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Chapter 4 Newton’s Laws of Motion PowerPoint® Lectures for University Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman Lectures by James Pazun Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley What are the properties of force(s)? • Combinations of “push” and “pull” Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley There are four common types of forces • The normal force— When an object rests or pushes on a surface, the surface pushes back. • Frictional forces—In addition to the normal force, surfaces can resist motion along the surface. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley There are four common types of forces II • Tension forces—When a force is exerted through a rope or cable, the force is transmitted through that rope or cable as a tension. • Weight—Gravity’s pull on an object. This force can act from large distances. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley What are typical sizes for common forces?—Table 4.1 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley How to denote a force—Figure 4.3 • Use a vector arrow to indicate magnitude and direction of the force. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Use the net (overall) force—Figure 4.4 • Several forces acting on a point have the same effect as their vector sum acting on the same point. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Decomposing a force into components • Fx and Fy are the parallel and perpendicular components of a force to a sloping surface. • Use F*Cosθ and F*Sinθ operations to find force components. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Notation and method for the vector sum—Figure 4.7 • We refer to the vector sum or resultant as the “sum of forces” R = F1 + F2 + F3 … Fn = ΣF. • Use Tanθ = Ry/Rx and R = (Rx2 + Ry2)1/2. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Superposition of forces—Example 4.1 • Adding all x components and all y components allows you to add many vectors. Example 4.1 has three. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newton’s First Law—Figure 4.9 • Simply stated— “objects at rest tend to stay at rest, objects in motion stay in motion.” • More properly, “A body acted on by no net force moves with constant velocity and zero acceleration.” Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newton’s First Law II—Figure 4.10 • Figure 4.10 shows an unbalanced force causing an acceleration and balanced forces resulting in no motion. • Refer to Conceptual Examples 4.2 and 4.3. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Inertial frames of reference—Figure 4.11 • When a car turns and a rider continues to move, the rider perceives a force. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newton’s Second Law—Figure 4.13 • An unbalanced force (or sum of forces) will cause a mass to accelerate. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley An object undergoing uniform circular motion • Refer to Figure 4.14. We have already seen the centripetal acceleration. But, if we measure the mass in motion, Newton’s Second Law allows us to calculate the centripetal force. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley The relationship of F, m, and a • Because a depends linearly on m and F, an acceleration will be directly proportional to the applied force. • Solution of the units gives a new combination of (kg*m)/s2 for the force. This is called… the Newton. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley The relationship of F, m, and a redux • Because a depends linearly on m and F, an acceleration will be inversely proportional to the object’s mass. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Using the Second Law—Example 4.4 • Refer to Example 4.4, using Figure 4.18. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Using the Second Law II—Example 4.5 • Refer to Example 4.5, using Figure 4.19. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newtons, kilograms, pounds, and slugs—Table 4.2 • Table 4.2 rightly points out that the pound is a force. The popular culture refers to it as a weight (which is actually a slug). • The Dyne is actually a cgs version of the Newton (sometimes used with fine work on tiny objects). Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Many have asked “how lethal is a coin dropped from atop a tall building”? • Urban legends have said that a penny dropped from the top of the Empire State Building can kill. • Conceptual Question 4.6 ponders this enigma with a euro. • Cable TV has allowed those two science guys who test such “myths” to debunk this one. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley g, and hence weight, is only constant on earth, at sea level • On Earth, g depends on your altitude. • On other planets, gravity will likely have an entirely new value. • Example 4.7 examines “apparent weight” in a rapidly stopping car. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newton’s Third Law • Exerting a force on a body results in a force back upon you. • Figure 4.25 shows “an action–reaction pair.” • See Example 4.8. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newton’s Third Law—Objects at rest • An apple on a table or a person in a chair—there will be the weight (mass pulled downward by gravity) and the normal force (the table or chair’s response). Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newton’s Third Law—Objects in motion • An apple falling or a refrigerator that needs to be moved—the first law allows a net force and mass to lead us to the object’s acceleration. Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Free-body diagrams—Figure 4.30 • A sketch then an accounting of forces Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley