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Chapter 4 Forces and Newton’s Laws of Motion Topics: • Motion and Newton’s first law • What is a force? • Identifying forces • Newton’s second law • Free-body diagrams • Newton’s third law Sample question: These ice boats sail across the ice at great speeds. What gets the boats moving in the first place? What keeps them from going even faster? Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-1 Example An elevator, lifted by a cable, is going up at a steady speed. • • Identify the forces acting on the elevator. Is FT,cable => elevator greater than (A), equal to (B), or less than Fg,Earth => elevator (C)? Or is there not enough information to tell (D)? Slide 4-23 Checking Understanding 10-year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is A. greater than the force of Sarah on Jack. B. equal to than the force of Sarah on Jack. C. less than the force of Sarah on Jack. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-27 Answer 10-year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is B. equal to than the force of Sarah on Jack. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-28 A Short Catalog of Forces: Weight w Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-12 Spring Force Fsp Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-13 Tension Force T Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-14 Normal Force n Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-15 Friction fk and fs Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-16 Drag D and Thrust Fthrust Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-17 Example 1 A block is dragged uphill by a rope. Identify all forces acting on the block. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-19 Example 2 The cable at left exerts a -30 N force. • Write the equation for the sum of the forces in the x-direction. What is the value of FT2? • Write the equation for the sum of the forces in the y-direction. What is the force of gravity acting on the ball? 30 T2 T1 Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-20 Exercise A ball, hanging from the ceiling by a string, is pulled back and released. Identify the forces acting on it just after its release. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-21 Newton’s Second Law Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-22 Newton 2 Demonstrations Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-19 Newton’s Third Law Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-25 Newton’s 3rd Law Demonstrations Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-19 Newton’s Laws of Motion Newton 0th Law Objects are dumb - They have no memory of the past and cannot predict the future. Objects only know what is acting directly on them right now Newton's 1st Law An object that is at rest will remain at rest and an object that is moving will continue to move in a straight line with constant speed, if and only if the sum of the forces acting on that object is zero. Newton's 2nd Law acceleration of an object = sum of forces acting on that object / the mass of the object Newton's 3rd Law Recall that a force is an interaction between two objects. If object A exerts a force on object B then object B exerts a force on object that is in the opposite direction, equal in magnitude, and of the same type. Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-19