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Unformatted text preview: Page 1 Physics 207 Lecture 5 Physics 207: Lecture 5, Pg 1 Physics 207, Physics 207, Lecture 5, Sept. 19 Lecture 5, Sept. 19 Agenda: Agenda: Assignment: Assignment: c For Monday: Read Chapter 6 For Monday: Read Chapter 6 c MP Problem Set 2 due tonight(!) MP Problem Set 2 due tonight(!) c MP Problem Set 3 due next week MP Problem Set 3 due next week c Finish Chapter 4 and Chapter 5 Finish Chapter 4 and Chapter 5 x Inertial reference frames x Free Body Diagrams x Non-zero net Forces (acceleration) x Friction Physics 207: Lecture 5, Pg 2 Newton Newton s First Law and s First Law and IRFs IRFs An object subject to no external forces moves with a constant velocity if viewed from an inertial reference inertial reference frame (IRF) frame (IRF) . If no net force acting on an object, there is no acceleration. c The above statement can be used to define inertial reference frames. x An IRF is a reference frame that is not accelerating (or rotating) with respect to the fixed stars. x If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector! x 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. c This expression is vector expression: F x , F y , F z c 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 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 Earths 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 b No acceleration No acceleration F B,T Normal force is always to a surface 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 = = +- = y Page 2 Physics 207 Lecture 5 Physics 207: Lecture 5, Pg 7 Lecture 5, Lecture 5, Exercise 1 , Exercise 1 , Newton Newton s 2 s 2 nd nd Law 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 W is the weight (force of the earth on the crate)....
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