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Lecture 7

Lecture 7 - Gravity and Kepler's Laws - October 1.docx

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Astronomy & Astrophysics
Michael Reid

October 1, 2013. Lecture 7 – Gravity and Kepler’s Laws Kepler’s Laws  First law: the orbit of each planet around the Sun is an ellipse with the Sun at one focus o “The Law of Inertia” o In plain words: if a force does not disturb you, you keep on doing what you are doing o Things in motion stay in motion, things at rest stay at rest o If other forces are not involved, would a rocket (in space) in motion keep on moving forever?  Yes, in any case, at constant speed o An object moves at a constant velocity if there is no force acting upon it  Second law: force = mass x acceleration (or force = rate of change in momentum) o If you want to accelerate a body, you need a force o Do our bodies exert a force of gravity on Earth?  Yes o Compared to the force of gravity the Earth exerts on you, the force of gravity you exert on the Earth is:  The same; the force is always equal but the mass is not o Why is the Earth not moving towards us?  Do we attract gravitationally Earth?  The difference is the mass!  Third law: for any force, there is always an equal and opposite reaction force o Pushing each other on an ice rink o When Canadian astronaut Julie Payette is on board the International Space Station (ISS), why does she float?  In orbit around Earth, constant free-fall o Orbits  If you drop a ball the gravitational pulls between Earth and the ball cause it to drop  If I throw a ball, it will fall along a curved trajectory  Why doesn’t the moon, which is also affected by Earth’s gravitational pull, crash into the Ea
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