What Keeps the Moon in its Orbit around the Earth? The Forces Which Keep the Moon in Orbit written by: VinceSummers•edited by: RC Davison •updated: 10/17/2014 The Moon has orbited Earth untold times. Why is it the Moon doesn't neither crashes into the Earth nor flies off into deep space? What keeps the Moon in orbit? What are the forces that enable it to do so? slide 1 of 5 Gravity and Newton The beautiful glowing Moon—our night-light with its specular brightness in various shades of gray—is held in orbit by the same forces that keep the planets and the comets in their orbits. It is the same force that enables the careful positioning of communications satellites encircling the Earth. That powerful force is gravity. Newton’s Law of Gravitation states, F = G (m1m2)/r2, where F is the force, G is the gravitational constant (6.6730 x 10-11 m3/kg-s2), m1 and m2 are the masses of two interacting objects (in this instance, the Earth and Moon), and r is the distance between them. slide 2 of 5 How Much Force?

The masses of the Earth and of the Moon are approximately 5.9736 x 1024 kg and 7.3477 x 1022 kg, respectively. The distance between Earth’s center and the Moon varies slightly, but is approximately 3.8440 x 108 m. This means the force due to gravity between the Earth and the Moon averages, F = (6.6730 x 10-11)(5.9736 x 1024)(7.3477 x 1022) / (3.8440 x 108)2 where the units have been left off for the sake of space. The result is, F = 1.9822 x 1020 kg-m/sec2 = 1.9822 x 1020 Newtons slide 3 of 5 Moon in orbit [Archive] - Cosmoquest Forum The force is the same.

cjbirch 2006-Aug-23, 04:06 PM The gravitational force between the Earth-Moon System, which tries to pull the moon towards the former, as it orbits about the Earth is known as Centripetal force. This force is balanced by Centrifugal force, which pulls on the Earth keeping the moon in motion. The balance between Centripetal and Centrifugal force are what keeps the Moon orbiting the Earth. This reasoning provides an understanding for how the Moon stays in orbit around the Earth. However you could also look at Einstein's Theory of GR to explain why the Moon orbits the Earth in the way it does. GR provides that objects with mass curve the spacetime within their vicinity and it is this curvature which influences the motions of other objects. The greater the objects mass and density, the larger the curvature of spacetime will be. It follows therefore that the Moon orbits the Earth because of the Earth's curvature of spacetime within the vicinity of the Moon. This relationship between mass and curvature cause the gravitational and Centripetal forces to exist, causing the Moon to orbit Earth. CJ 2006-Aug-23, 04:07 PM Why minus?

Because moon exerts a force on the earth which is in opposite direction to the force which pulls the moon towards the earth. ain't it so? hhEb09'1 2006-Aug-23, 04:38 PM Because moon exerts a force on the earth which is in opposite direction to the force which pulls the moon towards the earth. ain't it so?And they've equal magnitude. When you subtract...well, it depends on how you do the subtraction. You can get zero, or twice the quantity. adamsoltan 2006-Aug-23, 05:42 PM Because moon exerts a force on the earth which is in opposite direction to the force which pulls the moon towards the earth. ain't it so? That's the force that the Earth 'feels'. The original question was about forces acting on the Moon and those are, as already mentioned, the centripetal and the centrifugal forces. From the mechanics point of view it's the same as if you would start spinning around holding a string with a rock attached to the other end. The only differance is that with gravity you don't see the agent keeping the two objects together.

Hope this clarifies a bit. /Adam pghnative 2006-Aug-23, 05:57 PM Keep in mind that the Newtonian way of looking at this is that there is only one force involved: gravity. Of course, force = mass times acceleration. So the single force (gravity) is accelerating the moon*. The acceleration of the moon does not change its speed; instead, the acceleration changes the direction of the moon's travel. Thus the moon travels in an approximate circle around the earth instead of flying off. Good thing too, or we wouldn't have all of these Moon Hoax threads around here... *Of course, gravity is accelerating the Earth toward the moon, also, but we're just talking about the moon so far in this thread. Blue Fire 2006-Aug-23, 05:57 PM But within the concept of curved space due to mass, the moon would not be "held" in orbit around the Earth by any force of "attraction" but would instead be following a "straight line" in curved spacetime. Like a marble that has been sent rolling around a canonball resting on a trampoline.

