Chapter 4 Forces and Newton’s Laws of Motion Lecture Presentation Chapter 4 Forces and Newton’s Laws of Motion Chapter Goal: To establish a connection between force and motion. © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Slide 4-2 Chapter 4 Preview Reading Question 4.1 Looking Ahead If you are not wearing a seat belt and the car you are driving hits a fixed barrier, you will hit the steering wheel with some force. This is because A. The force of the collision has thrown you forward. B. The steering wheel has been pushed back toward you. C. You continue moving even after the car has stopped. Text: p. 97 © 2015 Pearson Education, Inc. Slide 4-3 © 2015 Pearson Education, Inc. Slide 4-4 Reading Question 4.1 Reading Question 4.2 If you are not wearing a seat belt and the car you are driving If you stand on a trampoline, it depresses under your weight. hits a fixed barrier, you will hit the steering wheel with When you stand on a hard stone floor, some force. This is because A. The floor does not deform under your weight; it is too A. The force of the collision has thrown you forward. stiff. B. The steering wheel has been pushed back toward you. B. The floor deforms—very slightly—under your weight. C. You continue moving even after the car has stopped. C. The floor deforms a slight amount if you are heavy enough. © 2015 Pearson Education, Inc. Slide 4-5 © 2015 Pearson Education, Inc. Slide 4-6 Reading Question 4.2 Reading Question 4.4 If you stand on a trampoline, it depresses under your weight. If you are standing on the floor, motionless, what are the When you stand on a hard stone floor, forces that act on you? A. The floor does not deform under your weight; it is too A. Weight force stiff. B. Weight force and normal force B. The floor deforms—very slightly—under your weight. C. Normal force and friction force C. The floor deforms a slight amount if you are heavy D. Weight force and tension force enough. © 2015 Pearson Education, Inc. Slide 4-7 © 2015 Pearson Education, Inc. Slide 4-8 Reading Question 4.4 Reading Question 4.5 If you are standing on the floor, motionless, what are the A skydiver has reached terminal velocity—she now falls at forces that act on you? a constant speed, so her acceleration is zero. Is there a net force on her? If so, what is the direction? A. Weight force B. Weight force and normal force A. There is a net force directed upward. C. Normal force and friction force B. There is no net force. D. Weight force and tension force C. There is a net force directed downward. © 2015 Pearson Education, Inc. Slide 4-9 © 2015 Pearson Education, Inc. Slide 4-10 Reading Question 4.5 Reading Question 4.6 A skydiver has reached terminal velocity—she now falls at An action/reaction pair of forces a constant speed, so her acceleration is zero. Is there a net force on her? If so, what is the direction? A. Points in the same direction. B. Acts on the same object. A. There is a net force directed upward. C. Are always long-range forces. B. There is no net force. D. Acts on two different objects. C. There is a net force directed downward. © 2015 Pearson Education, Inc. Slide 4-11 © 2015 Pearson Education, Inc. Slide 4-12 Reading Question 4.6 An action/reaction pair of forces A. Points in the same direction. B. Acts on the same object. C. Are always long-range forces. D. Acts on two different objects. Section 4.1 Motion and Forces © 2015 Pearson Education, Inc. Slide 4-13 © 2015 Pearson Education, Inc. What Causes Motion? What Causes Motion? • In the absence of friction, if the sled is moving, it will stay in motion. © 2015 Pearson Education, Inc. Slide 4-15 © 2015 Pearson Education, Inc. Slide 4-16 What Is a Force? What Is a Force? • A force is a vector . The general symbolr • A force is a push or a pull . for a force is the vector symbol F . The size or strength of such a force is its magnitude F. • A force acts on an object. • Contact forces are forces that act on an object by touching it at a point of contact. • Every force has an agent , something that acts or pushes or pulls. • Long-range forces are forces that act on an object without physical contact. © 2015 Pearson Education, Inc. Slide 4-17 © 2015 Pearson Education, Inc. Slide 4-18 Force Vectors Force Vectors Text: p. 100 © 2015 Pearson Education, Inc. Slide 4-19 © 2015 Pearson Education, Inc. Slide 4-20 Combining Forces QuickCheck 4.1 • Experiments showr rthat r when K several forces