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# Gravity

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• You may also wish to do the rock and paper demonstration described in the Think About It on p. 88.
• ### Gravity

1. 1. Gravity and the Rules of Attraction Newton vs. Einstein
2. 2. Mass Vs. Weight <ul><li>Mass is the amount of matter that makes up an object. </li></ul><ul><li>Mass of an object does not change (ex. A bar of gold will always have the same mass.) </li></ul><ul><li>Weight is the measurement of the force of gravity on an object (gravitational pull on the mass). </li></ul><ul><li>Ex. 1- scales measure the force that Earth exerts on the object. </li></ul><ul><li>Ex. 2 – the weight of a bar of gold will change on different planets. </li></ul>
3. 3. How is mass different from weight? <ul><li>mass – the amount of matter in an object </li></ul><ul><li>weight – the force that acts upon an object </li></ul>You are weightless in free-fall!
4. 4. Gravity – the force of attraction between any 2 masses, any 2 bodies, or any 2 particles. Gravity exists between all objects in the universe <ul><li>Gravitational pull is dependent on Mass- </li></ul><ul><li>Small mass = small gravitational pull </li></ul><ul><li>Large mass = big gravitational pull </li></ul><ul><li>Small gravitational pull = small weight </li></ul><ul><li>Large gravitational pull = large weight </li></ul><ul><li>Ex. The Moon is 1/6th the sixth the size of the Earth. So an object on the Moon would weigh 1/6 th as much. </li></ul>
5. 5. Thought Question On the Moon: <ul><li>My weight is the same, my mass is less. </li></ul><ul><li>My weight is less, my mass is the same. </li></ul><ul><li>My weight is more, my mass is the same. </li></ul><ul><li>My weight is more, my mass is less. </li></ul>
6. 6. On the Moon… <ul><li>My weight is the same, my mass is less. </li></ul><ul><li>My weight is less, my mass is the same. </li></ul><ul><li>My weight is more, my mass is the same. </li></ul><ul><li>My weight is more, my mass is less. </li></ul>
7. 7. Gravity on Earth <ul><li>Acceleration due to gravity on Earth = </li></ul><ul><li>9.8 meters/ second 2 </li></ul><ul><li>Free falling objects increase in speed 9.8 meters per second for every second they fall. </li></ul><ul><li>Rate of acceleration is NOT dependent on mass. </li></ul><ul><li>Ex. A golf ball and a shot put dropped at the same time from the same height will reach Earth at the same time. </li></ul><ul><li>For this to be true- what must we leave out? </li></ul>
8. 8. The Acceleration of Gravity <ul><li>All falling objects accelerate at the same rate (not counting friction of air resistance). </li></ul><ul><li>On Earth, g ≈ 9.8 m/s 2 : speed increases 9.8 m/s with each second of falling. </li></ul>
9. 9. One Exception for Gravity on Earth…… <ul><li>Earth has an atmosphere- so, Gravity is: </li></ul><ul><li>NOT independent of air resistance </li></ul><ul><li>Air Resistance slows down acceleration </li></ul><ul><li>In vacuum, all objects fall at the same rate. </li></ul><ul><li>Without a vacuum- objects fall at rates depending on their surface areas. </li></ul>
10. 10. Acceleration of Gravity of an object on Earth is independent of… <ul><li>1. The mass of the object </li></ul><ul><li>2. Velocity of the object parallel to the ground (I.e. horizontal velocity) </li></ul><ul><li>3. Then this means… </li></ul>
11. 11. If an object … <ul><li>..is traveling at a certain velocity parallel to the Earth’s surface, </li></ul><ul><li>it will fall to the Earth at the same rate as a stationary object (falling straight to Earth) dropped at the same height. </li></ul><ul><li>Ex. Simultaneously shoot a bullet from a gun and drop a bullet straight from the same height- both bullets will hit at the same time. </li></ul>
12. 12. The Effect of Gravity on Earth <ul><li>Keeps our Earth’s atmosphere, oceans, and inhabitants from drifting into space </li></ul><ul><li>Pulls rain into rivers and eventually into the oceans </li></ul><ul><li>Effects the way our muscles and bones develop and function. (ie. bench pressing, dunking a basketball.) </li></ul>
13. 13. <ul><li>Realized the same physical laws that operate on Earth also operate in the heavens </li></ul><ul><ul><li>one universe </li></ul></ul><ul><li>Discovered laws of motion and gravity </li></ul><ul><li>Much more: experiments with light; first reflecting telescope, electromagnetic spectrum, calculus… </li></ul><ul><li>Is most famous for his work- “Principia” which was two years of thought and scribbling, but was only a recounting of what he could remember. </li></ul>How did Newton change our view of the Universe? Sir Isaac Newton (1642-1727)