The canonball deforms the trampoline like the Earth deforms spacetime. And so the marble is just following the curvature of the trampoline with no force of attraction between the marble and the cannonball. Right? grav 2006-Aug-24, 12:37 AM But within the concept of curved space due to mass, the moon would not be "held" in orbit around the Earth by any force of "attraction" but would instead be following a "straight line" in curved spacetime. Like a marble that has been sent rolling around a canonball resting on a trampoline. The canonball deforms the trampoline like the Earth deforms spacetime. And so the marble is just following the curvature of the trampoline with no force of attraction between the marble and the cannonball. Right?But could this be demonstrated with an analogy that doesn't originally require gravity (or other similar force) already acting on the cannonball on the trampoline in order to create the "curved spacetime" in the first place?

hhEb09'1 2006-Aug-24, 02:00 AM But could this be demonstrated with an analogy that doesn't originally require gravity (or other similar force) already acting on the cannonball on the trampoline in order to create the "curved spacetime" in the first place?I think you're asking for an example, rather than an analogy, maybe? The point of the analogy is that the marble is not really attracted to the cannonball by the cannonball (at least, to the extent that it is), but is a result of the configuration of the path. But what you're modeling is the curvature of spacetime, and it's pretty hard to model curved time without actually doing it. :) Jens 2006-Aug-24, 03:30 AM Because moon exerts a force on the earth which is in opposite direction to the force which pulls the moon towards the earth. ain't it so? It sounds from your understanding that two masses of the same mass wouldn't orbit one another, because the force would amount to zero. But clearly that isn't true.

In actuality, the earth is pulled to the moon with the same force that the moon is pulled to the earth. It's just that the earth is so much more massive that it doesn't appear to move. But it does. PhantomWolf 2006-Aug-24, 03:43 AM The gravitational force between the Earth-Moon System, which tries to pull the moon towards the former, as it orbits about the Earth is known as Centripetal force. This force is balanced by Centrifugal force, which pulls on the Earth keeping the moon in motion. The balance between Centripetal and Centrifugal force are what keeps the Moon orbiting the Earth. Ummmm.... no. Centrifugal force doesn't exist. It is a mathematical result and in only found inside an accelerating frame of referance. ie, if you are sitting in a car as it rounds a corner, you experinece a "force" on you making you press up against the door of the car.

In reality though the force is being applied to the car making it turn (accelerate towards the centre of the bend), you are attempting to travel in a straight line and the car door intercepts you and applies a force on you to change your direction (accelerate you) to match its own new velocity. (remember acceleration is a change in velocity, and velocity has two components, magnitude (speed) and direction. A change of direction without a change in magnitude is still an acceleration.) Newton noted that "Any body set in motion will continue in that motion untill such time another foce acts upon it." If we apply this to the moon, its forward motion is unchanged as it orbits (the magnitude portion of its velocity does change due to Kepler's Laws of Orbital Motion and rotational inertia.) It just wants to travel in a straight line at a constant speed, and would do so if there was no other force acting on it. Now we add Gravity. This will accelerate the moon directly towards the centre of mass of the Earth-Moon system, in other words, the gravity well acts like a force attempting to push (or pull) the moon to the centre of mass.

