Schedule Today: Gravity Thursday: Light Tuesday, Feb 5: More light Thursday, Feb 7: Test 1 Today: Gravity Thursday: Light Tuesday, Feb 5: More light Thursday, Feb 7: Test 1

The Road to a Sun-Centered Solar System Ptolemy (90-168) -Almagest published ~150 Copernicus ( ) -Published in 1544 Tycho Brahe ( ) -Never published Johannes Kepler ( ) -Published his Laws of Motion starting in 1609 Galileo ( ) -Published “Dialogue of the Two Chief World Systems” in 1632 Ptolemy (90-168) -Almagest published ~150 Copernicus ( ) -Published in 1544 Tycho Brahe ( ) -Never published Johannes Kepler ( ) -Published his Laws of Motion starting in 1609 Galileo ( ) -Published “Dialogue of the Two Chief World Systems” in 1632

The Acceleration of Gravity (g) Galileo showed that g is the same for all falling objects, regardless of their mass.

Newton Newton was born about a year after Galileo died He nailed the foundation of the physics of our ordinary reality. He dominated Physics until Einstein. He was a bit of a misanthrope. ‒ Did poorly in school. Reports described him as 'idle' and 'inattentive'. ‒ He went to college at Cambridge when he was 18. ‒ Even then he was thought of as an average student. Newton was born about a year after Galileo died He nailed the foundation of the physics of our ordinary reality. He dominated Physics until Einstein. He was a bit of a misanthrope. ‒ Did poorly in school. Reports described him as 'idle' and 'inattentive'. ‒ He went to college at Cambridge when he was 18. ‒ Even then he was thought of as an average student.

Newton But at 20 (1663) math caught his fancy -In two years he ran through all of humankind’s mathematical knowledge and then proceeded to significantly advance it…. The plague closed the University between , sending Newton back home to a bit of enforced reflection. It was during this period that he puzzled-out most of his major contributions -Invention of calculus -Theory of gravity -The Laws of Motion -The inverse square law But at 20 (1663) math caught his fancy -In two years he ran through all of humankind’s mathematical knowledge and then proceeded to significantly advance it…. The plague closed the University between , sending Newton back home to a bit of enforced reflection. It was during this period that he puzzled-out most of his major contributions -Invention of calculus -Theory of gravity -The Laws of Motion -The inverse square law

Newton At 27 he became the Lucasian Professor of Mathematics at Cambridge In later life he held a number of major positions -Member of Parliament -Master of the Mint -President of the Royal Society -The first scientist to be knighted He was still very much a misanthrope -His response to any criticism was a bit extreme -He suffered two nervous breakdowns He could be a pretty nasty guy. -Take his dispute with Leibniz At 27 he became the Lucasian Professor of Mathematics at Cambridge In later life he held a number of major positions -Member of Parliament -Master of the Mint -President of the Royal Society -The first scientist to be knighted He was still very much a misanthrope -His response to any criticism was a bit extreme -He suffered two nervous breakdowns He could be a pretty nasty guy. -Take his dispute with Leibniz

Gravity Take a look around you….in our everyday experience we see gravity acting on all objects.

Newton made some simple observations based on his understanding of motion. -“A body remains stationary or moves in a straight line unless a force acts on it” -If a pen drops out of your hand, there HAS TO BE a force acting on that pen. Newton made some simple observations based on his understanding of motion. -“A body remains stationary or moves in a straight line unless a force acts on it” -If a pen drops out of your hand, there HAS TO BE a force acting on that pen.

Newton watched the Moon orbiting the Earth. -It is in orbit -it is moving in (essentially) a circle -Therefore…. force HAS TO BE acting on it to deflect it from a straight line. If you know the force HAS TO EXIST, then you can calculate what its strength has to be…. -About 1/3600 of the force of gravity on the Earth’s surface. -Other fact: Moon was about 60 times farther from the CENTER OF THE EARTH than the Earth’s surface Newton watched the Moon orbiting the Earth. -It is in orbit -it is moving in (essentially) a circle -Therefore…. force HAS TO BE acting on it to deflect it from a straight line. If you know the force HAS TO EXIST, then you can calculate what its strength has to be…. -About 1/3600 of the force of gravity on the Earth’s surface. -Other fact: Moon was about 60 times farther from the CENTER OF THE EARTH than the Earth’s surface

So the force on the Moon was 1/3600 as strong as the force acting on you at the surface of the Earth….. -He concluded that the force of the Earth’s gravity decreases by 1/r 2. The inverse square of distance -That is how gravity (and other fundamental forces) work, their intensity decreases by the inverse square of distance. INVERSE SQUARE LAWThis is called the INVERSE SQUARE LAW So the force on the Moon was 1/3600 as strong as the force acting on you at the surface of the Earth….. -He concluded that the force of the Earth’s gravity decreases by 1/r 2. The inverse square of distance -That is how gravity (and other fundamental forces) work, their intensity decreases by the inverse square of distance. INVERSE SQUARE LAWThis is called the INVERSE SQUARE LAW

How does this work? ALL matter produces a gravitation force! Why??? The force changes by 1/r 2 The inverse square law applies to ALL Forces….gravity, magnetism, electrical fields, light….! ALL matter produces a gravitation force! Why??? The force changes by 1/r 2 The inverse square law applies to ALL Forces….gravity, magnetism, electrical fields, light….! Gravity Light

What is the difference in terms of gravity between these pictures?

