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# 4 - Today Announcements No class on Thursday 2/4 Exam#1...

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ISP209s10 Lecture 4 -1- Today • Announcements: No class on Thursday 2/4 Exam #1 Thursday 2/11 – HW#2 (and extra credit #1) is due Wednesday Review Newton’s 1st and 2nd laws of motion Newton’s 3rd Law of motion Gravity, Planetary Orbits, and Newton’s Universal Law of Gravity and Unification

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ISP209s10 Lecture 4 -2- • Imagine we could turn off air resistance, friction, and gravity. How would things move? The Law of Inertia: A body that is subject to no external influences (also called external forces) will stay at rest if it was at rest to begin with and will keep moving if it was moving to begin with; in the latter case, its motion will be in a straight line at an unchanging speed. The Law of Inertia - Newton’s 1st Law Mass is the measure of an object’s inertia (I.e., resistance to changes in velocity)
ISP209s10 Lecture 4 -3- Intuition Alert! Intuition : When nothing pushes on an object, that object slows to a stop. I.e., you must push it to keep it going. Think of a thrown ball that eventually comes to rest. Physics : When nothing pushes on an object, that object coasts at constant velocity (I.e., doesn’t speed up, slow down, or change direction). Resolution: Objects such as a thrown ball experience “hidden” forces like air resistance, friction, or gravity. Once you eliminate these hidden forces, you will see that the law of inertia is “correct”.

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ISP209s10 Lecture 4 -4- Newton’s 2nd Law Acceleration = change in velocity /time What causes acceleration? A Force (a push or a pull) Force is a vector, it has a magnitude and a direction.
ISP209s10 Lecture 4 -5- Falling What happens when you drop a book, a ball, a beer bottle, a pencil, or any object at all from some height? They accelerate at a constant rate (g ~ 9.8 m/s 2 ) What is the force that is causing the acceleration? => GRAVITY v(t) = velocity at time t v 0 = velocity at t = 0 d(t) = distance object has fallen in time t Note that this Doesn’t depend on the mass, shape, size, color,… of the object

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ISP209s10 Lecture 4 -6- Gravity and Weight Phenomenon that produces an attractive (“pulling”) force between any and every pair of objects in the Universe. Depends on the masses and the distance between the pair of objects (and nothing more!) In our daily lives, the only object massive enough and near enough to have an obvious gravitational effect on us is Earth Weight = force on an object due to gravity Near the surface of earth,
ISP209s10 Lecture 4 -7- What is Momentum?

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