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# Astronomy Chapters 4 & 5 review.docx

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Department
Astronomy
Course
AST101H5
Professor
Jennifer Carlson
Semester
Fall

Description
Ch.4  an object is accelerating if either its speed or its direction is changing  gravity accelerates all objects by the same amount, regardless of their mass.—air resistance causes this difference in acceleration. Ex. If you drop a feather and a rock on the moon, where there is no air, both would fall at exactly the same rate.  The acceleration of a falling object is called acceleration of gravity.  An object must accelerate whenever a net force is present  Your mass is the same no matter where you are, but your weight can vary  Ppl and objects are weightless whenever they are falling freely, and astronauts are in constant state of free-fall Newton’s 3 laws of motion st  1 : in the absence of a net force, an object will move with constant vndocity—no net force, momentum does not change  2 :when net force is present, the amount of the acceleration depends on the objects mass and the strength of the net force—a net force changes momentum (usually changing velocity=acceleration) net force=mass*acceleration rd  3 :for any force, there is always an equal and opposite reaction force—forces come in opposite pairs (force=reaction force)  conservation of angular momentum: an objects angular momentum cannot change unless it transfers angular momentum to or from another object  earths angular momentum always stays the same as it orbits the sun, so it moves faster when its closer to the sun and slower when its farther from the sun. it needs no fuel to keep orbiting b/c no forces are acing in a way that could change its angular momentum (smaller radius= faster velocity) (larger radius= slower velocity)  earth keeps rotating—b/c—earth isn’t transferring any of the angular momentum of its rotation to another object, it keeps rotating at the same rate  earth is not exchanging substantial angular momentum w/ any other object, so its rotation and orbit must stay about the same  law of conservation of energy—energy cannot appear out of nowhere or disappear into nothingness. Objects can gain or loss energy only by exchanging energy w/ other objects  conservation of energy: energy can be transferred from one object to another or transformed from one type of energy to another, but the total amount is always the conserved ex. Sun became hot b/c of energy released by the gas that formed it  kinetic energy= energy of motion  radiative energy= energy carried by light  potential energy= stored energy  subcategory of kinetic energy is thermal energy—represents the collective kinetic energy of the many individual particles (atoms and molecules) moving randomly w/in a substance. Thermal energy- total kinetic energy of many individual particles  temperature measure the average kinetic energy of the particles  thermal energy depends on temp., a higher average kinetic energy for the particles in a substance must also lead to a higher total energy  an objects gravitational potential energy increases when it moves higher and decreases when it moves lower  thermal energy depends on both temp. and the density of a substance  E= mc this equation tells us that a small amount of mass contains a huge amount of energy Ex. Sun generates energy by converting a tiny fraction of its mass into energy through a similar process of nuclear fusion  Einstein’s formula tells us that mass can be converted into other forms of energy.  Where do objects get their energy? —b/c energy cannot be created or destroyed, objects always get their energy from other objects. We can always trace an objects energy back to the big bang, the beginning of the universe in which all matter and energy is thought to have come into existence  How gravity works? – universal law of gravitation—every object attracts every other object w/ a gravitational force that is directly proportional to the product of the objects’ masses and declines w/ the square of the distance b/w their centers  how does newton’s law of gravity extend kepler’s laws? 1. newton showed that kepler’s 1 2 laws apply to all orbiting objects, not just planets. 2. He showed that elliptical bound orbits are not the only possible orbital shape—orbits can also be unbound (taking shape of a parabola or a hyperbola) rd 3. Newton’s version of kepler’s 3 law allows us to calculate the masses of orbiting objects from their orbital periods and distances  Orbital energy: the sum of its kinetic and gravitational pot
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