Chapter 4. 4 Forces and Newton s Laws of Motion. Forces and Newton s Laws of Motion

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1 Chapter 4 Forces and Newton s Laws of Motion PowerPoint Lectures for College Physics: A Strategic Approach, Second Edition 4 Forces and Newton s Laws of Motion Slide 4-2 Slide 4-3 1

2 Slide 4-4 Weight is actually not the only one Slide 4-5 Slide 4-6 2

3 Reading Quiz 1. A net force is A. the sum of the magnitudes of all the forces acting on an object. B. the difference between two forces that are acting on an object. C. the vector sum of all the forces acting on an object. D. the force with the largest magnitude acting on an object. Slide 4-7 Answer 1. A net force is A. the sum of the magnitudes of all the forces acting on an object. B. the difference between two forces that are acting on an object. C. the vector sum of all the forces acting on an object. D. the force with the largest magnitude acting on an object. Slide 4-8 Reading Quiz 2. Which of the following is NOT one of the steps used to identify the forces acting on an object? A. Name and label each force the object exerts on the environment. B. Name and label each contact force acting on the object. C. Draw a picture of the situation. D. Identify the system and the environment. E. Name and label each long-range force acting on the object. Slide 4-9 3

4 Answer 2. Which of the following is NOT on of the steps used to identify the forces acting on an object? A. Name and label each force the object exerts on the environment. B. Name and label each contact force acting on the object. C. Draw a picture of the situation. D. Identify the system and the environment. E. Name and label each long-range force acting on the object. Slide 4-10 Reading Quiz 3. Which of these is not a force discussed in this chapter? A. The tension force. B. The normal force. C. The orthogonal force. D. The thrust force. Slide 4-11 Answer 3. Which of these is not a force discussed in this chapter? A. The tension force. B. The normal force. C. The orthogonal force. D. The thrust force. Slide

5 Reading Quiz 4. An action/reaction pair of forces A. point in the same direction. B. act on the same object. C. are always long-range forces. D. act on two different objects. Slide 4-13 Answer 4. An action/reaction pair of forces A. point in the same direction. B. act on the same object. C. are always long-range forces. D. act on two different objects. Slide 4-14 What Causes Motion? Slide

6 What Causes Motion? Slide 4-15 What Causes Motion? In the absence of any forces acting on it, an object will continue moving forever. Motion needs no cause. Slide 4-15 Seat Belts: An Application of Newton s First Law Slide

7 What Is a Force? A force is a push or a pull.... acts on an object.... requires an agent.... is a vector.... is a contact force or a long-range force. Slide 4-17 Force Vectors Slide 4-18 A Short Catalog of Forces: Weight w Slide

8 Spring Force F sp Slide 4-20 Tension ForceT Slide 4-21 Normal Force n Slide

9 Friction f k and f s Slide 4-23 Drag D and Thrust F thrust Slide 4-24 Identifying Forces Slide

10 Example Problem A block is dragged uphill by a rope. Identify all forces acting on the block. Slide 4-26 Example Problem Block A hangs from the ceiling by a rope. Another block B hangs from A. Identify the forces acting on A. Slide 4-27 Example Problem Block A hangs from the ceiling by a rope. Another block B hangs from A. Identify the forces acting on A. Slide

11 Example Problem Block A hangs from the ceiling by a rope. Another block B hangs from A. Identify the forces acting on A. Slide 4-27 Example Problem A ball, hanging from the ceiling by a string, is pulled back and released. Identify the forces acting on it just after its release. Slide 4-28 Newton s Second Law Slide

12 Example Problem An elevator, lifted by a cable, is going up at a steady speed. Identify the forces acting on the elevator. Is T greater than, equal to, or less than w? Or is there not enough information to tell? Slide 4-30 Example Problem An elevator, lifted by a cable, is going up at a steady speed. Identify the forces acting on the elevator. Is T greater than, equal to, or less than w? Or is there not enough information to tell? Slide 4-30 Free-Body Diagrams Slide

13 Newton s Third Law Slide 4-32 Checking Understanding An object, when pushed with a net force F, has an acceleration of 2 m/s 2. Now twice the force is applied to an object that has four times the mass. Its acceleration will be A. ½ m/s 2. B. 1 m/s 2. C. 2 m/s 2. D. 4 m/s 2. Slide 4-33 Answer An object, when pushed with a net force F, has an acceleration of 2 m/s 2. Now twice the force is applied to an object that has four times the mass. Its acceleration will be A. ½ m/s 2. B. 1 m/s 2. C. 2 m/s 2. D. 4 m/s 2. Slide

14 Checking Understanding A 40-car train travels along a straight track at 40 mph. A skier speeds up as she skis downhill. On which is the magnitude of the net force greater? A. The train. B. The skier. C. The net force is the same on both. D. There s not enough information to tell. Slide 4-35 Answer A 40-car train travels along a straight track at 40 mph. A skier speeds up as she skis downhill. On which is the magnitude of the net force greater? A. The train. B. The skier. C. The net force is the same on both. D. There s not enough information to tell. Slide 4-36 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

15 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 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. Then why is it that Sarah speeds up? Slide 4-38 Summary Slide 4-39 Summary Slide

16 Summary Slide