Lecture 13 Newton’s 2 nd Law (continued)

QUIZ Find this vector’s magnitude and angle with respect to the positive x axis and draw it Find the x- and y-components of the vector

Which of these is not a force discussed in this chapter? 1 Tension Force 2 Orthogonal Force 3. Normal Force 4. Thrust

Newton’s 2 nd Law

Units Weight is a force

What is the weight of a 170 lb person in Newtons? 1.756N N 3.1,666 N N

Using Newton’s 2 nd Law Need F NET Make a free body diagram Use particle model, object is a point dot located at the origin of coordinate system Draw vectors representing each force acting on object Draw vector representing F NET

Problem 4:34 Free body diagram of your physics textbook sliding across the table

Free body diagrams Make diagram of object showing all forces acting on it (as vectors). For now object will be represented by a point. Decide on the x and y-axes Find x- and y-components of each force Find Show F NET on diagram

Newton’s 3 rd Law If object A exerts a force F AB on object B, the object B exerts a force F BA = - F AB on object A. n W

Newton’s 3rd Objects need not be in contact. Example gravitational force sun exerts on the earth. One magnet on another.

Problem What is a? What force does the 7 kg object exert on the 28kg object? Vice versa? M=28 kg M=7 kg F = 35 N

Quiz An elevator, lifted by a cable, is going up at a steady speed. Identify the forces acting on the elevator. Make a free body diagram. Is T greater than, equal to, or less than w? Or is there not enough information to tell? Slide 4-23

Special Homework due Monday Workbook, section 5.2, pages 5-3 and 5-4, problems 7-12

Problems due Today Reading: 4: 5-8 Problems: 4: CQ1, MC21, 4, 7, 8, 9, 10, 11

Problems due Monday Reading: 5: 1-5 Problems: 4: 13, 14, 17, 18, 20, 25, 29, 30, 31, 33, 35, 38, 65