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Chapter 4: Forces and the Laws of Motion

Chapter 4: Forces and the Laws of Motion. Section 4-1: Changes in motion. lab. 10/31 do now. T or F? Statement A force is required to keep an object moving in a given direction. An upward moving object must be experiencing (or at least usually does experience) an upward force.

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Chapter 4: Forces and the Laws of Motion

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  1. Chapter 4: Forces and the Laws of Motion • Section 4-1: Changes in motion

  2. lab

  3. 10/31 do now • T or F? Statement • A force is required to keep an object moving in a given direction. • An upward moving object must be experiencing (or at least usually does experience) an upward force. • A rightward moving object must be experiencing (or at least usually does experience) a rightward force. • A ball is thrown into the air and is moving upwards and rightwards towards its peak. The ball experiences a force which is directed upwards and rightwards. • If a person throws a ball with his hand, then the force of the hand upon the ball is experienced by the ball for at least a little while after the ball leaves the hand. • A cannonball is shot from a cannon at a very high speed. The force of the explosion will be experienced by the cannonball for several seconds (or a least a little while). • If an object is at rest, then there are no forces acting upon the object.

  4. Objectives • Explain how force affects the motion of an object. • Distinguish between contact forces and field forces. • Interpret and construct free-body diagrams. • Homework: Castle Learning

  5. What is a force? • A force is a push or pull upon an object resulting from the object's interactionwith another object. • Force is the cause of an acceleration, or the change in an object’s velocity. • Figure 4-1 on page 124 • The ball is experiencing force in all three pictures. • Question: How can you tell that the ball experience at least one force in each picture? • By changing in the ball’s speed or direction.

  6. questions • What is the cause of acceleration? • What is the definition of acceleration? • Why is a projectile has no horizontal acceleration?

  7. The SI unit of force is Newton • 1 Newton is defined as the force needed to accelerate 1 kg mass at 1 m/s/s. 1 Newton = 1 kg ∙1 m/s2

  8. Contact forces vs. field forces • Demo • What made the ball change its motion? • What made the ball change direction at the floor and accelerated upward? • Contact forces – arises from the physical contact of two objects. • Field forces – exists between the two interacting objects are not in physical contact with each other.

  9. A force is a vector quantity 15 N, E • Force is a vector quantity • Force has a magnitude and a direction • Demo: which way is easier to pull? FA FB FAx FBx A B Smaller horizontal force Bigger horizontal force

  10. A force diagram is a diagram of the objects involved in a situation and the forces exerted on the objects. • Free-body diagrams isolate an object and the forces acting on it.

  11. Drawing Free-Body Diagrams • Free-body diagrams are used to show the relative magnitude and direction of all forces acting upon an object in a given situation. • The size of the arrow reflects the magnitude of the force. The arrow shows the direction that the force is acting. • It is generally customary to draw the force arrow from the center of the object outward in the direction that the force is acting. • Each force arrow in the diagram is labeled to indicate the exact type of force.

  12. Lab - Force and Changes in Motion. • Page 126 • Objective: observe the motion of a toy car before and after it strikes a book and diagram the forces involved in the collision. • Material: 1 toy car, 1 book

  13. Class work • Page 128 – section review #1-6 • Section 4-1: Diagram Skills

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