# Lecture Presentation. Chapter 4 Forces and Newton s Laws of Motion. Chapter 4 Forces and Newton s Laws of Motion. Reading Question 4.

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1 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. Slide 4-2 Chapter 4 Preview Looking Ahead Reading Question 4.1 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 Slide 4-3 Slide 4-4

2 Reading Question 4.1 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. Reading Question 4.2 If you stand on a trampoline, it depresses under your weight. When you stand on a hard stone floor, A. The floor does not deform under your weight; it is too stiff. B. The floor deforms very slightly under your weight. C. The floor deforms a slight amount if you are heavy enough. Slide 4-5 Slide 4-6 Reading Question 4.2 If you stand on a trampoline, it depresses under your weight. When you stand on a hard stone floor, Reading Question 4.4 If you are standing on the floor, motionless, what are the forces that act on you? A. The floor does not deform under your weight; it is too stiff. B. The floor deforms very slightly under your weight. C. The floor deforms a slight amount if you are heavy enough. A. Weight force B. Weight force and normal force C. Normal force and friction force D. Weight force and tension force Slide 4-7 Slide 4-8

3 Reading Question 4.4 If you are standing on the floor, motionless, what are the forces that act on you? A. Weight force B. Weight force and normal force C. Normal force and friction force D. Weight force and tension force Reading Question 4.5 A skydiver has reached terminal velocity she now falls at a constant speed, so her acceleration is zero. Is there a net force on her? If so, what is the direction? A. There is a net force directed upward. B. There is no net force. C. There is a net force directed downward. Slide 4-9 Slide 4-10 Reading Question 4.5 A skydiver has reached terminal velocity she now falls at a constant speed, so her acceleration is zero. Is there a net force on her? If so, what is the direction? A. There is a net force directed upward. B. There is no net force. C. There is a net force directed downward. 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. Slide 4-11 Slide 4-12

4 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 Slide 4-13 What Causes Motion? What Causes Motion? In the absence of friction, if the sled is moving, it will stay in motion. Slide 4-15 Slide 4-16

5 What Is a Force? A force is a push or a pull. A force acts on an object. Every force has an agent, something that acts or pushes or pulls. What Is a Force? A force is a vector. The general symbol for a force is the vector symbol F r. The size or strength of such a force is its magnitude F. Contact forces are forces that act on an object by touching it at a point of contact. Long-range forces are forces that act on an object without physical contact. Slide 4-17 Slide 4-18 Force Vectors Force Vectors Text: p. 100 Slide 4-19 Slide 4-20

6 Combining Forces Experiments show r rthat rwhen several forces F1, F2, F3, K are exerted on an object, the combine to form a net force that is the vector sum of all the forces: COMP: Symbols with overarrows are MathType 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? The net force is sometimes called the resultant force. It is not a new force. Instead, we should think of the original forces being replaced by F r net. A. B. C. D. Slide 4-21 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 Slide 4-23

7 Weight 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 vector always points vertically downward, no matter how the object is moving. Spring Force Springs come in in many forms. When deflected, they push or pull with a spring force. Slide 4-25 Slide 4-26 Tension Force When a string or rope or wire pulls on an object, it exerts a contact force that we call the tension force. The direction of the tension force is always in the direction of the string or rope. 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 object that is pressing against the surface. Slide 4-27 Slide 4-28

8 Normal Force The normal force is responsible for the solidness of solids. The symbol for the normal force is n r. COMP: n[overarrow] is MathType. Friction Friction, like the normal force, is exerted by a surface. The frictional force is always parallel to the surface. Kinetic friction, denoted by f r, acts as an object slides k across a surface. Kinetic friction is a force that always opposes the motion. Static friction, denoted by f r 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. Slide 4-29 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. Slide 4-31 Slide 4-32

