Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Motion and Newton’s first law What is a force? Identifying forces Newton’s second law Free-body diagrams Newton’s third law Chapter 4 Forces and Newton’s Laws of Motion Topics: 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? Slide 4-1

Example An elevator, lifted by a cable, is speeding up while moving upward. Identify the forces acting on the elevator. Is F T,cable => elevator greater than (A), equal to (B), or less than F g,Earth => elevator (C)? Or is there not enough information to tell (D)? Slide 4-23

Nellie hangs motionless, which means all the forces acting on her equal zero: The force due to gravity acting downward, her weight, is shown by the bold vector in A. Equilibrium dictates an equal force upward, supplied by the ropes, indicated by the dashed vector. This dashed vector has to be the resultant of tensions in the left and right sides of the rope. Their relative sizes are found by constructing a parallelogram, with the dashed vector as its diagonal (B). Aha, the relative magnitudes of these tensions are shown in C. The right side is under greater tension, and therefore is the most likely to break.

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. Slide 4-27

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. 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. Slide 4-28 Answer

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. A Short Catalog of Forces: Weight w Slide 4-12  

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Spring Force F sp Slide 4-13 

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Tension Force Slide 4-14

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Normal Force Slide 4-15

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 4-16 Friction f k and 

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Drag and Thrust Slide 4-17

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Example 1 A block is dragged uphill by a rope. Identify all forces acting on the block. Slide 4-19

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Newton’s Second Law Slide 4-22

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Newton 2 Demonstrations Slide 4-19

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Newton’s Third Law Slide 4-25

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Newton's Laws of Motion Slide 4-19 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. Newton’s 3rd Law Demonstrations Slide 4-19