However this is always occuring at an angle to the direction of motion of the moon, so what it does is change the veleocity of the moon by changing its direction rather than its magnitude (the magnitude will change slightly throughout the orbit due to the moon's orbit not being a perfect circle.) This shows us that no other force is required to keep it in orbit, just that applied by gravity. There is no "balancing of Centipedal and Centifugal forces keeping the moon in position," there is just gravity acting on an object in motion by drawing the moon towards a centre point, and the moon missing it because it is moving forwards too fast. hhEb09'1 2006-Aug-24, 03:59 AM This shows us that no other force is required to keep it in orbit, just that applied by gravity. There is no "balancing of Centipedal and Centifugal forces keeping the moon in position," there is just gravity acting on an object in motion by drawing the moon towards a centre point, and the moon missing it because it is moving forwards too fast.cjbirch was just looking at it from a different point of view, with regards to the so-called inertial forces.

If you push on an object, does it push back? The force that is felt on your hand is the inertial force, of the resisting object. The tendency of an object to move in a straight line is inertia, it will resist a force that tries to move it from that straight line with an inertial force. OTOH, it is very easy to get confused. :) Blue Fire 2006-Aug-24, 05:24 AM I think you're asking for an example, rather than an analogy, maybe? The point of the analogy is that the marble is not really attracted to the cannonball by the cannonball (at least, to the extent that it is), but is a result of the configuration of the path. But what you're modeling is the curvature of spacetime, and it's pretty hard to model curved time without actually doing it. :) Thanks for your response in my stead! I was unable to get back to this thread until now. And, I doubt I could have responded any better than you did anyway.

cjbirch 2006-Aug-24, 11:23 AM This shows us that no other force is required to keep it in orbit, just that applied by gravity. There is no "balancing of Centipedal and Centifugal forces keeping the moon in position," there is just gravity acting on an object in motion by drawing the moon towards a centre point, and the moon missing it because it is moving forwards too fast. Of course, it was just a different perspective for the same scenario! The moon is falling towards the Earth, just like if we dropped a stone onto the surface, and then we get to again why does the Moon travel around the Earth in an orbit rather than simply fallling onto it? Again it is true that if there were no gravity present the moon would travel in a straight line, at the same time the Moon's path along this straght line would cause it to move away from the Earth. Therefore without taking into account GR and the curvature of spacetime, we could simply assert that the amount the Moon falls towards the Earth due to gravity is enough to offset the straight-line path it would take if gravity were not acting in order to deflect it.

This is the balance I was referring too, simply using a different point of view. All being said we actually arrive at a position however that affirms that the Moon is in actuality, perpetually falling around the Earth. CJ Gravity in Orbit | How Things Fly Gravity in Orbit Forces of Flight on this Page Gravity in Orbit So, is there Gravity in Orbit? Space is nearly a perfect vacuum; beyond Earth’s atmosphere there is no air to produce lift or drag. Gravity is the main force to be dealt with in space, and thrust is the force that allows a spacecraft to get into space and maneuver. A spacecraft in orbit is not beyond the reach of Earth’s gravity. In fact, gravity is what holds it in orbit—without gravity, the spacecraft would fly off in a straight path. As the spacecraft orbits, it is actually falling, though it never reaches the ground. Why Doesn’t an Orbiting Spacecraft Fall to Earth? The faster an object travels, the more horizontal distance it covers as it falls, and the gentler the curve of its path. An orbiting spacecraft travels so fast that the curve of its path matches the curve of the Earth. So, the distance between the spacecraft and the Earth stays constant.

This interactive experience requires Adobe Flash Player 10.2 or higher. Why Doesn't the Moon Fall Down? | Science | Video | PBS LearningMedia Average rating: 5 stars - 1 rating Why Doesn't the Moon Fall Down? Astronomer Doris Daou explains how the forces of speed and gravity keep the Moon in a constant orbit around Earth in this animated video segment adapted from NASA. The Moon, Earth's natural satellite, seems to hover in the sky, unaffected by gravity. However, the reason the Moon stays in orbit is precisely because of gravity -- a universal force that attracts objects. With the right combination of speed and gravity, satellites can fall around, instead of into, the body that they orbit. In this animated video segment adapted from NASA, learn about how the Moon stays in orbit around Earth. This video is available in both English and Spanish audio, along with corresponding closed captions. Background Essay The solar system is full of satellites. Earth has many satellites, including the natural Moon as well as artificial satellites placed in orbit for scientific research, communication, and navigation purposes. Other planets have their own moons and artificial satellites, too. Earth itself is a satellite of the Sun, as are each of the other planets in our solar system.