9 Thrust Thrust is a force that occurs when a jet or rocket engine expels gas molecules at high speed. Thrust is a force opposite the direction in which the exhaust gas is expelled. Electric and Magnetic Forces Electricity and magnetism, like gravity, exert long-range forces. The forces of electricity and magnetism act on charged particles. These forces and the forces inside the nucleus won t be important for the dynamics problems we consider in the next several chapters. Slide 4-33 Slide 4-34 QuickCheck 4.2 QuickCheck 4.2 A ball rolls down an incline and off a horizontal ramp. Ignoring air resistance, what force or forces act on the ball as it moves through the air just after leaving the horizontal ramp? A. The weight of the ball acting vertically down. B. A horizontal force that maintains the motion. C. A force whose direction changes as the direction of motion changes. D. The weight of the ball and a horizontal force. E. The weight of the ball and a force in the direction of motion. A ball rolls down an incline and off a horizontal ramp. Ignoring air resistance, what force or forces act on the ball as it moves through the air just after leaving the horizontal ramp? A. The weight of the ball acting vertically down. B. A horizontal force that maintains the motion. C. A force whose direction changes as the direction of motion changes. D. The weight of the ball and a horizontal force. E. The weight of the ball and a force in the direction of motion. Slide 4-35 Slide 4-36

10 QuickCheck 4.3 QuickCheck 4.3 A steel beam hangs from a cable as a crane lifts the beam. What forces act on the beam? A. Gravity B. Gravity and tension in the cable C. Gravity and a force of motion D. Gravity and tension and a force of motion A steel beam hangs from a cable as a crane lifts the beam. What forces act on the beam? A. Gravity B. Gravity and tension in the cable C. Gravity and a force of motion D. Gravity and tension and a force of motion Slide 4-37 Slide 4-38 QuickCheck 4.4 QuickCheck 4.4 A bobsledder pushes her sled across horizontal snow to get it going, then jumps in. After she jumps in, the sled gradually slows to a halt. What forces act on the sled just after she s jumped in? A. Gravity and kinetic friction B. Gravity and a normal force C. Gravity and the force of the push D. Gravity, a normal force, and kinetic friction E. Gravity, a normal force, kinetic friction, and the force of the push A bobsledder pushes her sled across horizontal snow to get it going, then jumps in. After she jumps in, the sled gradually slows to a halt. What forces act on the sled just after she s jumped in? A. Gravity and kinetic friction B. Gravity and a normal force C. Gravity and the force of the push D. Gravity, a normal force, and kinetic friction E. Gravity, a normal force, kinetic friction, and the force of the push Slide 4-39 Slide 4-40

11 Identifying Forces Section 4.3 Identifying Forces Text: p. 105 Slide 4-42 Identifying Forces Section 4.4 What Do Forces Do? Slide 4-43

12 What Do Forces Do? The experimental findings of the motion of objects acted on by constant forces are: An object pulled with a constant force moves with a constant acceleration. Acceleration is directly proportional to force. Acceleration is inversely proportional to an object s mass. Example 4.4 Finding the mass of an unknown block When a rubber band is stretched to pull on a 1.0 kg block with a constant force, the acceleration of the block is measured to be 3.0 m/s 2. When a block with an unknown mass is pulled with the same rubber band, using the same force, its acceleration is 5.0 m/s 2. What is the mass of the unknown block? PREPARE Each block s acceleration is inversely proportional to its mass. Slide 4-45 Slide 4-46 Example 4.4 Finding the mass of an unknown block (cont.) SOLVE We can use the result of the Inversely Proportional Relationships box to write Section 4.5 Newton s Second Law ASSESS With the same force applied, the unknown block had a larger acceleration than the 1.0 kg block. It makes sense, then, that its mass its resistance to acceleration is less than 1.0 kg. Slide 4-47

13 Newton s Second Law Newton s Second Law A force causes an object to accelerate. The acceleration a is directly proportional to the force F and inversely proportional to the mass m: The direction of the acceleration is the same as the direction of the force: Slide 4-49 Slide 4-50 QuickCheck 4.9 QuickCheck 4.10 An object on a rope is lowered at constant speed. Which is true? Constant velocity Zero acceleration An object on a rope is lowered at a steadily decreasing speed. Which is true? A. The rope tension is greater than the object s weight. B. The rope tension equals the object s weight. C. The rope tension is less than the object s weight. D. The rope tension can t be compared to the object s weight. A. The rope tension is greater than the object s weight. B. The rope tension equals the object s weight. C. The rope tension is less than the object s weight. D. The rope tension can t be compared to the object s weight. Slide 4-51 Slide 4-52

