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# Lect03 - Physics 211 Lecture 3 Todays Agenda Reference...

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Physics 211: Lecture 3, Pg 1 Physics 211: Lecture 3 Physics 211: Lecture 3 Today Today ’s Agenda s Agenda z Reference frames and relative motion z Uniform Circular Motion ¾ Polar coordinates ¾ Centripetal acceleration!

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Physics 211: Lecture 3, Pg 2 Lecture 3, Lecture 3, Act 1 Act 1 Which statement is true: Which statement is true: A) If an object’s speed is constant then its acceleration must be zero. B) If an object’s acceleration is zero then its speed must be constant. C) If an object’s speed is constant then its velocity must be constant. D) An object’s velocity is always in the same direction as its acceleration.
Physics 211: Lecture 3, Pg 3 Inertial Reference Frames: Inertial Reference Frames: z A Reference Frame Reference Frame is the place you measure from. ¾ It’s where you nail down your (x,y,z) axes! z An Inertial Reference Frame ( IRF ) is one that is not accelerating . ¾ We will consider only IRF s in this course. z Valid IRF s can have fixed velocities with respect to each other. ¾ but not relative accelaration ¾ see what follows… Cart on track on track

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Physics 211: Lecture 3, Pg 4 Relative Motion Relative Motion z Consider a problem with two two distinct IRFs: ¾ An airplane flying on a windy day An airplane flying on a windy day . Î A pilot wants to fly from Champaign to Chicago. Having asked a friendly physics student, she knows that Chicago is 120 miles due north of Urbana. She takes off from Willard Airport at noon. Her plane has a compass and an air-speed indicator to help her navigate. ¾ The compass allows her to keep the nose of the plane pointing north. ¾ The air-speed indicator tells her that she is traveling at 120 miles per hour with respect to the air with respect to the air .
Physics 211: Lecture 3, Pg 5 Relative Motion. .. Relative Motion. .. z The plane is moving north in the IRF attached to the air : ¾ V p, a is the velocity of the plane w.r.t. the air. Air Air V p,a

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Physics 211: Lecture 3, Pg 6 Relative Motion. .. Relative Motion. .. z But suppose the air is moving east in the IRF attached to the ground. ¾ V a,g is the velocity of the air w.r.t. the ground (i.e. wind ). V a,g Air Air V p,a
Physics 211: Lecture 3, Pg 7 Relative Motion. .. Relative Motion. .. z What is the velocity of the plane in an IRF attached to the ground? ¾ V p,g is the velocity of the plane w.r.t. the ground. V p,g

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Physics 211: Lecture 3, Pg 8 Relative Motion. .. Relative Motion. .. Î V p,g = V p,a + V a,g Is a vector equation relating the airplane’s velocity in different reference frames. V p,g V a,g V p,a Tractor
Physics 211: Lecture 3, Pg 9 Lecture 3, Lecture 3, Act 2 Act 2 Relative Motion Relative Motion z You are swimming across a 50m wide river in which the current moves at 1 m/s with respect to the shore. Your swimming speed is 2 m/s with respect to the water.

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Lect03 - Physics 211 Lecture 3 Todays Agenda Reference...

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