Newtons 3 Law of Motion & Universal Gravity

Sir Isaac Newton January 4, 1643 - March 31 1727

  • The first law of motion is often times referred to as, The Law of Inertia. The first law states,
      • "An object at rest will stay at rest unless acted upon by an unbalanced force. An object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force." [1]

  • Newton's second law of motion further explained that force action upon an object causes the object to accelerate. Isaac Newton devised a formula to calculate force, acceleration, and the mass of an object in the famous formula,

Sir Isaac Newton is also credited for the discovery of his theory of Universal Gravity, not gravity itself. Newton could not have discovered the theory of universal gravity without the help of Tycho Brahe teaming up with Johannes Kepler. Sir Isaac Newton also contributed help from Robert Hooke. Newton simply took ideas from these three men and combined them with his own knowledge to form his theory of Universal gravitation.

M1 is the mass of the first object and M2 is the mass of the second object. D is the distance between their centers of mass. The greater the masses of m1 and m2, the greater the force of attraction between them are. The greater the distance between the objects, the weaker the force of attraction will be.

Here is a video for a visual link that covers Sir Isaac Newtons three laws of motion:

First Law of Motion

The word “Inertia” means "amount of resistance to change in velocity". The law of inertia allows a person to predict the movement of any object. You know that if an object is at rest then it will desire to continue at rest. If an object is in motion you are able to predict that the object will crave to continue in motion. Although Sir Isaac Newton perfected the law of inertia, Galileo Galilei formulated the concepts in which Newton was able to expand upon. Galileo could not have done all his work without the help of Aristotle. Newton expanded Galileo’s ideas who in turn expanded on Aristotle’s ideas.

Before Sir Isaac Newton presented his Laws of Motion in the “Principia Mathematica Philosophiae Naturalis”, Galileo first formulated the concepts of inertia and acceleration due to the effects of gravity off of Aristotle who distinguished two types of motion.

It started in the 5th century when Greeks, such as, Aristotle, studied motion. Aristotle then divided motion in two different types of categories. The first being natural motion and the second being violent motion. Aristotle believed that natural motion on earth was to be either straight up or straight down. He came to think it was "natural" for heavy objects such as a rock or a cannonball to naturally seek the ground, or fall towards the earth. While lighter objects such as steam and fire seem to naturally rise or float up. Violent motion on the other hand is imposed motion. Violent motion is caused by external force acting on an object already in its natural state. It is the result of an object being pushed or pulled. Aristotle found that objects in their natural resting areas could not move anymore by themselves unless pushed or pulled by an external force.

Nicolas Copernicus formulated that the earth was not the center of the universe in the sixteenth century. At the time Copernicus was thought as a lunatic for his observations and his theories unfathomable. Copernicus believed that the Earth along with the other known planets at the time revolved around the Sun. To avoid persecution he theorized privately until his death. He sent his ideas to "De Revolutionibus" on May 24, 1543, the day of his death.

Galileo supported Copernicus belief about the Sun being the center of the universe as opposed to the Earth being the center. Galileo demolished the idea that a force was necessary to keep an object in motion. Galileo found the idea of friction. Friction is a force that acts between two objects that touch as they move past each other. Friction is only willingly to work as hard as it needs to. Every subject creates some sort of friction no matter how little the friction may be. Friction is caused by microscopic irregularities in the surfaces of the objects touching that slow down an objects motion. If we were to rid two surfaces of these irregularities, friction would then be absent, and the object in motion would absolutely need no outside force to continue to stay in motion. Galileo knew that if there is friction an outside force would be needed to keep an object in motion.

He tested his theories of friction by rolling balls on different inclines. He noted
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that a ball rolling down a slope picks up more speed because it is working alongside with gravity. If a ball is rolling up an inclined plane in a direction opposite of gravity then the ball will slow down. He also found that if a ball rolling on a flat-level surface does not roll with or against gravity. He found that a ball rolling on a horizontal plane has virtually constant velocity. Galileo figured if friction were to be absent the ball would have perpetual motion without a needed outside force to keep it in motion. He found the smoother the surface was the weaker the friction would be. If you have little friction the longer the ball will roll. Although if there were to be no friction, motion would be endless, it is essential to everyday life. Without friction simply walking would be a difficult task to accomplish. As a person walks the persons legs pushes against the floor causing friction. Friction against different objects varies. There is more friction to stop a car crashing into a concrete median than a steel median. The rubber tires rubbing against the concrete median will slow the car down faster than opposed to a rubber tire rubbing against a steel median.

