http wwwaplusphysicscom courseshonorsdynamics FBDhtml Unit 3 Dynamics Objectives and Learning Targets 1 Define a force and distinguish between contact forces and field forces 2 Draw ID: 652607
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Slide1
Dynamics – Free Body Diagrams
http://www.aplusphysics.com/courses/honors/dynamics/FBD.html
Unit #3 DynamicsSlide2
Objectives and Learning Targets
1. Define a force and distinguish between contact forces and field forces.
2. Draw and label a free body diagram showing all forces acting on an object.
3. Determine
the resultant of two or more vectors graphically and algebraically.
4. Draw scaled force diagram using a ruler and protractor.5. Resolve a vector into perpendicular components: both graphically and algebraically.6. Use vector diagrams to analyze mechanical systems (equilibrium and nonequilibrium).
Unit #3 DynamicsSlide3
Free Body Diagram
Unit #3 Dynamics
Fortunately, we have a terrific tool for analyzing the forces acting upon objects. This tool is known as a free body diagram. Quite simply,
a free body diagram is a representation of a single object, or system, with vector arrows showing all the external forces acting on the object
. These diagrams make it very easy to identify exactly what the net force is on an object, and they’re also quite simple to create:Slide4
Free Body Diagram
Unit #3 Dynamics
1. Isolate the object of interest. Draw the object as a point particle representing the same mass.
2. Sketch
and label each of the external forces acting on the object.
3. Choose a coordinate system, with the direction of motion as one of the positive coordinate axes.4. If all forces do not line up with your axes, resolve those forces into components using trigonometry (note that the formulas below only work if the angle is measured from the horizontal).5. Redraw your free body diagram, replacing forces that don’t overlap the coordinates axes with their components.Slide5
Free Body Diagram Example
Unit #3 Dynamics
As an example, picture a glass of soda sitting on the dining room table. You can represent the glass of soda in the diagram as a single dot. Then, represent each of the vector forces acting on the soda by drawing arrows and labeling them. In this case, you can start by recognizing the
force of gravity on the soda, known more commonly as the soda’s weight
. Although you could label this force as
Fgrav, or W, get in the habit right now of writing the force of gravity on an object as mg. You can do this because the force of gravity on an object is equal to the object’s mass times the acceleration due to gravity, g.Slide6
Free Body Diagram Example
Unit #3 Dynamics
Of course, since the soda isn’t accelerating, there must be another force acting on the soda to balance out the weight. This force, the force of the table pushing up on the soda, is known as the normal force (F
N
)
. In physics, the normal force refers to a force perpendicular to a surface (normal in this case meaning perpendicular). The force of gravity on the soda must exactly match the normal force on the soda, although they are in opposite directions, therefore there is no net force on the soda. The free body diagram for this situation could be drawn as shown at right.Slide7
Example Problem #1
Unit #3 Dynamics
Question: Which diagram represents a box in equilibrium?
Answer: (2) all forces are balanced for a net force of zero.Slide8
Example Problem #2
Unit #3 Dynamics
Question: If the sum of all the forces acting on a moving object is zero, the object will
1. slow
down and stop
2. change the direction of its motion3. accelerate uniformly4. continue moving with constant velocityAnswer: (4) continue moving with constant velocity in accordance with Newton’s 1st Law of Motion.Slide9
Types of Force
Gravity = FG
, W, or mg – always straight downNormal = F
N
– perpendicular surface force
Friction = Ff – surface force opposite velocity of objectAir Resistance = FD – opposes velocitySpring = Fsp
= can be push or pull
Applied =
F
A
– general push or pull on object
Tension =
F
T
= pull from a rope, cord or wire
Buoyant =
F
buoy
– displayed fluid (liquid or gas) force
Magnetic =
F
B
-
from moving charged objects
Electric
=
F
E
-
from moving charged objects
Unit #3 Dynamics