14 QuickCheck 4.10 Units of Force An object on a rope is lowered at a steadily decreasing speed. Which is true? Decreasing downward velocity Acceleration vector points up points up A. The rope tension is greater than the object s weight. B. The rope tension equals the object s weight. C. The rope tension is less than the object s weight. D. The rope tension can t be compared to the object s weight. The basic unit of force is called a newton. One newton is the force that causes a 1 kg mass to accelerate at 1 m/s 2. 1 pound = 1 lb = 4.45 N Slide 4-53 Slide 4-54 Example 4.6 Racing down the runway A Boeing 737 a small, short-range jet with a mass of 51,000 kg sits at rest. The pilot turns the pair of jet engines to full throttle, and the thrust accelerates the plane down the runway. After traveling 940 m, the plane reaches its takeoff speed of 70 m/s and leaves the ground. What is the thrust of each engine? Example 4.6 Racing down the runway (cont.) PREPARE If we assume that the plane undergoes a constant acceleration (a reasonable assumption), we can use kinematics to find the magnitude of that acceleration. Then we can use Newton s second law to find the force the thrust that produced this acceleration. FIGURE 4.23 is a visual overview of the airplane s motion. Slide 4-55 Slide 4-56

15 Example 4.6 Racing down the runway (cont.) SOLVE We don t know how much time it took the plane to reach its takeoff speed, but we do know that it traveled a distance of 940 m. We can solve for the acceleration by using the third constant-acceleration equation in Synthesis 2.1: (v x ) f2 = (v x ) i2 + 2a x x The displacement is x = x f x i = 940 m, and the initial velocity is 0. We can rearrange the equation to solve for the acceleration: Example 4.6 Racing down the runway (cont.) We ve kept an extra significant figure because this isn t our final result we are asked to solve for the thrust. We complete the solution by using Newton s second law: F = ma x = (51,000 kg)(2.61 m/s 2 ) = 133,000 N The thrust of each engine is half of this total force: Thrust of one engine = 67,000 N = 67 kn Slide 4-57 Slide 4-58 Free-Body Diagrams Section 4.6 Free-Body Diagrams Text: p. 112 Slide 4-60

16 QuickCheck 4.13 QuickCheck 4.13 A ball, hanging from the ceiling by a string, is pulled back and released. Which is the correct free-body diagram just after its release? A ball, hanging from the ceiling by a string, is pulled back and released. Which is the correct free-body diagram just after its release? A. B. C. D. E. A. B. C. D. E. Slide 4-61 Slide 4-62 QuickCheck 4.14 QuickCheck 4.14 A car is parked on a hill. Which is the correct free-body diagram? A car is parked on a hill. Which is the correct free-body diagram? C. Slide 4-63 Slide 4-64

17 QuickCheck 4.15 QuickCheck 4.15 A car is towed to the right at constant speed. Which is the correct free-body diagram? A car is towed to the right at constant speed. Which is the correct free-body diagram? D. Slide 4-65 Slide 4-66 Newton s Third Law Section 4.7 Newton s Third Law Motion often involves two or more objects interacting with each other. As the hammer hits the nail, the nail pushes back on the hammer. A bat and a ball, your foot and a soccer ball, and the earth-moon system are other examples of interacting objects. Slide 4-68

18 Interacting Objects Reasoning with Newton s Third Law An interaction is the mutual influence of two objects on each other. The pair of forces shown in the figure is called an action/reaction pair. An action/reaction pair of forces exists as a pair, or not at all. Slide 4-69 Slide 4-70 Runners and Rockets The rocket pushes hot gases out the back, and this results in a forward force (thrust) on the rocket. QuickCheck 4.17 A mosquito runs head-on into a truck. Splat! Which is true during the collision? A. The mosquito exerts more force on the truck than the truck exerts on the mosquito. B. The truck exerts more force on the mosquito than the mosquito exerts on the truck. C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito. D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck. E. The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito. Slide 4-71 Slide 4-72

19 QuickCheck 4.17 Summary A mosquito runs head-on into a truck. Splat! Which is true during the collision? A. The mosquito exerts more force on the truck than the truck exerts on the mosquito. B. The truck exerts more force on the mosquito than the mosquito exerts on the truck. C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito. D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck. E. The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito. Text: p. 118 Slide 4-73 Slide 4-74 Summary Summary Text: p. 118 Text: p. 118 Slide 4-75 Slide 4-76

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