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Friction does not only come through solids but fluids and air resistance too. It is much harder to run through water than it is to run on

Fluid friction occurs as an object pushes aside the fluid it is trying moving through. Air resistance, which is friction acting on something moving or cutting through air, is a common form of friction. Let's say a person rides a bike against the wind, he would find himself working harder to push the pedals and accelerate forward as opposed to riding with the wind.

Galileo's conclusion was further supported by another line of reasoning. He described two inclined planes facing each other, would reach the same nearly the same height. If friction were absent then the ball would meet at the exact same height. He noted that the ball tended to attain the same height. Additional reductions of the angle of the upward plane gave the exact same results. With this experiment he found that the ball always went farther and tended to reach the same height. Galileo then ran into a question. What if the angle of the incline of the second plane was horizontal and even? How far would the ball roll until it stopped? He realized that only friction would keep the ball from perpetual motion. Galileo stated that this tendency of a moving body to want to keep moving is natural and that every material object resists changes to its state of motion. We now call this the law of inertia! Galileo’s findings about motion and his new concept of inertia discredited Aristotle's theory of motion.

Without any force, an object at rest stays at rest and an object in motion will continue in motion. In the absence of a net force objects do not change their state of motion. The combination of all forces action on an object is called the net force. The net force is what changes an object's state of motion from resting to moving or moving to resting.

Although many of Aristotle's ideas about the causes of motion were false, it prompted physicist such as Sir Isaac Newton to figure out "why”

Newton simply restated Galileo's idea of inertia by stating:

      • "Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it. Objects want to stay in their natural state."

For example, a person is driving a car down a country road. As the person is driving around fifty-five miles per hour a deer ventures out onto the road. The person, to avoid hitting the deer quickly steps on the brakes. The person’s body lurches forward as his seat belt constrains him. The person lurched forward because he was in motion and wanted to continue in motion with the car but the car abruptly stopped.

Another example of Newton’s law of inertia can be experienced on a plane ride. As a passenger is seated down and the plane begins to ascend, the passenger is then pushed back into the seat. The passenger was at rest therefore the passenger wants to stay in rest but the plane is moving. Because the plane is moving and the passenger wants to stay at rest he is then pushed back and then later becomes part of the man-plane machine that are moving together.

Mass is a measure of the inertia of an object. The more mass an object obtains, the greater its inertia and the more force it will take to change its state of motion. Mass is the quantity of matter in an object. Do not confused mass for weight though. Weight is the force of gravity on an object.

A person on earth has the same amount of mass on the moon. Yet the weight of the person on earth is greater than the moon because the moon has a lower gravitational pull.

Let's say a stapler is sitting on top of a desk. What net forces are acting on the
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stapler? The answer is not simply gravity. If the only force acting on the stapler was gravity the object would be in free fall. The fact that the stapler is at rest is evidence that another force must be action on it. This force must exactly balance the staplers’ weight and produce a net force of zero. There is a force that supports the force of the table. The support force is often referred to as the normal force. The table pushes up on the stapler with a force equal to the staplers’ weight. When an object is at rest, with the net force on it being zero, the object is in a state of equilibrium. When an object is at rest or moving without acceleration, or a constant velocity, the object is said to be in a state of equilibrium. All the forces that are applied to the object are canceled out by each other. This means all the vectors such as horizontal and vertical forces are balanced out.

An example of an object at equilibrium is a ladder leaning up against a wall. The wall pushes back the ladder with the same amount of force as the ladder in order to support its weight. In this instance the step in the ladder would also match the force of a person climbing up the ladder. Balance plays a major form of equilibrium because it is the ability of an object to support its center of mass.

In this video above, When the men and women are punched by the boxer you will notice that their face wants to stay in place yet they are forced to move by an outside force which is the boxer.

