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sy11_oct10_07hc

# sy11_oct10_07hc - "Professor Goddard does not know the...

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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.

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Physics 207: Lecture 11, Pg 2 Partial Survey Summary Partial Survey Summary Lecture Too many slides that come too quickly More problem solving on white board Too much time spent on “interactive problems but, when used, not enough time spent on explanation More demos
Physics 207: Lecture 11, Pg 3 Another example with friction and pulley Another example with friction and pulley 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. Assume the pulleys are massless and frictionless. What is T 1 ? M M M T 1 friction coefficients μ s =0.4 and μ k =0.2

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Physics 207: Lecture 11, Pg 4 Physics 207, Physics 207, Lecture 11, Oct. 10 Lecture 11, Oct. 10 Agenda: Agenda: Assignment: Assignment: Read through Chapter 10 MP HW5 available now, due Wednesday 10/17, 11:59 PM Chapter 9: Momentum & Impulse Chapter 9: Momentum & Impulse Momentum conservation Collisions Impulse
Physics 207: Lecture 11, Pg 5 Impulse & Linear Momentum Impulse & Linear Momentum Transition from forces to conservation laws Transition from forces to conservation laws Newton’s Laws Conservation Laws Conservation Laws 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

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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? F is in the x-direction F = ma so a = F/m = 5 m/s 2 v = v 0 + 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?
Physics 207: Lecture 11, Pg 7 Twice the mass Twice the mass Same force Same time Half the acceleration Half the velocity ! ( 5 m/s )

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Physics 207: Lecture 11, Pg 8 Example 1 Example 1 Notice that the final velocity in this case is inversely proportional to the mass (i.e., if thrice the mass….one-third the velocity). It would seems that mass times the velocity always gives the same value. (Here is it always 20 kg m/s.)
Physics 207: Lecture 11, Pg 9 Example 1 Example 1 There many situations in which the product of “mass times velocity” is a constant and so we give a special name, “momentum” and associate it with a conservation law.

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