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Introduction to Force Chapter 4.1: Changes in Motion.

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Presentation on theme: "Introduction to Force Chapter 4.1: Changes in Motion."— Presentation transcript:

1 Introduction to Force Chapter 4.1: Changes in Motion

2 What is force? a push or pull upon an object resulting from the object's interaction with another object  For ex., you exert force on a ball when you kick it  You exert a force on the chair you are sitting in right now Whenever there is an interaction between two objects, there is a force acting on each of the objects. When the interaction ceases, the two objects no longer experience a force. Forces only exist as a result of an interaction.

3 Units (yes, more units to know) The SI unit of force is the newton (N)  Named after Sir Isaac Newton Definition:  N = the amount of force, that when acting on a mass, produces an acceleration  1 N = 1 kg * 1 m/s 2

4 Two categories of forces 1. Contact  types of forces in which the two interacting objects are physically in contact with each other.  Examples: frictional force, tensional force, air resistance force, normal forces, and applied forces 2. Action at a Distance  types of forces in which the two interacting objects are not in physical contact with each other, but are able to exert a push or pull despite the physical separation.  Examples: gravitational force, magnetic force, and electric force

5 Force Diagrams Force is a vector quantity  A vector quantity is a quantity which has both magnitude and direction. To fully describe the force acting upon an object, you must describe both its magnitude (size) and its direction.  Arrows represent all the forces acting in a situation

6 Free Body Diagrams Used to show the relative magnitude and direction of all forces acting upon an object in a given situation. The size of the arrow in a free-body diagram is reflective of the magnitude of the force. The direction of the arrow reveals the direction in which the force acts. Each force arrow in the diagram is labeled to indicate the type of force. It is customary in a free-body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction in which the force is acting. Hint: gravitational force is usually a downward arrow.

7 Examples Each force arrow in the diagram is labeled to indicate the type of force. It is customary in a free- body diagram to represent the object by a box and to draw the force arrow from the center of the box outward in the direction in which the force is acting.

8 Practice #1 Draw a free body diagram for the following: An egg is free-falling from a nest in a tree. Neglect air resistance.

9 Practice #1 Answer Draw a free body diagram for the following: An egg is free-falling from a nest in a tree. Neglect air resistance. The only force acting on a free-falling egg is the downward force of gravity.

10 Practice #2 Draw a free body diagram for: A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces.

11 Practice #2 Answer Draw a free body diagram for: A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces We must draw the applied force which starts the book in motion as well as the force of friction which will be opposing the direction of motion (both in the horizontal direction). Furthermore, gravity will pull the book toward the table and the table will push back up on the book to oppose the force of gravity (vertical direction).

12 Practice #3 Draw a free body diagram for: A skydiver is descending with a constant velocity. Consider air resistance.

13 Practice #3 Answer A skydiver is descending with a constant velocity. Consider air resistance. Answer: The force pulling the skydiver toward the ground is gravity. Air resistance opposes the skydiver’s fall and opposed the force of gravity. Constant velocity means that there is no acceleration, so there is no net force (the two forces cancel each other out – they’re equal and opposite)


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