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Gravity

Gravity. And---Sir Isaac Newton: The Universal Law of Gravitation. The Falling Apple. Newton reasoned the moon is falling for the same reason the apple falls – they are both pulled by the Earth’s gravity. Newton’s Excellent Idea.

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Gravity

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  1. Gravity And---Sir Isaac Newton: The Universal Law of Gravitation

  2. The Falling Apple • Newton reasoned the moon is falling for the same reason the apple falls – they are both pulled by the Earth’s gravity.

  3. Newton’s Excellent Idea • Now came Newton's truly brilliant insight: if the force of gravity reaches to the top of the highest tree, might it not reach even further; in particular, might it not reach all the way to the orbit of the Moon! Then, the orbit of the Moon about the Earth could be a consequence of the gravitational force, because the acceleration due to gravity could change the velocity of the Moon in just such a way that it followed an orbit around the earth. Source: http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

  4. The Falling Moon • Newton’s First law: • An object (the moon) will remain in motion in a straight line unless acted on by an outside unbalanced force (gravitational attraction) • The force that pulls an apple to the ground also keeps the moon in orbit.

  5. First, What is gravity? • An attractive force between two objects that depend upon the masses of the objects and the distance between them. • https://www.youtube.com/watch?v=mezkHBPLZ4A • https://www.youtube.com/watch?v=EwY6p-r_hyU

  6. The Falling Earth • Newton’s theory of gravitation confirmed the Copernican theory of the solar system. • A planets tangential speed allows the planets to orbit the sun without being pulled into it.

  7. Newton’s law of Universal Gravitation First, let’s get some background information https://www.youtube.com/watch?v=7gf6YpdvtE0

  8. Newton’s law of Universal Gravitation

  9. The Gravitational Constant Henry Cavendish 1798 He used a device with two objects on a rod that was hung on a wire. These masses were attracted to two larger objects, twisting the rod slightly. The measurement of the masses, distance between them and the torque on the wire allowed G to be determined.

  10. The Universal Gravitational Constant – A G-thing • G is a constant of proportionality. • The magnitude is the force exerted by two 1-kg masses 1 meter apart. • G = 6.67 x 10-11 Nm2/kg2

  11. Weight and Mass

  12. Gravity and Distance: The Inverse Square Law • A quantity varies as the inverse square of its distance from its source • A localized source spreads evenly throughout the surrounding space.

  13. The Weight of an Apple?

  14. Gravity and Orbits • Centripetal acceleration • ac= v2/r • Centripetal Force • Fc= mac • The force in circular motion is directed towards the center - centripetal

  15. So why is the earth round? • Because of gravitation – earth attracted itself together. • https://www.youtube.com/watch?v=T4WjyTV98lg Is the Earth Flat? • https://www.youtube.com/watch?v=uFIs7AZ8pOw How do we know it is round.

  16. Reviewing • Objects orbiting around the Earth are actually falling toward Earth but have great enough tangential velocity to avoid hitting the earth. • Everything pulls on everything else with a force that depends on the mass of the objects and distances between them. • Gravitation decreases according to the inverse square law. • http://studyjams.scholastic.com/studyjams/jams/science/solar-system/sgravity-and-inertia.htm

  17. Weightlessness

  18. The velocity of a satellite keeps it in orbit • Even when moving, the satellite is actually accelerating toward the Earth (this is what keeps it in its circular path) • Its acceleration results in a curved path which is the same as the curve of the Earth • Gravity is providing the centripetal force

  19. Perception of Weightlessness • There is still gravity acting in a satellite (about 8.9 m/s2), so why do we feel weightless? • In an free falling elevator, if the FA is equal to the FG, there is no FN • No force is felt feel weightless – called apparent weightlessness

  20. Weightlessness that you feel in a satellite is like the weightlessness in an elevator • The satellite and everything on it are all accelerating toward the earth at the same rate

  21. Circular Motion • When an object moves in a circle at constant speed, we describe it as undergoing uniform circular motion. • Its speed is constant, but its velocity is not because velocity includes direction and the object’s direction is clearly changing.

  22. Circular Motion • A changing velocity means acceleration. • The pull on the string is always directed perpendicular to the velocity. • The pull accelerates the ball into a circular path, even though the ball does not speed up or slow down. • The pull changes only the direction of the velocity, not the magnitude.

  23. Centripetal Acceleration • The acceleration arising from the change in direction of the velocity vector is called the centripetal acceleration and is determined mathematically by:

  24. Gravity can be a Centripetal Force

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