# Newton's Laws. Before Isaac Newton

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1 Newton's Laws

2 Before Isaac Newton Newton's Laws There were facts and laws about the way the physical world worked, but no explanations After Newton There was a unified system that explained those facts and laws and many other things besides Newton published that system in Mathematical Principles of Natural Philosophy in 1686 Among other things, the Principia explained motion And to understand the universe, you need to understand motion Because everything in the universe moves

3 Prelude to Newton's Laws How to describe motion? Position where it is Velocity how fast and in what direction it is going Acceleration how fast and in what direction its velocity is changing when something speeds up, it is accelerating when something slows down, it is accelerating (deceleration = negative acceleration) there can even be acceleration without a change of speed! don t believe it? watch this

4 Prelude to Newton's Laws There is a very important type of acceleration in astronomy: the acceleration due to gravity Consider the ball dropped off the building at right It accelerates at a rate of about ten meters per second, per second, or 10 m/s 2 (more exactly, 9.8 m/s 2 ) This is called the acceleration of gravity, symbolized by g

5 Prelude to Newton's Laws In a given gravity field, all objects experience the same gravitational acceleration A piece of paper and a brass mass A brass mass and a similar-shaped wad of paper A hammer and a feather

6 Prelude to Newton's Laws The Hammer and the Feather USAF Col David R. Scott Apollo 15 July 26th - August 7th, 1971

7 Prelude to Newton's Laws The Hammer and the Feather USAF Col David R. Scott Apollo 15 July 26th - August 7th, 1971

8 Prelude to Newton's Laws The Hammer and the Feather USAF Col David R. Scott Apollo 15 July 26th - August 7th, 1971 The hammer and feather fell because they felt a force from gravity The force of gravity = weight There would be no weight without mass But is mass the same as weight?

9 Prelude to Newton's Laws Mass versus Weight Mass = a measure of the amount of matter in an object Mass does not change no matter where the object is Weight = the result of a force exerted on the object The amount of weight changes with the strength of gravity There are two kinds of weight: "Normal" weight = force exerted by gravity Normal weight depends on the strength of gravity "Apparent" weight = force of gravity + other forces

10 Prelude to Newton's Laws Normal Weight versus Apparent Weight

11 Prelude to Newton's Laws Normal Weight versus Apparent Weight

12 Prelude to Newton's Laws Normal Weight versus Apparent Weight

13 Prelude to Newton's Laws Normal Weight versus Apparent Weight Todo Universo by Lulu Santos

14 Prelude to Newton's Laws An object s motion is specified by its position, velocity, and acceleration Newton s Laws of Motion describe why and how things move Newton s Laws are related to the concept of momentum

15 Prelude to Newton's Laws Galileo identified momentum as a fundamental physical property of any moving object that has mass At that time, it was called impetus Momentum tends to keep an object moving with the same speed and direction In other words, with the same velocity

16 Prelude to Newton's Laws A mathematical expression for momentum is momentum p mass velocity Momentum is a vector quantity, with both magnitude and direction More momentum harder to change the object s direction and speed But while speed and direction can be hard to change, it can be done With a push from a net force Net forces can consist of more than one force

17 Prelude to Newton's Laws Momentum and Force If forces are applied so that they balance, then the net force is zero But if they don t balance, then there is a nonzero net force And that changes momentum, which is p = m. V

18 Prelude to Newton's Laws Momentum and Force Now if momentum changes, but mass stays the same, the velocity must have changed So there must have been an acceleration Therefore, a net force, which changes momentum, causes an acceleration And that leads us to Newton's Laws of Motion

19 Newton s Laws of Motion 1.If the net force on an object is zero, the object s velocity is constant 2.A nonzero net force on an object changes the object's momentum, accelerating it in the direction of the force: 3.For every force, there is an equal but opposite reaction force Newton's laws reflect a property of momentum called Conservation of Momentum

20 Conservation of Momentum "The total amount of momentum in the universe is constant A more useful way to say it: "The total amount of momentum in an isolated system is constant So how do Newton's laws of motion reflect Conservation of Momentum?