Second Law of Motion

Acceleration is how quickly motion is able to change. It is the change in velocity per certain time interval. The cause of acceleration is force. Acceleration depends on the mass being pushed or pulls by an external force. For example, push on an empty shopping cart. Then push equally as hard on a very full and heavy shopping cart. The loaded shopping cart will accelerate much less than the lighter and empty shopping cart. The same force applied to two times as much mass result in about half the acceleration.

Sir Isaac Newton was one of the first to realize that acceleration produced when we move something depends not only on how hard someone pushes or pulls on an object but also the objects mass in which you're pushing or pulling on. Newton then found his second law of motion. The simplest way to convey his second law is by an equation. Force equals mass multiplied by acceleration. The unit in which force is under is Newton’s or (N). The actual SI units that make up a Newton is kg/m/s^2. A Kilogram is the unit for mass while meters per second squared for acceleration.

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Galileo showed that falling objects accelerate equally, regardless of their mass. This statement is true when there is little or no air resistance compared with the mass of the falling object. For example, a 10-kg ball and a 1-kg stone are dropped at the same time from on top of a building. Which one will reach the ground first? The two balls will fall together and strike the ground at practically the same time. Galileo demonstrated his idea of free fall on the leaning tower of Pisa. Galileo again demolished one of Aristotle’s ideas that an object that weighs more than another should fall faster than the lighter object. Although Galileo's assumptions were true he couldn't explain why the accelerations were equal. This is where Newton steps in with his second law. Newton goes on to explain with his second law that the 10-kg ball and the 1-kg ball will both produce the same acceleration. Why? The answer is simply because there is more force acting on the bigger ball so it lags behind. When he did let go of the two objects the heavier object did hit the ground first. Although the heavier cannon hit the ground first it was only by a split second. That split second was due to air resistance.

If you place a penny and a feather in a vacuum, both will fall with equal accelerations. Now let's let a little air in. If you drop the penny and the feather from the exact same height the penny will still fall quickly to the bottom because the net force only decreases a tiny bit while the feather flutters to the bottom because of air resistance. Air resistance diminishes the net forces acting on the object. When the air resistance on the feather is equal to its weight, the net force is thus zero. When Acceleration can no longer occur, the object has reached its terminal speed or terminal velocity when the object is in free fall.

A heavier person will experience a greater terminal speed than a lighter person would. The greater the weight is, the more effective in cutting
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through the air. Body orientation also makes a huge difference. If your body is sprawled out, the air is more likely to encounter the body because the surface area is increased. If your body is sticking straight your terminal velocity increases because you're able to "cut" through the air quicker.

Place a book standing from one of its corner edges on your hands and it digs into your palm. Hold a book flat on its back and there is less pressure on your hand. The force of gravity pulls the book down and creates the pressure you feel on your hand. No matter how you hold the book, whether by the corner or flat on its back, it will always have the same weight or applied force. The book may feel heavier or lighter based on how you hold the book because the weight may be spread out on your hand or at a single point in contact. If the contact area is smaller, the book would cause more pressure on that single area of your hand, which causes the book to feel heavier when actually it weights the same as it would if you held it flat on your hand. Pressure and force are not the same concept. Pressure is quantity of force per contact area.

Third Law of Motion

Sir Isaac Newton last law of motion and his third law, commonly known as the "Law of Reciprocal Action" are easily stated as, for every action there is an equal or opposite reaction. In Newton’s third law we must remember the Law of conservation of energy that states, “Energy is neither created nor destroyed" Energy is simply converted in various ways such as heat and noise. The interaction between two objects is always involved in some type of force. There are two equations to convey this:

      • “FB = - FA
      • FA + FB = 0”

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Picture a hammer striking a nail into a piece of wood. The hammer exerts force onto the nail while the nail exerts a force on the hammer. This is what stops the hammer from going all the way through the nail and straight through the wood. By the direction of which the nail penetrates, you could tell that the hammer exerts a stronger force. Energy is converted in this action and reaction by the noise the hammer-nail make. This law applies forces in mass. Although the nail is smaller than the hammer, it exerts a reaction force equal to the mass of the hammers force.

In Newton’s third law of reciprocal action, he found that a force is not a thing in itself but part of a mutual action, an interaction between two objects. His third law of action and reaction or:

      • “Whenever one object exerts a force on a second object the second object exerts and equal or opposite force on the first object.”