14. 14. Newton was a neat guy…. <ul><li>Described as “odd, brilliant beyond measure, solitary, joyless, prickly to the point of paranoia, famously distracted.” </li></ul><ul><li>Capable of riveting strangeness </li></ul><ul><li>Once inserted a “bodkin” – a long needle used for sewing leather- into his eye socket and rubbed it around “betwixt my eye and the bone as near to the backside of my eye as I could” just to see what would happen. </li></ul><ul><li>A hair sample analyzed in the 1970s found a high level of mercury – 40 times the natural level found for most people. </li></ul>
15. 15. Newton: Ideas About Gravity <ul><li>Gravity is apparent everywhere on Earth from the mountains to the valleys </li></ul><ul><li>He wondered – Does gravity… </li></ul><ul><li>Extend to the Moon? </li></ul><ul><li>Account for Earth and Moon movement? </li></ul><ul><li>Motions of other planets ? </li></ul>
16. 16. Variables <ul><li>Two variables can change the force of gravity: </li></ul><ul><li>The mass of the objects </li></ul><ul><li>The distance between the objects </li></ul>
17. 17. Universal Law of Gravitation <ul><li>Every mass attracts other mass through the force of gravity. Anything with a mass has its own gravity, although it may be very small *: note that gravity obeys the inverse square rule </li></ul>
18. 18. Breaking down the equation… <ul><li>Fg = force of gravity </li></ul><ul><li>G = gravitational constant = 6.67 x 10 -11 N . m 2 /kg 2 </li></ul><ul><li>M 1 = mass of the first object </li></ul><ul><li>M 2 = mass of the second object </li></ul><ul><li>d = distance between the centers of each object </li></ul>
19. 19. This means that: <ul><li>As objects get farther apart, the effect of gravity drops. </li></ul><ul><li>Ex. An object 1 km from Earth is pulled much more strongly than an object 3km away. </li></ul><ul><li>This drop is an “inverse proportion” </li></ul>
20. 20. Questions to consider! <ul><li>1. Why do tides occur in the lakes and oceans on Earth? </li></ul><ul><li>2. How do planets in our solar system stay in orbit? </li></ul><ul><li>3. How does the Sun’s force travel through so much empty space? </li></ul><ul><li>4. What would happen if a person stepped out into space and opened a package of Skittles? M&Ms? </li></ul>
21. 21. Einstein and Relativity That Anyone Can Understand <ul><li>First things first – to understand where Al was coming from, we must remember the following: </li></ul><ul><li>The speed of light is the universal speed limit (299,792.458 km/ sec.) </li></ul><ul><li>The flow of time is dependent upon the state of the clock </li></ul><ul><li>Space and time are interconnected. (adding a fourth dimension) </li></ul><ul><li>The speed of light in empty space can not be reached or exceeded by any massive object in the universe. </li></ul>
22. 22. Einstein’s New Theory of Gravity <ul><li>Einstein felt that Newton’s view of gravity worked well on Earth, but not so well in space. </li></ul><ul><li>Einstein felt that gravity should be described as a field in space rather than a strange property of matter. </li></ul><ul><li>Any object that has mass must have gravity. </li></ul><ul><li>Gravity is the result of the interaction between matter and space – between the Earth and the space immediately surrounding the Earth. </li></ul>
23. 23. Spacetime and Einstein's Gravity <ul><li>The mass of an object creates a “warp” or “distortion” of space time. </li></ul><ul><li>The greater the mass, the greater the distortion. </li></ul><ul><li>The size of the distortion determines the motion of the objects around it. </li></ul>
24. 24. Gravity is Curved Space <ul><li>For planets orbiting the Sun – they are effected by the largest warp in space time. </li></ul><ul><li>The Sun is the most massive, so it has the most gravity. </li></ul><ul><li>This ‘gravity’ tells that planets how to move. </li></ul>
25. 25. If Spacetime was a Trampoline <ul><li>Picture Spacetime as a trampoline holding 100 people of equal weight. </li></ul><ul><li>If everyone has their own space, the trampoline should remain evenly distributed. (with no large lumps.) </li></ul><ul><li>What would happen if everyone walked into the middle, creating a large warp in spacetime? </li></ul>
26. 26. As the people move towards the center- the “warp” grows larger. Eventually, even the furthest person is effected by the warped spacetime
27. 27. Gravity, Escape Velocity, and Black Holes <ul><li>Now that we understand a little more about gravity: ponder these questions- </li></ul><ul><li>How fast would an object have to go to escape the gravity of Earth? </li></ul><ul><li>Is it possible to have an object so massive that the escape velocity is beyond the speed of light? </li></ul><ul><li>What about a “Theory of Everything?” </li></ul>