21 Newton's Laws and Conservation of Momentum 1. If the net force on an object is zero, the object s velocity is constant therefore its momentum is constant (= conserved ). 2. If there is a nonzero net force on the object... then the net force accelerates it in the direction of the force: changing its momentum: Things are changing here, so there is no conservation yet But with the third law 3. For every force there is an equal but opposite reaction force" The momentum is kept constant, because if object 1 s momentum is changed by a force from object 2... the reaction force exerted on object 2 by object 1... changes object 2 s momentum by an equal but opposite amount. In other words, momentum is conserved.

22 Here s a familiar example of Conservation of Momentum:

23 Rockets are important parts of space programs

24 Rockets are important parts of space programs What do you think makes rockets launch?

25 It s Conservation of Momentum Before launch, the rocket body and the fuel inside together have zero momentum After launch, the fuel shoots out the exhaust with large momentum To conserve momentum, the rest of the rocket must move in the opposite direction

26 The type of momentum we have been talking about so far is technically called linear or translational momentum, p = mv This is to distinguish it from another type of momentum that is very important in astronomy Linear momentum is a property of objects that are moving or translating through space The other type of momentum is a property of objects that are rotating It s called angular momentum

27 Angular Momentum Like linear momentum, angular momentum is a vector, with magnitude and direction The magnitude is given by the formula in the figure: L = mvr What about the direction? It is perpendicular to the rotation, in a direction given by the right hand rule Curl the fingers of your right hand in the direction of rotation Your thumb points in the direction of the angular momentum vector Angular momentum vectors sometimes appear in depictions of spinning celestial bodies

28 Angular Momentum

29 Angular Momentum Like linear momentum, angular momentum is conserved Conservation of Linear Momentum says: in the absence of a net force, the linear momentum of a system remains constant In other words, to change linear momentum, a net force must be applied To change angular momentum, you must apply a twisting force, also called a torque So Conservation of Angular Momentum can be expressed: in the absence of a net torque, the angular momentum of a system remains constant

30 This figure shows a good example of conservation of angular momentum magnitude A bicycle wheel and a spinnable platform can demonstrate conservation of angular momentum direction Spinning objects like ice skaters continue to spin as a unit and don t come apart because they are held together by intermolecular forces, which are mostly electromagnetic Angular Momentum

31 If the ball spins at a constant rate it has constant angular momentum The string exerts a centripetal force on the ball The centripetal force causes the ball s linear momentum and velocity to change constantly and it spins in a circle

32 If the ball spins at a constant rate it has constant angular momentum The string exerts a centripetal force on the ball The centripetal force causes the ball s linear momentum and velocity to change constantly and it spins in a circle If the string breaks, the ball flies off on a tangent

33 Orbital Motion Planets orbit the Sun in roughly circular (actually elliptical) orbits Sort of like balls on strings But there are no strings There still must be a centripetal force, though So what causes them to orbit? GRAVITY

34 And that brings us back to Sir Isaac and the apple... from University of Tennessee Astronomy 161 web site

35 from University of Tennessee Astronomy 161 web site But it didn't happen exactly that way... According to Newton himself much later, he did see an apple fall But it didn't fall on his head and knock that equation into it Instead, the story goes, he noticed that even apples from the very tops of the trees fall to the ground...

36 from University of Tennessee Astronomy 161 web site But it didn't happen exactly that way... According to Newton himself much later, he did see an apple fall But it didn't fall on his head and knock that equation into it Instead, the story goes, he noticed that even apples from the very tops of the trees fall to the ground... Then he looked up and saw the Moon, even higher than that

37 But it didn't happen exactly that way... According to Newton himself, much later, he did see an apple fall But it didn't fall on his head and knock that equation into it Instead, the story goes, he noticed that even apples from the very tops of the trees fall to the ground... Then he looked up and saw the Moon, even higher than that and wondered...