Still, despite the many different kinds of satellites, the physical force that keeps all satellites in orbit is the same -- gravity. Gravity is an attractive force between any objects with mass. When an object that is not supported by anything falls due to the force of gravity of another object, it is said to be in free fall. An orbit is essentially a path of constant free fall. For example, the Moon is falling towards Earth. However, it also has an orbital speed from the momentum gained during its formation that allows it to fall around Earth with a trajectory that follows the same curve as Earth's surface. Because these paths are parallel, the Moon perpetually falls around Earth without ever touching it. In our solar system, the time it takes each planet to complete an orbit around the Sun varies greatly. Inner planets have much shorter orbits - in distance and in time -- than outer planets. For example, Mercury, the closest planet to the Sun, completes an orbit around the Sun in 88 days, while Earth takes one year to make a complete orbit. Pluto, whose average distance from the Sun is about 40 times greater than Earth's, takes 248 years to complete its orbit.

In addition, outer planets also take a long time to orbit the Sun because they have slower average orbital speeds than inner planets. Closer to the Sun, the pull of gravity is very strong, so a planet needs a lot of speed to stay in orbit. If the planet does not move fast enough, gravity would cause it to fall into the Sun. In general, the stronger the force of gravity, the more speed is needed to keep the planet falling around, instead of into, the object it is orbiting. Because the force of gravity weakens with distance, there is less gravity farther away from the Sun, so less speed is needed to keep an object in orbit. Mercury has an average orbital speed of 48 km/s, whereas Earth's average orbital speed is 30 km/s and Pluto's average orbital speed is only 4.7 km/s. Why doesn't the Moon hit Earth even though it is "falling" toward it? What would happen if the Moon moved faster than it does now? How is the Sun able to have so many planetary bodies in orbit around it? Why do planets closer to the Sun than Earth not fall into the Sun from the pull of gravity?

If someone asked you if it's true that the Moon is always falling towards the Earth, what would you tell them? What keeps the moon in orbit around the earth? | Socratic What keeps the moon in orbit around the earth? Start with a one sentence answer Then teach the underlying concepts Don't copy without citing sources Write a one sentence answer... Answer: I want someone to double check my answer Describe your changes (optional) 200 Cancel The earth 's gravity. Explanation: Like any object in orbit, an orbit is maintained, as according to Newton's first law of motion, by a combination of speed and gravity. The object, the moon, wants to move off in a straight line however the earth's gravity, outside force, moves it slightly out of line. Was this helpful? Let the contributor know! Yes Does centrifugal force hold the Moon up? Original by Don Koks 2003. Does centrifugal force hold the Moon up? In the 1960s, Wernher von Braun put together a series of articles about space flight, some of which were published in Popular Science Monthly. Eventually they were collected and made into the book Space Frontier, (1st ed., Holt, Rinehart and Winston).

It's a very readable book, and talks about how rockets work, and flight and safety in space. In one of the articles, von Braun explains why a satellite is able to stay up while in Earth orbit. He begins the article by asking what would happen if we could throw an object horizontally, but at faster and faster speeds, such as in the picture shown here. "Eventually", he writes, "the curvature of the downward-bent trajectory would become equal to the curvature of the earth." This is almost well and good. (It's not quite right to say that the curvatures of the orbit and Earth's surface are the same, even for circular motion, but this is a minor error compared with what comes next.) The important point is that as the bullet moves faster and faster, a magical speed is reached where the curved Earth drops away from the bullet precisely as fast as the bullet falls to the ground; added to which, the direction of "down" keeps changing. As a result, the bullet never gets any closer to the ground—it's in orbit. This is actually a marvellous special feature of an inverse square force like gravity; it would not be guaranteed to happen if gravity were not inverse square.

