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Unformatted text preview: Page 1 Physics 207 – Lecture 11 Physics 207: Lecture 11, Pg 1 "Professor Goddard does not know the relation between action and reaction and the need to have something better than a vacuum against which to react. He seems to lack the basic knowledge ladled out daily in high schools." New York Times editorial, 1921, about Robert Goddard's revolutionary rocket work . "Correction: It is now definitely established that a rocket can function in a vacuum. The 'Times' regrets the error." New York Times editorial, July 1969. Physics 207: Lecture 11, Pg 2 Partial Survey Summary Partial Survey Summary Lecture c Too many slides that come too quickly c More problem solving on white board c Too much time spent on “interactive problems but, when used, not enough time spent on explanation c More demos Physics 207: Lecture 11, Pg 3 Another example with friction and pulley Another example with friction and pulley c Three 1 kg masses are connected by two strings as shown below. There is friction between the stacked masses but the table top is frictionless. c Assume the pulleys are massless and frictionless. c What is T 1 ? M M M T 1 friction coefficients μ s =0.4 and μ k =0.2 Physics 207: Lecture 11, Pg 4 Physics 207, Physics 207, Lecture 11, Oct. 10 Lecture 11, Oct. 10 Agenda: Agenda: Assignment: Assignment: c Read through Chapter 10 c MP HW5 available now, due Wednesday 10/17, 11:59 PM • Chapter 9: Momentum & Impulse Chapter 9: Momentum & Impulse x Momentum conservation x Collisions x Impulse Physics 207: Lecture 11, Pg 5 Impulse & Linear Momentum Impulse & Linear Momentum c Transition from forces to conservation laws Transition from forces to conservation laws Newton’s Laws b Conservation Laws Conservation Laws b Newton’s Laws They are different faces of the same physics phenomenon. NOTE: We already have studied “impulse” and “momentum” but we have not explicitly named them as such Physics 207: Lecture 11, Pg 6 Lecture 11, Example 1 Lecture 11, Example 1 A 2 kg cart initially at rest on frictionless horizontal surface is acted on by a 10 N horizontal force along the positive x-axis for 2 seconds what is the final velocity? c F is in the x-direction F = ma so a = F/m = 5 m/s 2 c v = v + a t = 0 m/s + 2 x 5 m/s = 10 m/s (+x-direction) What if the mass had been 4 kg? What is the new final velocity? Page 2 Physics 207 – Lecture 11 Physics 207: Lecture 11, Pg 7 Twice the mass Twice the mass c Same force c Same time c Half the acceleration c Half the velocity ! ( 5 m/s ) Physics 207: Lecture 11, Pg 8 Example 1 Example 1 c Notice that the final velocity in this case is inversely proportional to the mass (i.e., if thrice the mass….one-third the velocity)....
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This note was uploaded on 10/30/2011 for the course PHYS 207 taught by Professor Winnokur during the Spring '06 term at University of Wisconsin.
- Spring '06