38 Could the same force that causes an apple to fall to the ground cause the Moon to orbit Earth? Hold on! An apple falls straight to the ground the Moon does not! But Newton wasn't thinking of things that fall straight down He was thinking of projectiles And projectiles do fall

39 Object dropped from rest at Earth's surface falls ~5 m in 1 s Earth's surface drops ~5 m in 8,000 m So if a cannonball travels 8,000 m/s (5 mi/sec or 18,000 mi/h) parallel to the surface, it will never hit it will go into orbit This speed is called orbital velocity or orbital speed

40 Newton realized that the Moon might orbit Earth for the same reason as a cannonball with orbital velocity: Earth s surface curves away at the same rate as the Moon falls But was it the same force that causes it to fall? To get at that, Newton asked whether the Moon falls 5 m in one second, like the cannonball It doesn t it only falls a little more than 1 mm in one second That s only 1/3600th as far as the cannonball So the Moon is not pulled on as hard as the earthly cannonball Maybe Aristotle was right Maybe things work differently up there in the heavens

41 But Newton would not accept that He agreed that the Moon was not pulled on as hard as the cannonball But not because things work differently up there Instead, he proposed that the same force gravity attracts them both It s just that the Moon isn t attracted as much because it s farther away

42 Newton was able to determine how gravity depends on distance: The Moon is about 60 Earth radii from Earth s center so it s about 60 times farther away than Earth s surface is The Moon falls 1/3600th of the distance that the cannonball falls 3600 = 60x60 So Newton concluded that force of gravity decreases with increasing distance as the inverse square of the distance: The Moon is 60X farther away, so it feels 1/(60x60) of the force

43 from University of Tennessee Astronomy 161 web site So it didn t really happen the way the cartoon depicts But it seems not to have happened according to Newton s story either Turns out that correspondence between Newton and Robert Hooke show that Hooke suggested the inverse square relation to Newton around 1680 But Hooke was thinking of the motion of planets around the Sun Newton took it further than that, both mathematically and conceptually Newton said gravity worked between any two masses, as described in the Law of Universal Gravitation

44 Law of Universal Gravitation Newton did not know the value of G In 1798, Henry Cavendish first measured it The current accepted value is Even without a precise value for G, Newton was able to derive Kepler s laws from his Laws of Motion and this Law of Gravitation

45 Kepler s First Law Planets move in elliptical orbits with the Sun at one focus

46 Kepler s Second Law Planets in orbit sweep out equal areas in equal times

47 Kepler s Third Law More distant planets orbit the Sun at slower average speeds, obeying the relationship p 2 = a 3 p = orbital period in years a = average distance from Sun in AU

48 Newton s Version of Kepler s Third Law p 2 = a 3 Kepler s version Newton s version As in Kepler s version, p is the period and a is the average orbital distance But Newton s version is more general than Kepler s Kepler s only works for the Sun and our planets Newton s works for any orbiting objects In Newton s version: M 1 and M 2 are the masses of the orbiting objects (in kilograms) G is the gravitational constant (in m 3 /kg s 2 ) p is in seconds, and a is in meters

49 Newton found that objects orbit in ellipses around their common center of mass, not their geometrical centers Two objects of the same mass orbit around a common focus halfway between

50 Newton found that objects orbit in ellipses around their common center of mass, not their geometrical centers Two objects of the same mass orbit around a common focus halfway between Two objects of different mass orbit around a common focus closer to the larger mass

51 Newton found that objects orbit in ellipses around their common center of mass, not their geometrical centers Two objects of the same mass orbit around a common focus halfway between Two objects of different mass orbit around a common focus closer to the larger mass The common focus for objects of very different mass, like the Sun and a planet and similar systems, is inside the larger mass

52 Types of Allowed Orbits Newton also found that elliptical orbits were not the only ones possible An elliptical orbit is an example of a bound orbit There are also unbound orbits Objects on bound orbits go around and around Objects on unbound orbits pass by once and never return Unbound orbits have more orbital energy than bound orbits To understand what orbital energy is, we need to learn about some types of energy

53 Types of Energy The types of energy needed to understand orbital energy are: Kinetic energy Energy of motion KE = 1 2 mv2 Gravitational potential energy Energy of position PE g = mgh

54 Orbital Energy Consider Kepler s 2 nd Law: Planets sweep out equal areas in equal times So they go faster when they are closer And so have more kinetic energy, KE = 1 2 mv2, when closer But they have less gravitational potential energy, PE g = mgh, when they are closer The sum, KE + PE g, is the orbital energy And it is conserved: orbital energy = KE + PE g = constant

55 Orbital Energy Because orbital energy is conserved orbits are stable Orbits can change but only by adding or taking away energy from the object One way to do this is with a gravitational encounter