In general, orbits are ellipses, and one such is drawn on the left. A picture just like this was originally included by Sir Isaac Newton in his Principia of 1687. After this fine start, von Braun then proceeds to muddy the water. He says that as the bullet is shot at ever faster speeds, "its trajectory will be less deflected because the centrifugal force is increased by its higher speed, and more effectively counteracts the Earth's gravitational pull." At this point physicists baulk. Centrifugal force? What has that got to do with satellite motion? Next, von Braun draws a picture of a satellite in Earth orbit. Acting on the satellite are two forces: gravity, pulling the satellite toward Earth, and this centrifugal force, pushing the satellite away. He writes "A circular orbit occurs whenever a small mass, travelling through the gravitational field of a big one, happens to have a speed at which the centrifugal force is precisely strong enough to balance the large body's gravitational pull." And later, "If the balance between gravitational and centrifugal force is not perfect, [...] the small body will describe an elliptical path around the large one." What would Newton say?

He too would draw the forces acting on the satellite, and would then proceed to apply his "force = mass × acceleration"; but first, he'd want to choose an "inertial frame" within which to do this, since his laws only work in inertial frames. An inertial frame is one in which, if we throw a ball, it moves away from us with constant velocity (i.e. constant speed in a straight line). Since this doesn't quite happen on Earth, the frame Newton would choose would be something more all-encompassing, outside of Earth. One good approximation would be the frame of the Solar System, within which the Sun is at rest and Earth revolves fairly accurately in a circle around it, once a year. An inertial frame like this is presumably what von Braun is using, because anything noninertial won't tie in too well with his picture of Earth plus satellite. In an inertial frame, if there really were two equal-but-opposite forces on the satellite as von Braun drew them, then the total force on it would be zero. So it wouldn't accelerate; it would move in a straight line with constant speed.

Since the orbiting satellite doesn't move in a straight line, neither von Braun's picture nor his explanation can be right. Nothing holds the Moon up! In reality, nothing holds the Moon up. As Newton's inertial frame analysis predicts, the Moon is completely under gravity's thrall; in other words, it falls, because in such a frame there's only one force on the Moon: gravity. Gravity accelerates it. That doesn't mean its speed must necessarily change, or that it must get closer to Earth (although actually both of these things do occur slightly during the month, but that's not an important point). If Newton's F=ma is solved for the general case of falling under gravity, the motions that result are lines, circles, ellipses, parabolae, and hyperbolae. In one of those great correspondences between Nature and pure mathematics, these are precisely the curves that result if we take a cone and slice it in any direction. Even if the Moon's orbit were circular, its direction of travel would still be changing, which is one kind of acceleration.

(Remember that acceleration is a change in velocity, meaning that acceleration can change an object's speed, or it can change merely the direction of motion, or both.) The Moon, and every other satellite, fall just as surely as an apple does when pulled down by gravity. Whereas the apple changes its speed but not its direction of motion, the Moon changes its direction of motion, but not its speed. The real difference between a satellite and an apple falling from a tree, is that for the fast sideways-moving satellite, the direction of "down" is always changing. But the satellite really is falling, and in fact a near-Earth satellite has almost the same acceleration that a falling apple has. If it's above us now, then in about 45 minutes, for a low satellite, it will have fallen so far down that it'll be on the other side of Earth. By then, the direction of down has reversed completely, and the satellite will again fall down for those who live on the opposite side of Earth, returning to us about 90 minutes after we first saw it. Of course, it never hits Earth because of its ever-present sideways motion.

