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Chapter 4- Forces and Motion

Chapter 4- Forces and Motion. FOCUS QUESTION(S). What is force? List some examples of force. FOCUS QUESTION(S). What are Newton’s three laws of motion?. Objective(s). 1. Explain how force affects the motion of an object. 2. Distinguish between contact forces and field forces.

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Chapter 4- Forces and Motion

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  1. Chapter 4- Forces and Motion

  2. FOCUS QUESTION(S) • What is force? • List some examples of force

  3. FOCUS QUESTION(S) What are Newton’s three laws of motion?

  4. Objective(s) • 1. Explain how force affects the motion of an object. • 2. Distinguish between contact forces and field forces. • 3. Interpret and construct free-body diagrams.

  5. Erupting Volcano!! Think about the following questions:What is this object? Where is it? Why does it look like that? IO is a moon of Jupiter Competing forces between Jupiter and the other Galilean moons cause the center of Io compress and melt. Consequently Io is the most volcanically active body in the solar system.

  6. Other examples of forces

  7. What is a force? • IPC definition: A push or a pull exerted on some object • Better definition: Force represents the interaction of an object with its environment • The Unit for Force is a Newton

  8. Two major types of forces • Contact Forces: Result from physical contact between two objects • Examples: Pushing a cart, Pulling suitcase • Field Forces: Forces that do not involve physical contact • Examples: Gravity, Electric/Magnetic Force

  9. Force is a vector! (yay more vectors ) • The effect of a force depends on magnitude and direction

  10. Force Diagrams (p. 126) • Force Diagram: A diagram that shows all the forces acting in a situation

  11. Free Body Diagrams p.127 • Free Body Diagrams (FBDs) isolate an object and show only the forces acting on it • FBDs are essential! They are not optional! You need to draw them to get most problems correct!

  12. How to draw a free body diagram Situation: A tow truck is pulling a car (p. 127) We want to draw a FBD for the car only.

  13. Step 1: Draw a shape representing the car (keep it simple) Step 2: Starting at the center of the object, Draw and label all the external forces acting on the object Force of Tow Truck on Car= 5800 N Steps for drawing your FBD

  14. Force of Tow Truck on Car= 5800 N Gravitational force (Weight of car)= 14700 N Add force of gravity

  15. Normal Force = 13690 N Force of Tow Truck on Car= 5800 N Gravitational force (Weight of car)= 14700 N Add force of the road on the car(Called the Normal Force)

  16. Normal Force = 13690 N Force of Tow Truck on Car= 5800 N Force of Friction= 775 N Gravitational force (Weight of car)= 14700 N Finally add the force of friction acting on the car

  17. Fkick Fg A Free Body Diagram of a Football Being Kicked

  18. FN= 500 N Ff= 30 N Fapp= 185 N Fg= 500 N A person is pushed forward with a force of 185 N. The weight of the person is 500 N, the floor exerts a force of 500 N up. The friction force is 30 N.

  19. Forces you will need

  20. Sample Problem p. 128 #3 • Draw a free body diagram of a football being kicked. Assume that the only forces acting on the ball are the force of gravity and the force exerted by the kicker.

  21. Practice Problem • A large, square box of exercise equipment sits on a storeroom floor. A rope is tied around the box. Assume that if the box moves along the floor, there is a backward force that resists its motion. • Suppose that the box remains at rest. In the space provided, draw a free body diagram for the box. Label each force involved in the diagram.

  22. A large, square box of exercise equipment sits on a storeroom floor. A rope is tied around the box. Assume that if the box moves along the floor, there is a backward force that resists its motion. • Suppose a warehouse worker moves the box by pulling the rope to the right horizontal to the ground. In the space provided, draw a free-body diagram for the box. Label each force involved in the diagram

  23. A large, square box of exercise equipment sits on a storeroom floor. A rope is tied around the box. Assume that if the box moves along the floor, there is a backward force that resists its motion. • Suppose the warehouse worker moves the box by pulling the rope to the right at a 50° angle to the ground. In the space provided, draw a free body diagram for the box. Label each force involved in the diagram

  24. FOCUS QUESTION(S) • Explain how force affects the motion of an object. • 2. Distinguish between contact forces and field forces. • A Chair is pushed forward with a force of 185N. The gravitational force of the earth on the chair is 155N downward, and the floor exerts a force of 155N upward on the chair. Draw a free-body diagram showing the forces acting on the chair.

