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
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Checking Understanding A.The string tension and the friction force acting on A. B.The normal force on A due to B and the weight of A. C.The normal force on A due to B and the weight of B. D.The friction force acting on A and the friction force acting on B. Which pair of forces is an action/reaction pair? Slide 5-32
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. D. The friction force acting on A and the friction force acting on B. Which pair of forces is an action/reaction pair? Slide 5-33 Answer
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 2-34 Newton’s Zeroeth Law of Motion Objects are dumb - They have no memory of the past and cannot predict the future. Objects only know what acts directly on them in a given moment.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Newton's First Law of Motion Every object continues in a state of rest or a state of motion with a constant speed in a straight line unless acted on by an unbalanced net force. Or v = constant (magnitude and direction) iff net force = 0 N.
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. What Causes Motion? In the absence of any forces acting on it, an object will continue moving forever. Motion needs no “cause.” Slide 4-8
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. What holds it up? Forces & Kitchen Magnets Use a free body diagram to determine the force that holds up a kitchen magnet. What is the magnitude of this force?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. F net = 0 Problems Determine tension in each cable. (Hint: There is more than one way to define the system.)
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. F net = 0 Problems The object hung from the cable has a weight of 25 N. Write the equation for the sum of the forces in the y-direction. What is the tension in the cable?
Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. F net = 0 Problems The cable at left exerts a -30 N force. a. Write the equation for the sum of the forces in the x- direction. What is the value of T 2 ? b. Write the equation for the sum of the forces in the y-direction. What is the force of gravity acting on the ball? 30 T2T2 T1T1