The Moon is much farther away where gravity is weaker, so it takes fully two weeks to fall to the other side of Earth. So what is centrifugal force? So why was the concept of centrifugal force invented? Centrifugal force was invented to allow us to do proper bookkeeping in a noninertial frame, if we insist on using such a frame to work with Newton's laws (and there might be a good reason for wanting to do so). For a simple example of a noninertial frame, consider what happens when you stand in a bus while the driver brakes. For a few moments, every passenger moves forward. The heavier ones feel a strong force that acts on their large mass, while the lighter ones feel a small force acting on their small mass. All feel the same acceleration. This is called a fictitious force, because it's a force that we invoke to explain why we are suddenly accelerated forward. In the (almost) inertial frame of the outside street, this force doesn't exist. The real force there is a single simple one provided by the friction of the ground on the bus tyres, and transmitted through its brakes to its body.

This force accelerates the bus backwards—or, to use the more intuitive expression, decelerates the bus. Unless the passengers hang on, they will continue to move forwards until something inside the bus stops them. Depending on our choice of frame then, there are two forces to choose from when analysing why the passengers are impelled forward: In the inertial street frame, there is a braking force that pushes backwards on the bus. This is a bona-fide force, in the sense that it's produced in an inertial frame. It acts on the bus only, so unless we hold on, we'll continue to move forward at constant velocity. In the noninertial frame of the decelerating bus, the force is a mysterious force that acts on us, but not on the bus. It pushes us forwards, and has a strength that is proportional to how massive we are. Our fellow passenger, twice as massive as ourself, will feel twice as much force, but by F=ma, both we and they will feel the same acceleration. This is certainly a more complicated force to explain than the simple one that pushes on the bus in the street frame.

Such a mass-dependent force is an indicator that something is awry—we have chosen to use a noninertial frame, and if we demand Newton's laws to hold in such a frame, then we'll have to invoke this fictitious force. It's worth pointing out that although the force impelling us forward in the bus's frame when the brakes are applied is conventionally called fictitious, it is real enough for the bus passengers! It might be a less natural force to use from a fundamental point of view of explaining why Nature works in the way that she does, but that does not mean that a fictitious force is not "real", that it is some kind of make-believe thing that will go away if we concentrate hard enough. The word "fictitious" merely means that the force vanishes when we shift our frame of reference to one that is inertial, because inertial frames hold a very special position in physics. But as a means of quantifying how the passengers in a bus lurch forward when the brakes are applied, a fictitious force is entirely appropriate and as real as any other—in the frame of the bus. There it must be included in Newton's laws, as all forces must.

Centrifugal force is such a fictitious force, invented purely to balance the books, when we analyse a situation in a rotating frame. Because circular motion involves acceleration (perhaps just a changing direction—not necessarily a changing speed), a rotating frame is also accelerated, just as the bus was. We feel a centrifugal force when the bus goes around a corner. If we made a better choice of frame (say, the almost-inertial street frame), then we'd see the situation for what it is: the bus feels a simple force due to the friction of the road on its tyres, while the passengers feel no force at all—until their natural constant velocity motion gets checked by the body of the bus, which pushes on them to get them around the corner along with the bus. But in the noninertial bus frame, there is a strange force that pushes sideways on the passengers until they press against the inside wall of the bus. So centrifugal force is a fictitious force invoked to make Newton's laws work in a rotating frame.

(For that matter, Coriolis force is another fictitious force that we introduce in Earth's gently rotating frame, to explain why large air masses, moving due to pressure differences, feel forces that work to create winds.) Centrifugal force has absolutely nothing at all to do with the motion of a satellite, as long as we do our calculations within an inertial frame. If we choose a noninertial frame, then we'll certainly need to invoke a centrifugal force. But a noninertial frame isn't fundamental—it has no use for explaining why the satellite orbits Earth. A good intuitive case in point is that of geosynchronous satellites. These are placed in orbit about 36,000 km above Earth's equator, at just the right height that they orbit Earth in one day. Since Earth rotates once in the same period, these satellites always hover above the same point on its surface. From an inertial frame (say, the Solar System), there is nothing special about geosynchronous satellites. They feel only gravity, and respond to it by falling, while retaining the sideways motion given to them when they were initially released from the rocket that sent them up.