56 Gravitational Encounters Comet comes in on a high-energy unbound orbit Gravity of Jupiter slows it down Loss of energy makes it adopt a lower-energy bound orbit Gravitational encounters like this, aka gravitational slingshots, are important in space travel: Cassini trip to Saturn How are spacecraft trajectories plotted? NEWTON S LAWS Newton s laws also help us understand tides

57 Tides, Tidal Friction, and Synchronous Rotation Why do tides occur? They are caused by the gravity of the Moon How does that work? The Moon pulls harder on the nearer side This stretches Earth out, making two tidal bulges on opposite sides

58 Tides, Tidal Friction, and Synchronous Rotation The Moon goes around Earth slower than Earth rotates So any point on Earth should have two high tides and two low tides each day But they aren t exactly 12 hours apart Why?

59 Tides, Tidal Friction, and Synchronous Rotation It s because the Moon orbits around Earth So at a given location, Earth has to go through more than one sidereal rotation to get back to the same tide It also depends on the shape of the coast and the shape of the bottom

60 Tides, Tidal Friction, and Synchronous Rotation For example, the tidal variation in mid-ocean is 2 meters (6 6 ) or less At Jacksonville Beach it is about 4 feet from low to high Elsewhere, the variation can be much greater

61 Tides, Tidal Friction, and Synchronous Rotation For example, the tidal variation in mid-ocean is 2 meters (6 6 ) or less At Jacksonville Beach it is about 4 feet from low to high Elsewhere, the variation can be much greater For example, in the Bay of Fundy

62 Tides, Tidal Friction, and Synchronous Rotation For example, the tidal variation in mid-ocean is 2 meters (6 6 ) or less At Jacksonville Beach it is about 4 feet from low to high Elsewhere, the variation can be much greater For example, in the Bay of Fundy

63 Tides, Tidal Friction, and Synchronous Rotation For example, the tidal variation in mid-ocean is 2 meters (6 6 ) or less At Jacksonville Beach it is about 4 feet from low to high Elsewhere, the variation can be much greater For example, in the Bay of Fundy This is high tide there

64 Tides, Tidal Friction, and Synchronous Rotation For example, the tidal variation in mid-ocean is 2 meters (6 6 ) or less At Jacksonville Beach it is about 4 feet from low to high Elsewhere, the variation can be much greater For example, in the Bay of Fundy This is low tide

65 Tides, Tidal Friction, and Synchronous Rotation For example, the tidal variation in mid-ocean is 2 meters (6 6 ) or less At Jacksonville Beach it is about 4 feet from low to high Elsewhere, the variation can be much greater For example, in the Bay of Fundy The tides can vary by as much as 40 feet!

66 Tides, Tidal Friction, and Synchronous Rotation This is due to the shape of the bay When in a confined space like a bay or a bathtub water wants to slosh back and forth with a particular frequency In the Bay of Fundy, the tides roll in and out at the same frequency the water wants to slosh

67 Tides, Tidal Friction, and Synchronous Rotation This is due to the shape of the bay When in a confined space like a bay or a bathtub water wants to slosh back and forth with a particular frequency In the Bay of Fundy, the tides roll in and out at the same frequency the water wants to slosh So the sloshing amplifies the tides and leads to the huge variation in water height between low and high tides

68 Tides, Tidal Friction, and Synchronous Rotation This is due to the shape of the bay When in a confined space like a bay or a bathtub water wants to slosh back and forth with a particular frequency In the Bay of Fundy, the tides roll in and out at the same frequency the water wants to slosh So the sloshing amplifies the tides and leads to the huge variation in water height between low and high tides

69 Tides, Tidal Friction, and Synchronous Rotation This is due to the shape of the bay When in a confined space like a bay or a bathtub water wants to slosh back and forth with a particular frequency In the Bay of Fundy, the tides roll in and out at the same frequency the water wants to slosh So the sloshing amplifies the tides and leads to the huge variation in water height between low and high tides

70 Tides, Tidal Friction, and Synchronous Rotation This is due to the shape of the bay When in a confined space like a bay or a bathtub water wants to slosh back and forth with a particular frequency In the Bay of Fundy, the tides roll in and out at the same frequency the water wants to slosh So the sloshing amplifies the tides and leads to the huge variation in water height between low and high tides

