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sy05_sept19_07hc

sy05_sept19_07hc - Physics 207 Lecture 5 Physics 207...

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Page 1 Physics 207 – Lecture 5 Physics 207: Lecture 5, Pg 1 Physics 207, Lecture 5, Sept. 19 Agenda: Agenda: Assignment: circle6 For Monday: Read Chapter 6 circle6 MP Problem Set 2 due tonight(!) circle6 MP Problem Set 3 due next week MP Problem Set 3 due next week circle6 Finish Chapter 4 and Chapter 5 xrhombus Inertial reference frames xrhombus Free Body Diagrams xrhombus Non-zero net Forces (acceleration) xrhombus Friction Physics 207: Lecture 5, Pg 2 Newton Newton’ s First Law and IRFs An object subject to no external forces moves with a constant velocity if viewed from an inertial reference frame (IRF) . If no net force acting on an object, there is no acceleration. circle6 The above statement can be used to define inertial reference frames. xrhombus An IRF is a reference frame that is not accelerating (or rotating) with respect to the “fixed stars”. xrhombus If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector! xrhombus The surface of the Earth may be viewed as an IRF Physics 207: Lecture 5, Pg 3 Newton Newton’ s Second Law s Second Law The acceleration of an object is directly proportional to the net force acting upon it. The constant of proportionality is the mass. circle6 This expression is vector expression: F x , F y , F z circle6 Units The metric unit of force is kg m/s 2 = Newtons (N) The English unit of force is Pounds (lb) Physics 207: Lecture 5, Pg 4 Contact (i.e., normal) Forces Certain forces act to keep an object in place. These have what ever force needed to balance all others (until a breaking point). F B,T Physics 207: Lecture 5, Pg 5 Non Non- contact Forces contact Forces All objects having mass exhibit a mutually attractive force (i.e., gravity) that is distance dependent At the Earth’s surface this variation is small so little “g” (the associated acceleration) is typically set to 9.80 or 10. m/s 2 F B,G Physics 207: Lecture 5, Pg 6 No net force No net force barb2right No acceleration No acceleration F B,T Normal force is always to a surface 0 0 0 net = = = = = y x F F a m F F r r r F B,G (Force vectors are not always drawn at contact points) mg N N mg F y = = - = 0 y

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Page 2 Physics 207 – Lecture 5 Physics 207: Lecture 5, Pg 7 Lecture 5, Lecture 5, Exercise 1 , Newton Newton’ s 2 nd Law A. P + C < W B. P + C > W C. P = C D. P + C = W P is the upward force being exerted on the crate by the person C is the contact force on the crate by the floor, and Wis the weight (force of the earth on the crate). Physics 207: Lecture 5, Pg 8 Lecture 5, Example Gravity and Normal Forces A woman in an elevator is accelerating upwards The normal force exerted by the elevator on the woman is, (A) greater than (B) the same as (C) less than the force due to gravity acting on the woman Physics 207: Lecture 5, Pg 9 Lecture 5, Example Example Gravity and Normal Forces Gravity and Normal Forces A woman in an elevator is accelerating upwards The normal force exerted by the elevator on the woman is,
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sy05_sept19_07hc - Physics 207 Lecture 5 Physics 207...

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