But from the point of view of someone standing on Earth's surface and defining himself to be at rest, a geosynchronous satellite just hovers in one place, as if it were somehow overcoming the gravity force that acts upon it. Remember though, that the rotating Earth is a noninertial frame, and we will need to invoke a centrifugal force to make Newton's F=ma work. So we can certainly say that in this unnatural frame there is a centrifugal force holding the satellite up. But it's precisely because noninertial frames are so unnatural, that we are forced into inventing this fictitious force to keep the books balanced. If we want to calculate things in this frame, then that's fine. Keep the centrifugal force, and everything will work out right. And for whatever reason, it might well be useful to do our calculations in this noninertial frame. But to explain why the satellite stays up, we need to choose an inertial frame. In that frame, the centrifugal force vanishes, the satellite no longer stays up, and the question evaporates. Is gravity itself a fictitious force?

A few lines up, I wrote that the force we feel when the bus is braking is weird, in that its strength is proportional to our own mass. But what about gravity? That also has a strength that is proportional to our mass! Could gravity be a fictitious force too? Yes, that's exactly how gravity is viewed these days. This is the content of Einstein's General theory of Relativity. Einstein conjectured that perhaps we've been looking at things in the wrong way. Newton viewed the orbit of a satellite, or the parabolic flight of a projectile, or the fall of an apple, to be complicated motions caused by the action of this mysterious force called gravity. But Einstein turned the problem on its head, and decided that satellites, projectiles, and apples are all following a motion that is as simple as any motion can be, provided they are viewed on the stage of a curved spacetime. The ones whose motion through spacetime is very complicated are ourselves, standing on Earth's surface. This change of view wasn't just made for the sake of a different viewpoint. Einstein's change in viewpoint enabled him to make predictions that differ to corresponding ones made by Newton's theory.

And it is Einstein's predictions that have been verified experimentally to a very high accuracy. So gravity can be treated as a fictitious force. But it's a sort of "higher order" fictitious force when compared to the everyday one we experience in a bus: unlike the very uniform force that pushes us forward in a braking bus, gravity can't be made to completely vanish by a simple change of frame. Added to that, the mathematics of curved spacetimes is more complicated than using F=ma. So for everyday use, it's completely sufficient to treat gravity as a good old bona-fide force, just like the mechanics books tell us to do. Astronomy803 - Gravity and Motion Gravity and Motion 2.D.1.d: Recognize that the pull of gravity causes the pattern of motion of celestial objects Objectives 1.2.1 Identify what determines the strength of the force of gravity between two objects. 1.2.2 Describe two factors that keep the moon and Earth in orbit. Big ideas Earth revolves around the sun in the same way the moon orbits Earth. Gravity attracts all objects toward each other.

Enduring Understanding The strength of the force of gravity between two objects depends on the masses of the objects and the distance between them. Inertia and gravity keep Earth in orbit around the sun and the moon in orbit around Earth The law of universal gravitation states that every object in the universe attracts every other object. Essential Questions How are gravity and weight related? Why doesn’t Earth simply fall into the sun? What determines the strength of the force of gravity between two objects? What two factors combine to keep the moon and earth in orbit? OVERVIEW Earth revolves around the sun in a nearly circular orbit. The moon orbits Earth in the same way, But what keeps Earth and the moon in orbit? Why don't they just fly off into space? The first person to answer these questions was the English scientist Isaac Newton. Late in his life, Newton told a story of how watching an apple fall from a tree in 1666 had made him think about the moon’s orbit. Newton realized that there must be force acting between Earth and the moon that kept the moon in orbit. A force is push or a pull. Most everyday forces require objects to be in contact.

