Vectors, Forces and Equilibrium pp Mr. Richter

Agenda  Warm-Up  Any last progress reports?  Notes:  Vectors and Scalars  Vectors vs. Scalars Activity  Drawing Free-Body Diagrams (FBDs)  Equilibrium

Objectives: We Will Be Able To…  Distinguish between vector and scalar quantities.  Draw and analyze free-body diagrams.  Understand the behavior of an object in equilibrium.

Warm-Up:  A book lies at rest on a table. List all of the forces that act on the book? In what directions do those forces go?

Vectors and Scalars

 All measured quantities (force, speed, temperature, etc.) can be broken up into two categories: vectors and scalars.  A vector quantity is a measurement that includes both magnitude (size) and direction.  Like force. How hard you push and in what direction.  A scalar quantity is a measurement that includes only magnitude, but no direction.  Like temperature. No direction necessary.

Vectors and Scalars: Example  Here is a simple way to remember the difference between vectors and scalars.  Vvvvvvelocity is a vvvvvector because direction is important.  30 km/h North  Sssssspeeed is a sssssscalar because direction is not important.  30 km/h. Your speedometer does not tell you what direction you’re going.

Vectors and Scalars  At your table, place the measurements to the right into two columns, one for vectors and one for scalars. If you’re not sure, discuss at your table and make an educated guess. We will discuss as a class in a few minutes.  Force  Temperature  Speed  Velocity  Time  Acceleration  Distance  Power  Energy  Mass

Vectors and Scalars Vectors  Force  Velocity  Acceleration  Displacement Scalars TTemperature SSpeed TTime DDistance PPower EEnergy MMass

Free-Body Diagrams (FBDs)

 We have spent a lot of time looking at situations where only one force is present. FBDs allow physicists to analyze the effect of more than one force acting on an object at a time.  Rules for FBDs. (Example on the next slide) 1.Draw only the forces acting ON the object. No forces of the object acting on other things. 2.Draw all forces as arrows coming FROM the center of the object in the appropriate direction. 3.Draw force arrows proportional to the magnitude of the forces. Small forces = short arrows.

Drawing Free Body Diagrams  Draw a FBD of a book lying flat on a table. What are the forces? 1.Force of gravity pulling the book down. 2.Force of the table pushing the book up. Force of Table on Book Force of Gravity on Book

Equilibrium

 Reminder: the net force is the sum of all of the forces acting on an object.  When the net force acting on an object is zero, the object is in equilibrium.  In other words, either:  there are no forces acting on the object at all, or…  all of the forces cancel each other out

Equilibrium  Can an object be in equilibrium and still be moving?  Yes. A net force of zero just means the object is not accelerating.  If an object is traveling at a constant velocity (same speed and direction) it is also in equilibrium.

Wrap-Up: Did we meet our objectives?  Distinguish between vector and scalar quantities.  Draw and analyze free-body diagrams.  Understand the behavior of an object in equilibrium.

Homework  Due Wednesday:  p 113 #1, 2, 5  p 130 Reviewing Concepts #1, 5  Due Thursday  p116 #a, b