F 1 , F 2 , F 3 , The net force on an object points to the left. are exerted on an object, the Two of three forces are shown. Which is the combine to form a net force missing third force? that is the vector sum of all COMP: Symbols with the forces: overarrows are MathType • The net force is sometimes called the resultant force. It is not a new force. Instead, we should think of the originalr forces being replaced by F net . A. B. C. D. © 2015 Pearson Education, Inc. Slide 4-21 © 2015 Pearson Education, Inc. Slide 4-22 QuickCheck 4.1 The net force on an object points to the left. Two of three forces are shown. Which is the missing third force? Section 4.2 A Short Catalog of Forces A. B. C. D. Vertical components cancel © 2015 Pearson Education, Inc. Slide 4-23 © 2015 Pearson Education, Inc. Weight Spring Force • The gravitational pull of the earth on an object on or near the surface of the earth is called weight . • The agent for the weight forces is the entire earth pulling on an object. • An object’s weight • Springs come in in many forms. When deflected, they vector always points push or pull with a spring force. vertically downward, no matter how the object is moving . © 2015 Pearson Education, Inc. Slide 4-25 © 2015 Pearson Education, Inc. Slide 4-26 Tension Force Normal Force • The force exerted on an object that is pressing against a surface is in a direction perpendicular to the surface. • The normal force is the force exerted by a surface (the agent) against an • When a string or rope or wire pulls on an object, it exerts a object that is pressing contact force that we call the tension force . against the surface. • The direction of the tension force is always in the direction of the string or rope . © 2015 Pearson Education, Inc. Slide 4-27 © 2015 Pearson Education, Inc. Slide 4-28 Normal Force Friction • The normal force is responsible for the “solidness” of • Friction, like the normal force, is exerted by a surface. solids. • The frictional force is always parallel to the surface. r r • The symbol for the normal force is n . • Kinetic friction, denoted by f k , acts as an object slides across a surface. Kinetic friction is a force that always COMP: n[overarrow] “opposes the motion.” is MathType. r • Static friction, denoted by f s , is the force that keeps an object “stuck” on a surface and prevents its motion relative to the surface. Static friction points in the direction necessary to prevent motion. © 2015 Pearson Education, Inc. Slide 4-29 © 2015 Pearson Education, Inc. Slide 4-30 Friction Drag • The force of a fluid (like air or water) on a moving object is called drag . • Like kinetic friction, drag points opposite the direction of motion. • You can neglect air resistance in all problems unless a problem explicitly asks you to include it. © 2015 Pearson Education, Inc. Slide 4-31 © 2015 Pearson Education, Inc. Slide 4-32 Thrust Electric and Magnetic Forces • Thrust is a force that occurs when a jet or rocket engine • Electricity and magnetism, like gravity, exert long-range expels gas molecules at high speed. forces. • Thrust is a force opposite the direction in which the • The forces of electricity and magnetism act on charged exhaust gas is expelled. particles. • These forces—and the forces inside the nucleus—won’t be important for the dynamics problems we consider in the next several chapters. © 2015 Pearson Education, Inc. Slide 4-33 © 2015 Pearson Education, Inc. Slide 4-34 QuickCheck 4.2 QuickCheck 4.2 A ball rolls down an incline and A ball rolls down an incline and off a horizontal ramp. Ignoring off a horizontal ramp. Ignoring air resistance, what force or air resistance, what force or forces act on the ball as it moves forces act on the ball as it moves through the air just after leaving through the air just after leaving the horizontal ramp? the horizontal ramp? A. The weight of the ball acting vertically down. A. The weight of the ball acting vertically down. B. A horizontal force that maintains the motion. B. A horizontal force that maintains the motion. C. A force whose direction changes as the direction of motion C. A force whose direction changes as the direction of motion changes. changes. D. The weight of the ball and a horizontal force. D. The weight of the ball and a horizontal force. E. The weight of the ball and a force in the direction of motion. E. The weight of the ball and a force in the direction of motion.