Newton realized that the force that holds the moon in orbit is different in that it acts over long distances between objects that re not in contact. Gravity and introduction Can you remove the bottom penny? Materials – 25 pennies, thin plastic ruler or thin spatula Procedure: Place 25 or so pennies in a stack on a table. Write down your prediction of what will happen if you attempt to knock the bottom penny out of the stack. Quickly slide a ruler along the surface of the table and strike the bottom penny. Observe what happens to the stack of pennies. Repeat step 3 several times, knocking more pennies from the bottom of the stack Think in over: Developing Hypothesis Explain what happened to the stack of pennies as the bottom penny was knocked out of the stack. GRAVITY Newton hypothesized that the force that pulls an apple to the ground also pulls the moon toward Earth, keeping it in orbit. This force, called gravity, attracts all objects toward each other. In Newton’s day, most scientists thought that forces on Earth were different from that elsewhere in the universe. Although Newton did not discover gravity, he was the first person to realize that gravity occurs anywhere.

Newton’s law of universal gravitation states that every object in the universe attracts every other object. The force of gravity is measured in units called newtons, named after Isaac Newton. The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them. Gravity, Mass, and Weight According to the law of universal gravitation, all of the objects around you, including Earth and even this book, are pulling on you, just as you are pulling on them. Why don’t you notice a pull between you and the book? Because the strength of gravity depends in part on the masses of each of the objects. Mass is the amount if matter in an object. Because Earth is so massive, it exerts a much greater force on you than this book does. Similarly, Earth exerts a gravitational force on the moon, large enough to keep the moon in orbit. The moon also exerts a gravitational force on Earth. The force of gravity on an object is known as it weight. Unlike mass, which doesn’t change, an object’s weight can change depending on its location. For example, on the moon you would weigh about one sixth of your weight on Earth.

This is because the moon is much less massive than Earth, so the pull of the moon’s gravity on you would be far less than that of Earth’s gravity. The strength of the force of gravity between two objects depends on their masses and the distance between them. To see the difference between mass and weight and its relative connection to gravity, watch the video. Gravity and Distance The strength of gravity is affected by the distance between two objects as well as their masses. The force of gravity decreases rapidly as distance increases. For example, if the distance between two objects were doubled, the force of gravity between them would decrease to one fourth of its original value. INERTIA AND ORBITAL MOTION If the sun and Earth are constantly pulling on one another because of gravity, why doesn’t Earth fall into the sun? Similarly, why doesn’t the moon crash into Earth? The fact that such collisions have not occurred shows that there must be another factor at work. That factor is called inertia. The tendency of an object to resist a change in motion is inertia. You feel the effects of inertia every day. When you are riding in a car and it stops suddenly, you keep moving forward.

If you didn’t have a seat belt on, your inertia would cause you to bump into the car’s windshield or the seat in front of you. The more mass an object has, the greater its inertia. An object with greater inertia is more difficult to star or stop. Isaac Newton stated his ideas about inertia as a scientific law. Newton’s first law of motion says that an object at rest will stay at rest and an object in motion will stay in motion with a constant speed and direction unless acted on by a force. Orbital Motion Why do Earth and the moon remain in their orbits? Newton concluded that two factors – inertia and gravity – combine to keep Earth in orbit around the sun and the moon in orbit around Earth. A combination of gravity and inertia keeps the moon in orbit around Earth. If there were no gravity, inertia would cause the moon to travel in a straight line. What would happen to the moon if it were not moving in orbit? Earth Orbits Circular Orbit Gravity supplies the necessary centripetal force to hold a satellite in orbit about the earth. The circular orbit is a special case since orbits are generally ellipses, or hyperbolas in the case of objects which are merely deflected by the planet's gravity but not captured.

Setting the gravity force from the univeral law of gravity equal to the required centripetal force yields the description of the orbit. The orbit can be expressed in terms of the acceleration of gravity at the orbit . The force of gravity in keeping an object in circular motion is an example of centripetal force . Since it acts always perpendicular to the motion, gravity does not do work on the orbiting object if it is in a circular orbit.

Question: What force keeps the moon in orbit around Earth?
Answer: Gravity