Forces and Newton’s Laws of Motion 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? Slide 4-1

Reading Quiz 1. What is a “net force?” Slide 4-2 Answer: It is the vector sum of all the forces acting on an object. Slide 4-2

Reading Quiz List at least three of the steps used to identify the forces acting on an object. A. Identify “the system” and “the environment.” B. Draw a picture of the situation. C. Draw a closed curve around the system. D. Locate every point on the boundary of this curve where the environment touches the system. E. Name and label each contact force acting on the object. F. Name and label each long-range force acting on the object. Answer: See Tactics Box 4.2 Slide 4-3

Reading Quiz 3. Which of these is not a force discussed in this chapter? A. The tension force. C. The orthogonal force. B. The normal force. D. The thrust force. Answer: C Slide 4-4

Answer 3. Which of these is not a force discussed in this chapter? C. The orthogonal force. Answer: C Slide 4-5

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. Answer: D Slide 4-6

Answer 4. An action/reaction pair of forces D. act on two different objects. Answer: D Slide 4-7

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

Seat Belts: An Application of Newton’s First Law Slide 4-9

What Is a Force? A force... ... is a push or pull. ... acts on an object. ... requires an agent. ... is a vector. ... is a contact force or a long-range force. Slide 4-10

Force Vectors Slide 4-11

A Short Catalog of Forces: Weight w  A Short Catalog of Forces: Weight w  Slide 4-12

 Spring Force Fsp Slide 4-13

Tension Force Slide 4-14

Normal Force Slide 4-15

Friction fk and  Slide 4-16

Drag and Thrust Slide 4-17

Identifying Forces Slide 4-18

Example A block is dragged uphill by a rope. Identify all forces acting on the block. Slide 4-19

Example Block A hangs from the ceiling by a rope. Another block B hangs from A. Identify the forces acting on A. 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. Slide 4-21

Newton’s Second Law Slide 4-22

Example 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 4-23

Free-Body Diagrams Slide 4-24

Newton’s Third Law Slide 4-25

Identifying Forces for Interacting Objects Slide 4-26

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 greater than the force of Sarah on Jack. equal to than the force of Sarah on Jack. less than the force of Sarah on Jack. Answer: B 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 equal to than the force of Sarah on Jack. Answer: B Slide 4-28