# jan12_10 - The Physics of Astronomy(Chapter 4 of Text...

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The Physics of Astronomy (Chapter 4 of Text ) Understanding Motion, Energy, and Gravity

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Announcements All Tutorial Group changes, late registrations, etc. are handled by your College Registrar, who will give you a form to be signed by me. I do that after class, or in my office (any time, contact me, Prof. Mochnacki first). Room AB 127.
• Assignment 1 closes before midnight tonight. Assignment 2 comes online early tomorrow morning. • Assignment 2 will use material from the first 3 lectures, plus your own reading.

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Term Test The Term Test will now be held on Tues. 2 nd March 2010 at 4.10 pm . Room assignments will be made after Reading Week. The change was necessary because we were being split up into too many rooms, and we have only 20 teaching assistants!
4.1 Describing Motion Our goals for learning: • How do we describe motion? • How is mass different from weight?

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How do we describe motion? Precise definitions to describe motion: Speed : Rate at which object moves speed = distance time units of m s example: speed of 10 m/s Velocity : Speed and direction example: 10 m/s, due east Acceleration : Any change in velocity units of speed/time (m/s 2 ) (An acceleration may involve change of direction , not “speed” per se)
The Acceleration of Gravity • All falling objects accelerate at the same rate (not counting friction of air resistance). • On Earth, g ≈ 10 m/s 2 : speed increases 10 m/s with each second of falling.

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The Acceleration of Gravity ( g ) • Galileo showed that g is the same for all falling objects, regardless of their mass.
Momentum and Force Momentum = mass × velocity • A net force changes momentum, which generally means an acceleration (change in velocity) • Rotational momentum of a spinning or orbiting object is known as angular momentum

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In which case is there NO net force? 1.
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jan12_10 - The Physics of Astronomy(Chapter 4 of Text...

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