He found two forces, Action and reaction force. These two actions are equal in strength and opposite in direction. Another example to help identify action and reaction is a boy skateboarding. When he pushes himself forward, he exerts a force on the ground in the opposite direction in the way he wants to move. In the same sense, the ground also pushes against him and the skateboard-man machine to propel him in the opposite direction.

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Action and Reaction has to do a lot about differences of mass. On a big scale, let's say you jump up. You push the Earth down and in turn the Earth pushes you up! Why can't you see theEarth push you up? Why can you see yourself move up? Your mass compared to the earth is immensely smaller, very much so smaller. Although the Earth does move Earth's acceleration is negligible. Because Earth has a huge mass, we don't sense it's infinitesimally small acceleration. With Newton’s third law we can understand how a helicopter gets its lift. The helicopters blades are shaped to force air downwards, and the air forces the blades upwards. Which one is action and reaction? This upward reaction force is called lift. When the lift equals the weight of the helicopter, the helicopter hovers in place. When the lift is greater than the weight, the helicopter ascends. The way wings are enhanced affects the lift.

The most difficult question about Sir Isaac Newton’s third law is simply, do the action and reaction cancel each other out. Action and reaction forces can never cancel each other out at all. Because of the two forces' characteristic of having opposite directions to each other, they do cause the object in the system to stay at rest in some instances. For example, a person pushing on the wall is not in motion because the force of that person is pushed back by the wall pushing back in the opposite direction. It is possible for an action force to seem to cancel with another action force.

Another example may appear while playing a game of soccer. Joe kicks the ball, so he exerts a force on the ball and the ball accelerates forward. The ball then comes in contact with Jeff’s foot. The wall then will have exerted a force on Jeff’s foot which in turn, stops the balls acceleration. Later, the two boys kick the ball at the exact same time but in opposite direction of each other’s forces. The reaction from the ball would still push back on both Joes and Jeff’s foot so there would be no sort of acceleration. The ball then would stay motionless.

Universal Gravity

Sir Isaac Newton is credited for the discovery of Universal gravity, not gravity itself. Newton could not have discovered the laws of universal gravity without the help of Tycho Brahe teaming up with Johannes Kepler also with contribution from Robert Hooke. Newton simply took ideas from these three men and combined them with his own to form the law of universal gravity.

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It all started with an apple. According to a popular legend, this idea struck Newton while he was sitting underneath an apple tree on his mother's farm pondering the forces of nature and why it works. Newton understood the concept of inertia developed by Galileo in Newton’s law of motions. Newton knew that without any outside forces, such as friction, moving objects will continue to move at a constant speed in a straight vector. Newton also knew that if an object changes speed or direction, there is a force responsible. Newton knew that circular motion is accelerated motion, which requires some sort of force. But what was this unknown force? Newton had the insight to see that the moon is falling towards Earth, just as the apple falls down towards Earth. The two objects are pulled by Earth's gravity. Newton then realized that the apple falling down towards Earth is the same force that causes the moons circular path around Earth.

Newton went on to compare the apple falling from the tree to the “falling” moon. Newton knew that if the moon did not orbit around Earth then it would fly off in a tangent with its orbit. Newton hypothesized that the moon was simply an object circling around Earth under the attraction of gravity as opposed to the moon revolving around the Earth to a cannon ball that is fired from a mountain that is above Earth’s atmosphere. If the cannonball were to be shot with a small horizontal speed it would follow a parabolic path and eventually hit the Earth. Although, If the cannon ball was fired a bit faster, its path would curve less and hit the Earth farther away. Newton then went on to reason that if the cannonball was fired fast enough, it would circle around Earth just as the moon does. The cannon ball would then be in orbit, just like the moon.

The moon does not fall towards Earth because of tangential velocity. The tangential velocity is sufficient to ensure the circular motion around Earth instead of falling down into the Earth like an cannonball. Both the moon and the cannon ball have a component velocity exactly parallel to Earth’s crust.

