Physics 207: Lecture 9, Pg 1

Physics 207, Physics 207, Lecture 9,Lecture 9, Oct 5 Oct 5 ‘‘0909

TODAYTODAY

Circular motion: 4.5, 4.6, 8.2, 8.3Circular motion: 4.5, 4.6, 8.2, 8.3 Momentum: 9.1, 9.2Momentum: 9.1, 9.2 Exams to be returned in discussionExams to be returned in discussion this weekthis week

Assignments:Assignments: This week’s lab: Equilibrium Force (M-2) & Circular Motion (M-6) HW 5 due Friday, Oct 9, 6 pm

Physics 207: Lecture 9, Pg 2

Circular MotionCircular Motion

Car driving around curved track Planets orbiting Sun (actually elliptical motion) Electrons orbiting nucleus (simplest “Bohr model” for atom)

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Prof. Timbie whirls a tennis ball around hisProf. Timbie whirls a tennis ball around hishead on a string. If the string breaks, thehead on a string. If the string breaks, the

ball:ball:

A. Continues to follow a circular pathB. Follows a straight lineC. Follows a slightly curved path

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What makes a rocket take off?What makes a rocket take off?

A. Newton’s third lawB. Conservation of MomentumC. Hot gases push against air

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"Professor Goddard does not know the relation betweenaction and reaction and the need to have something betterthan a vacuum against which to react. He seems to lackthe basic knowledge ladled out daily in high schools."New York Times editorial, 1921, about Robert Goddard'srevolutionary rocket work.

"Correction: It is now definitelyestablished that a rocket canfunction in a vacuum.The 'Times' regrets the error."New York Times editorial, July 1969.

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Newton’s Third Law