  25. FBD Worksheet

  26. FOCUS QUESTION(S) • Two soccer players kick a ball at the same instant. One player kicks with a force of 65 N to the north, while the other player kicks with a force of 88 N to the east. In what direction does the ball travel?

  27. The muscle responsible for closing the mouth is the strongest muscle in the human body. It can exert a force greater than that exerted by a man lifting a mass of 400 kg. Richard Hoffman of Florida recorded the force of biting at 4.33 x 103 N. If each force shown in the diagram below has a magnitude equal to the force of Hoffman’s bite, determine the net force.

  28. FOCUS QUESTION(S) • The gravitational force of earth on a cake on a plate is 8.9 N. The plate exerts a force of 11.0 N. Draw a free- body diagrams of the cake.

  29. Newton’s first Law SECTION 2

  30. Objective(s) • 1. Explain the relationship between the motion of an object and the net external force acting on it. • 2. Determine the net external force on an object. • 3. Calculate the force required to bring an object into equilibrium.

  31. Newton’s 1st Law of Motion • The Law of Inertia • An object at rest remains at rest, and an object in motion continues in motion with constant velocity (constant speed in straight line) unless the object experiences a net external force • The tendency of an object not to accelerate is called inertia

  32. Acceleration • The net external force (Fnet) is the vector sum of all the forces acting on an object • If an object accelerates (changes speed or direction) then a net external force must be acting upon it

  33. Equilibrium • If an object is at rest (v=0) or moving at constant velocity, then according to Newton’s First Law,Fnet=0 • WhenFnet =0, the object is said tobe inequilibrium

  34. How do we use this information?Sample Problem p. 133 #2 • A crate is pulled to the right with a force of 82.0 N, to the left with a force of 115 N, upward with a force of 565 N and downward with a force of 236 N. • A. Find the net external force in the x direction • B. Find the net external force in the y direction • C. Find the magnitude and direction of the net external force on the crate.

  35. Step 1: Draw a FBD Fup = 565 N Fright = 82 N Fleft = 115 N Fdown = 236 N

  36. R = 331 N at 84.3 North of West 329 N 33 N Find the vector sum of forces • A. 82 N + (-115 N )= -33 N • B. 565 N + (-236 N) = 329 N • C. Find the resultant of the two vectors from part a and b.

  37. Newton’s 1st Law • Review Newton’s 1st Law: • When Fnet=0, an object is in equilibrium and will stay at rest or stay in motion • In other words, if the net external force acting on an object is zero, then the acceleration of that object is zero

  38. Practice Problems • Practice 4A page 133 Classwork • Section 4.2 review page 135 Homework.

  39. FOCUS QUESTION(S) • What is Newton 2nd and 3rd Law of Motion? • The net force exerted by a woodpecker’s head when its beak strikes a tree can be as large as 4.90 N, assuming that the bird’s head has a mass of 50.0 g. Assume that two different muscles pull the woodpecker’s head forward and downward, exerting a net force of 4.90 N. If the forces exerted by the muscles are at right angles to each other and the muscle that pulls the woodpecker’s head downward exerts a force of 1.70 N, what is the magnitude of the force exerted by the other muscle? Draw a free-body diagram showing the forces acting on the woodpecker’s head.

  40. Chapter 4 Section 3

  41. Objective(s) • 1. Describe the acceleration of an object in terms of its mass and the net external force acting on it. • 2. Predict the direction and magnitude of the acceleration caused by a known net external force. • 3. Identify action-reaction pairs. • 4. Explain why action-reaction pairs do not result in equilibrium.

  42. Newton’s 2nd Law (p.137) • The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass

  43. Example p. 138 # 4 • A 2.0 kg otter starts from rest at the top of a muddy incline 85 cm long and slides down to the bottom in 0.50 s. What net external force acts on the otter along the incline?

  44. Solving the problem • To calculate Fnet, we need m and a • M=2.0 kg • What is a? • Vi= 0 m/s, t=0.50 s, • displacement=85 cm=.85 m • Welcome back kinematic equations! 

  45. Newtons’ 3rd Law • Forces always exist in pairs • For every action there is an equal and opposite reaction

  46. Action- Reaction Pairs Some action-reaction pairs:

  47. Although the forces are the same, the accelerations will not be unless the objects have the same mass.

  48. Practice Practice page 138 HW section 4.3 review page 140

  49. FOCUS QUESTION(S)

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