Newton must now test his theory for his hypothesis to evolve into a scientific theory. Newton’s test was to see if the moon would fall beneath its otherwise straight path was in correct proportion of an apple falling down and hitting Newton on top of his head. Newton stated that the mass of the moon should not affect how fast it would fall because mass has no effect at all on free falling object. How far the moon will fall and how far the apple will fall, should relate only to their distance from Earth’s center. If the distance of the fall for the moon and the apple are proportional to each other, then the hypothesis for universal gravitation is correct and should be taken seriously.

By using geometry, Newton was able to calculate how far the circle of the moon’s orbit lies below the straight distance that the moon would travel. Later scientists were able to prove that Sir Isaac Newton’s calculations turned out to be around 1.4-mm of today’s accepted measurement. Newton was unsure of the distance between the Earth and the moon. He couldn’t figure out whether or not to use the distance between their centers or their surface. Because Newton was uncertain about this, he put off his theories of gravity for twenty years.

Copernican theory of the solar system was proved true by Newton's theory of gravitation. Copernicus’s theory will no longer be made fun of. Earth is not even the center of the solar system. The sun occupies the center of the solar system. Earth, along with many other planets simply orbit around the Sun. The planets don’t crash into the sun because they have tangential velocities. If the planets tangential velocity were obliterated their motion would be heading straight towards the Sun. Any objects in the solar system without sufficient tangential velocities have long ago crashed into the sun.

Sir Isaac Newton discovered the law of universal gravitation that states every object attracts every other object with a force that for any two objects is directly proportional to the mass of each object. Newton did not discover gravity at all but simply universal gravitation. The greater the masses, the greater the force of attraction between the two objects will be. Newton concluded that the force does decrease as the square distance between the center of mass of each object increases. There will be less attraction between two objects the farther away they are from each other. Newton devised a symbolic expression for his law of universal gravity:

      • F= m1m2 /d

M1 is the mass of the first object and M2 is the mass of the second object. D is the distance between their centers of mass. The larger the distance between the two objects are, the weaker the force of attraction will be.

The proportionality form of the theory of universal gravitation can be illustrated as an equation when the constant of proportionality G is introduced. Big G stands for universal gravitational constant. Then the equation transforms into

      • F ~ G( m1m2 /d2)

The force of gravity between two objects is found by multiplying their masses, dividing by the square of the distance between their centers, and then multiplying the result by the constant of G. The magnitude of G is given by the magnitude of the forces between the two masses of 1 kilogram each. This is an extremely weak force.

Gravity was first measured 150 years after Newton's discovery of universal gravity by Henry Cavendish. Cavendish accomplished this fete by measuring the small force between lead masses with an tremendously sensitive torsion balance. Later Philipp Von Jolly attached a spherical flask of mercury to one arm of a sensitive balance, an easier way than Cavendish. After the balance was put in equilibrium, a 60ton lead sphere was rolled beneath the flask. The flask was slightly pulled downward. The gravitational force F between the lead mass and the flask of mercury was equal to the weight that had to be placed on the opposite end of the balance to reach equilibrium.

The greater the distance from Earth's center, the less an object will weigh. Weight is gravity acting on an object. No matter how far the distance, Earth's gravity will not drop to zero. Knowing these two facts, and noting that 3,600 = 60 x 60, led Newton to his famous inverse square law:
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      • “The force of gravitational attraction between two bodies decreases with increasing distance between them as the inverse of the square of that distance, so if the distance is doubled, the force is down by a factor of four.”

The circle which consisted of fixed stars and fixed planets were made by God, perfect. Everything inside the orbit of the Moon was to be of the earthly circle. Tycho Brahe, born in 1546-1601 disagreed with this theory.

Nicholaus Copernicus’ work on helio=centric nature of the Universe had been published in 1543. At this point Galileo did not discover the moons of Jupiter. Copernicus’ ideas were thought of as crazy and were not accepted. To avoid persecution Copernicus never sent in his theories until the day of his death. On November 11, 1572, Tycho Brahe noticed a new star in the heavens. He noted that the new star was so bright that it could be seen in daylight. What he saw was a nova.

Tycho Brahe strongly believed in Copernicus’ theory about the Solar system. With the invention of the telescope by Galileo, it was possible to study the surfaces of planets and stars and planets not visible to the naked eye. In 1610, Galileo used his telescope to discover that Earth is not the only planet with an orbiting moon. He found that Jupiter had more moons than Earth, four at the time. Aided by Tycho Brahe’s observation, Johannes Kepler figured out that the orbit of the plants were not circular but elliptical. This observation supported Copernicus’ theory.

