Thinking About Force in school, you have no doubt learned about various kinds of forces, gravity, friction, magnetism, and so on all observable forces. Newton’s work dealt with things he could observe with the instruments he had avail- able to him in the 1600s.
Sometimes Newton did “thought experiments” in his head. In the next few pages, you can think about forces and do a few simple physical and mental ex- ercises to help you understand them.Feel the ForceFORCES are simply the constant pushing or pulling on things everywhere. Hook one end of a rubber band to a stationary object, such as a knob on a closed door. Pull on the other end of the rubber band.It might seem that your right arm is doing all the pulling, but the rubber band is pulling back, too.
A force can feel like a pull, but it can feel like a push, as well. And that con- tinuous push/pull tells us another fact about force: forces always work in pairs. If a soccer goalie blocks a ball with his chest, the goalie is pushing the ball away.GravityDID you ever watch a baby sit in a highchair and drop things to the floor? You might get annoyed at having to pick them up, but for the baby, the process is fascinating. No mat ter what that baby pushes off the tray, ob jects always fall down.Sure, you say, that’s because gravity pulls them in that direction. Remember though, there are two forces at play.
For instance, are you holding this book up as you sit at a desk? The book would fall down onto the desktop, but your hands are pulling it up. Are you slouched back in your chair? If the back of the chair didn’t push you up, you’d fall over backward.A similar process occurs when you stand on the earth. Your body pulls the earth to ward you and the earth pulls back, although you cannot feel this at all. Why not?
Let’s say that you weigh 100 pounds. A 100-pound mass of you is pulling on the earth. However, the entire mass of the earth is pulling on you, always and all at once. In other words, Earth’s gravity is the force that pulls you down. But at the same time, your body makes its own tiny tug to pull the earth toward you.