July 9th, Lecture 3.docx

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Astronomy & Astrophysics
Marija Stankovic

JULY 9 , LECTURE 3, AST201H1 Kepler’s First Law (insert slide) Stretched out circle. Difference between circle and the ellipse, two points, focus. How far apart they are will tell us how stretch out it will be. Circle is essentially low eccentricity (e=0) High eccentricity (e=0.5) Kepler’s Third Law P oc a 3 Orbital period, P (time taken to complete one orbit) oc (proportional to) a (orbital semimajor axis, a. Half of the longest axis of an ellipse) The further away, the smaller the speed. Could describe how things orbited. Couldn’t say why they orbited that way. Gravity Every mass attracts every other mass. Gravity is a universal force between all objects having mass. Between solar systems, between galaxies. Earth has a much larger mass, so that is why it is so strong. Gravity is unique because it only ever adds up- there is no “anti-gravity”. This property allows Gravity to dominate all other forces on large scales, even though it is by far the weakest force. F= G(M X1M / 2 X d) Our mass and Earth’s mass and distance between us and the Earth. Force is the same. Nothing changes. There is gravity on the moon. It has mass. More pull you have, the higher your weight is. Smaller mass, smaller gravitational pull, less of a pull so you weigh more. (insert slide) Two different planets. Gravity only depends on the mass of these two objects and the distance between them. How much matter is inside this object. Depends entirely upon distance times distance. Increase separation between these objects two times, decrease four times. Larger the separation, force of gravity is weaker. The larger the distance, the weaker force of gravity. Square of this distance to get how much times. If you move it three times away, it will be 9 times weaker. If move it three times closer, it will be 9 times stronger. It just matters if you have mass, doesn’t have a requirement. (insert slide) Space shuttle orbiting. Mass of the planet stays the same, but planet gets bigger. STAYS THE SAME. In all of these cases, M1 is mass of planet and M2 will be the mass of the shuttle If the size of the earth will double (with the mass unchanged), the gravitational attraction of the Sun would not change. If I throw the ball very slowly, it will fall along a curving trajectory. At v= orbitale ball falls without hitting Earth! It’s in orbit.circularcircling the Earth. escape,he ball breaks free of Earth’s gravity. Escape speed doesn’t depend on the direction in which the object is thrown. V> V escape unbound orbit (insert slide) Different shapes of orbits.
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