The theories related to universal gravitation formulated by Newton were based on the works done by Galileo Galilei. His works began with his desire to uncover the reason why light objects fall slower than heavy objects. With the formulation of the concept of gravitation, Sir Isaac Newton was able to provide a scientific explanation to the astronomical theories developed by Johannes Kepler.

Kepler’s laws give a description of the motion of the planets orbiting the sun. His laws are the orbit of every planet is an ellipse with the Sun at one of the two foci. A foci is a pair of special points used in describing conic sections. The four types of conic sections are, circle, ellipse, parabola, and hyperbola. The second law states,

      • “A line joining a planet and the sun sweeps out equal areas during equal time intervals.”

The third law states,

      • “The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis and its orbit. “

His third law was not discovered until many years later when it was published in 1619. A century later, Sir Isaac Newton proved that relationships like Kepler’s would apply for universal gravitation. Kepler’s laws and observations asserted that the Earth orbited the sun. Kepler generally supported the heliocentric theory of Nicolaus Copernicus. Kepler’s laws challenged the long-accepted rules of Aristotle’s.

Kepler, like Brahe believed in Copernicus’ picture of the solar system. One day, Kepler remembered that there were just five perfect Platonic solids, and this gave a reason for there being six planets, not only earth. The orbit spheres were maybe just such that between two successive ones a perfect solid would just fit. Kepler realized Tycho’s work could settle the question so he went to work with Tycho in 1600. Tycho then passed away in 1601, a year later. After Tycho’s death, Kepler acquired the data from Tycho and worked with them for nine years. Some experts say that Kepler stole Tychos’ ideas.
Kepler found that Tychos geometric scheme was wrong. Instead he found his famous three laws of planetary motion. The plants move in an elliptical orbit around the sun, instead of a circle. In their orbit around the sun, the planets would sweep out in equal areas at equal times. [4]

Kepler’s three laws of planetary motion aided Sir Isaac Newton in his law of universal gravitation.
Kepler was one of the first to state that the way to understand the motion of the planets had to do with a force from the sun. Keplers thought opposed the thoughts of Galileo’s experiments. Kepler thought that a continuous force was needed to maintain motion. Kepler went on to visualize the force from the sun is like a rotating beam pushing the planet around its orbit.

Once Kepler had Tycho’s data, due to Tycho’s death, he went on to determine the exact orbit of Mars. Kepler noticed that the orbit was very close to a circle but not a circle. He found that the sun was not at the center of the circle but it was at a point almost one-tenth of a radius away. He also found that Mars varied in speed as it orbited instead of one constant speed. Mars moves fastest when it is closer to the sun and slowest when it is further away from the sun. [5]

Kepler proved Aristotelian doctrine inaccurate. He stated that gravity was a mutal tendency between materal bodies toward contact. The Earth draws a rock like the rock draws the Earth. Heavy bodies are attracted by the Earth not because it’s the center of the universe, but because it has a lot more mass. [6]
To prove he really understood, he wrote:

      • "If two stones were placed anywhere in space near to each other, and outside the reach of force of (other bodies), then they would come an intermediate point, each approaching the other in proportion to the other's mass." [7]

Kepler was completely accurate about gravity. Kepler realized that gravity was the cause of tides. He claimed that the tides were attracted by the moons gravitational pull. He wrote: [8]

      • "If the earth ceased to attract the waters of the sea, the seas would rise and flow into the moon..." and went on to add: "If the attractive force of the moon reaches down to the earth, it follows that the attractive force of the earth, all the more, extends to the moon and even farther..."[9]

The crucial reason he was unsuccessful to make the connection was that he believed the planets needed a constant pushing force, to keep them going in their orbits. Galileo's insight about projectiles was extended to the planets by Isaac Newton.[10]

18 July 1635 – 3 March 1703 Image Courtesy of:

Robert Hooke was a significant influence in the advancement of physics as well as Sir Isaac Newton. In the end Sir Isaac Newton could not have found his theory of gravitation without the help of Robert Hooke. Robert Hooke is often in the shadows of Newton, not gaining much fame. They both influenced each other far more than either would ever know.. Each intellectual man deserves their own identity. Although, when Hooke is mention Newton is brought up, rarely the other way around. A large part of this is because they were enemies. Newton had more influence over the Royal Society than Hooke. [11]