Gravity Remember gravity is measured from the center of an object -We are 6378 km from the center of the Earth -Space shuttle astronauts in space are only 200 km farther away….. -In inverse square terms that is 1.03 times Remember gravity is measured from the center of an object -We are 6378 km from the center of the Earth -Space shuttle astronauts in space are only 200 km farther away….. -In inverse square terms that is 1.03 times

Gravity Work it out -If the distance is 1.03 times the distance at the Earth’s surface…. -R 2 = /R 2 x G = 9.2 m/s 2 -The force of gravity is only 6% less in low Earth orbit Work it out -If the distance is 1.03 times the distance at the Earth’s surface…. -R 2 = /R 2 x G = 9.2 m/s 2 -The force of gravity is only 6% less in low Earth orbit

The reason astronauts are “weightless” in orbit is because they are in FREEFALL!

Remember: We live on a sphere When you drop something it heads toward the center of the Earth IF you put a bit of horizontal velocity on that something…. When you drop something it heads toward the center of the Earth IF you put a bit of horizontal velocity on that something….

Newton’s three laws of motion Newton’s first law of motion: An object moves at constant velocity unless a net force acts to change its speed or direction.

Newton’s second law of motion: Force = mass  acceleration

Newton’s third law of motion: For every force, there is always an equal and opposite reaction force.

How do gravity and energy together explain orbits? Orbits cannot change spontaneously. An object’s orbit can only change if it somehow gains or loses orbital energy = kinetic energy + gravitational potential energy (due to orbit). Orbits cannot change spontaneously. An object’s orbit can only change if it somehow gains or loses orbital energy = kinetic energy + gravitational potential energy (due to orbit).

 So what can make an object gain or lose orbital energy? Friction or atmospheric drag A gravitational encounter. Rockets….  So what can make an object gain or lose orbital energy? Friction or atmospheric drag A gravitational encounter. Rockets….

Center of mass Objects attracted by gravity orbit around their center of mass

Why Does Gravity Work? Einstein’s Relativity Quantum Gravity (String theory, Loop quantum gravity, Causal Dynamical Triangulation……and many more) Brans–Dicke theory of gravity Induced gravity In the modified Newtonian dynamics (MOND) The self-creation cosmology theory of gravity Nonsymmetric gravitational theory (NGT) Tensor–vector–scalar gravity (TeVeS) Gravity as an entropic force Superfluid vacuum theory Einstein’s Relativity Quantum Gravity (String theory, Loop quantum gravity, Causal Dynamical Triangulation……and many more) Brans–Dicke theory of gravity Induced gravity In the modified Newtonian dynamics (MOND) The self-creation cosmology theory of gravity Nonsymmetric gravitational theory (NGT) Tensor–vector–scalar gravity (TeVeS) Gravity as an entropic force Superfluid vacuum theory

Why are there Tides? Take the Bay of Fundy in Nova Scotia -Tides there can range 50 feet between high and low High (and low) tides happen basically twice a day Take the Bay of Fundy in Nova Scotia -Tides there can range 50 feet between high and low High (and low) tides happen basically twice a day

Tidal Bores in the Severn Estuary

Tides  Gravitational force decreases with (distance) 2  The Moon’s pull on Earth is strongest on the side facing the Moon, and weakest on the opposite side.  The Earth gets stretched along the Earth-Moon line.  The oceans rise relative to land at these points.

Tides Every place on Earth passes through high tides twice per day as the Earth rotates. High tides occur every 12 hours 25minutes remember, the Moon moves! The Sun’s tidal effect on Earth is not as strong. Remember the inverse square law….the Sun is a long way away.

Tides

Tides are strongest when the Sun & Moon pull in the same direction (new & full phases) –high tide is higher than usual (spring) Weakest when the Sun & Moon pull at right angles (first & last quarter phases) –high tide is lower than usual (neap) Spring and Neap Tides

Tidal Friction This fight between Moon’s pull & Earth’s rotation causes friction. Earth’s rotation slows down (1 sec every 50,000 yrs.) Conservation of angular momentum causes the Moon to move farther away from Earth. -The Moon is moving away from Earth at of 38 mm per year! This fight between Moon’s pull & Earth’s rotation causes friction. Earth’s rotation slows down (1 sec every 50,000 yrs.) Conservation of angular momentum causes the Moon to move farther away from Earth. -The Moon is moving away from Earth at of 38 mm per year!

Tidal Friction -It adds up! 4 billion years ago 1 day may have only been 5 or 6 hours long. -The moon may have been 1/10 the distance: 22 Earth radii away instead of 221 -How big would the tides be if the Moon was 110 Earth radii away? -It adds up! 4 billion years ago 1 day may have only been 5 or 6 hours long. -The moon may have been 1/10 the distance: 22 Earth radii away instead of 221 -How big would the tides be if the Moon was 110 Earth radii away?