71 Tides, Tidal Friction, and Synchronous Rotation This is due to the shape of the bay When in a confined space like a bay or a bathtub water wants to slosh back and forth with a particular frequency In the Bay of Fundy, the tides roll in and out at the same frequency the water wants to slosh So the sloshing amplifies the tides and leads to the huge variation in water height between low and high tides

72 Tides, Tidal Friction, and Synchronous Rotation The Sun also affects the tides, but because of its distance, only about 1/3 as much as the Moon Occasionally the Sun and the Moon work together to produce unusually extreme tides

73 Tides, Tidal Friction, and Synchronous Rotation The Sun also affects the tides, but because of its distance, only about 1/3 as much as the Moon Occasionally the Sun and the Moon work together to produce unusually extreme tides When the Sun, Earth, and Moon are in a line there is a spring tide

74 Tides, Tidal Friction, and Synchronous Rotation The Sun also affects the tides, but because of its distance, only about 1/3 as much as the Moon Occasionally the Sun and the Moon work together to produce unusually extreme tides When the Sun, Earth, and Moon are in a line there is a spring tide When they form a right angle there is a neap tide

75 Tides, Tidal Friction, and Synchronous Rotation But in fact, the tidal bulge is not lined up with Earth and Moon This is due to friction between the solid Earth and the water above

76 Tides, Tidal Friction, and Synchronous Rotation Earth s rotation pulls the tidal bulges along This causes them to run slightly ahead of the Earth-Moon line If Earth didn t rotate faster than the Moon orbits, the bulges would be right on the Earth-Moon line

77 Tides, Tidal Friction, and Synchronous Rotation The Moon s gravity pulls back on the bulge, slowing Earth s rotation As a result, the length of a day increases ~2 ms/century (1 s/50,000 y)

78 Tides, Tidal Friction, and Synchronous Rotation What about the effect of the Moon on Earth? If the Moon pulls on Earth s tidal bulge, what does Newton s 3 rd Law say? It says the bulge exerts an equal but opposite force on the Moon This pulls the Moon ahead in its orbit, increasing its orbital energy As a result, the Moon moves away from Earth by ~4 cm per year So the Moon is almost 2 m farther away than when Apollo 11 landed

79 Tides, Tidal Friction, and Synchronous Rotation How does this affect the angular momentum of the Earth-Moon system? If Earth s rotation slows, it loses angular momentum But the angular momentum of the system is conserved So the angular momentum lost by Earth is gained by the Moon Which is why its orbital energy increases, and it moves away

80 Tides, Tidal Friction, and Synchronous Rotation Earth s rotation is slowed only slightly by this process But Earth s tidal force causes a tidal bulge on the Moon and Earth pulls on it, too The Moon is much less massive, so its rotation has been affected much more

81 Tides, Tidal Friction, and Synchronous Rotation So over time, the Moon s rotation has slowed so much that it matches its orbital period It is now in synchronous rotation with its orbit This is why it always shows the same face to us

82 Tides, Tidal Friction, and Synchronous Rotation So over time, the Moon s rotation has slowed, and it now matches its orbital period It is now in synchronous rotation with its orbit This is why it always shows the same face to us Well, almost the same face, but not quite, due to the librations seen above

83 Tides, Tidal Friction, and Synchronous Rotation Librations are caused by three things: The eccentricity of the Moon s orbit and Kepler s 2 nd Law ( longitudinal ) The tilt of the Moon s orbit ( latitudinal ) Our movement from side to side as Earth rotates ( diurnal )

84 Tides, Tidal Friction, and Synchronous Rotation This rotational locking is common out there The Moon is in synchronous rotation around Earth

85 Tides, Tidal Friction, and Synchronous Rotation Pluto and Charon are in synchronous rotation with each other Eventually, Earth and the Moon will be this way, too in about 50 billion years long after the Sun becomes a red giant and then a white dwarf

86 Tides, Tidal Friction, and Synchronous Rotation Others are somewhat different, with sometimes surprising results Click here to see Mercury s 2:3::orbital:rotational resonance with the Sun

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