Hooke and Newton first confrontation occurred in 1672. Newton had written a paper over white light being composed of many other colors. It was when the paper was present to the Royal Society just prior to Newton’s reception as a Fellow of the Society. Newton was met with Hooke. Hooke also had his own theory about light. Hooke went into detail about it in the Micrographia. He claimed that Newton needed to be more detailed in his theory. [12]

The situation was made worse for Newton later on. Christian Huygens, Ignace Pardies and the Jesuits of Liege joined Hooke. In March 1673, wrote to the secretary of the Royal Society, Henry Oldenburg. Newton wrote to ask to withdraw from the society. Through much persuasion, Oldenburg convinced Newton to stay. Oldenburg also offered an apology for this misbehavior of Hooke. Newton had successfully established his place in the Royal Society, which gave him the advantage over Hooke. Hooke was beginning to live in the shadows of Newton. [13]

The next confrontation between Hooke and Newton surfaced in 1684. This confrontation concerned Newton’s Principia. Hooke actually played a large role in Newton’s work, although Newton claimed that Hooke had no part in his Principia at all.

There are several accounts of Newton and Hooke exchanging letters. Both these intelligent men made an attempt to settle their differences. Hooke blamed Oldenburg for the problems and that the two scientist should lay aside their differences to avoid further misunderstandings. Newton reluctantly agreed. The topic of the first letters between them was an old trajectory problem. What path would an object follow falling to the Earth? Newton suggested that the trajectory could be a spiral. Robert Hooke then took Newton’s false opinion and announced it to the Royal Society.

Newton refused to contact Hooke after this incident. Hooke then wrote a third letter to Newton, one of the most important letters of all. In this particular letter, written in January 6, 1680, Hooke spoke of his theories of gravity. Hooke wrote, “But my supposition is that the Attraction always is in a duplicate proportion to the Distance from the Center Reciprocal, and consequently that the Velocity will be in a sub duplicate proportion to the Attraction and Consequently as Kepler supposes Reciprocal to the Distance." [14]

The Principia was published in 1687 by Newton, without recognition to Hooke. [15]

In Kepler’s Attempt to Prove the Motion of the Earth written in 1674, Kepler offered a theory of planetary motion based on inertia and centrifugal force and in inward gravitational attraction to the center of the Solar System. In 1679, Hooke wrote a letter to Newton suggesting that this attraction would vary inversely as the square of the distance from the Sun. Although Hooke’s theory was correct he lacked the mathematical ability to give it a quantitative expression. The next 20 years of Hookes life was spent on the interest of gravity.

Sir Isaac Newton made large contributions to physics and astronomy. Although Newton is the front man for his three laws of motion and universal gravitation, he had help from many people. Without Aristotle, Galileo, Tycho Brahe, Johannes Kepler, and Robert Hooke’s guidance Newton would not have found many of his laws. Even great physicists need a little help.


  1. ^ Paul G. Hewitt 1/31/2005 Inertia
  2. ^ Paul G. Hewitt 1/31/2005 F=MA
  3. ^ Paul G. Hewitt 1/31/2005 Inertia
  4. ^ Michael Fowler Johannes Kepler
  5. ^ WIKI KIDS Mars
  6. ^ Michael Fowler Johannes Kepler
  7. ^ Michael Fowler Johannes Kepler
  8. ^ Michael Fowler Johannes Kepler
  9. ^ Michael Fowler Johannes Kepler
  10. ^ Michael Fowler Johannes Kepler
  11. ^ Kathy Miles and Charles F. Peters II June 1996 Robert Hooke & Newton
  12. ^ Kathy Miles and Charles F. Peters II June 1996 Robert Hooke & Newton
  13. ^ Kathy Miles and Charles F. Peters II June 1996 Robert Hooke & Newton
  14. ^ Kathy Miles and Charles F. Peters II June 1996 Robert Hooke & Newton
  15. ^ Kathy Miles and Charles F. Peters II June 1996 Robert Hooke & Newton