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sy05_sept19_07hc

# sy05_sept19_07hc - Physics 207 Lecture 5 Sept 19 Physics...

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Physics 207: Lecture 5, Pg 1 Physics 207, Physics 207, Lecture 5, Sept. 19 Lecture 5, Sept. 19 Agenda: Agenda: Assignment: Assignment: For Monday: Read Chapter 6 For Monday: Read Chapter 6 MP Problem Set 2 due tonight(!) MP Problem Set 2 due tonight(!) MP Problem Set 3 due next week MP Problem Set 3 due next week Finish Chapter 4 and Chapter 5 Finish Chapter 4 and Chapter 5 Inertial reference frames Free Body Diagrams Non-zero net Forces (acceleration) Friction

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Physics 207: Lecture 5, Pg 2 Newton’s First Law and Newton’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 . The above statement can be used to define inertial reference frames. An IRF is a reference frame that is not accelerating (or rotating) with respect to the “fixed stars”. If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector! The surface of the Earth may be viewed as an IRF
Physics 207: Lecture 5, Pg 3 Newton’s Second Law Newton’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. This expression is vector expression: F x , F y , F z

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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-contact Forces Non-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

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Physics 207: Lecture 5, Pg 6 No net force No net force 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 F B,G (Force vectors are not always drawn at contact points) mg N N mg F y = = + - = 0 y
Physics 207: Lecture 5, Pg 7 Lecture 5, Lecture 5, Exercise 1 , Exercise 1 , Newton’s 2 Newton’s 2 nd nd Law Law A. P + C < W B. P + C > W C. P = C D. P + C = W A woman is straining to lift a large crate, without success. It is  too  heavy. We denote the forces on the crate as follows:  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).  Which of following relationships between these forces is true,  while the person is trying unsuccessfully to lift the crate? ( Note:  force up is positive & down is negative )

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Physics 207: Lecture 5, Pg 8 Lecture 5,
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sy05_sept19_07hc - Physics 207 Lecture 5 Sept 19 Physics...

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