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Free Body DiagramsSession A19
Amos FreemanChristian College Geelong
a.freeman@ccg.vic.edu.au
So what are your backgrounds?
1. I studied Physics extensively beyond high school.2. I studied Physics a little bit beyond high school.3. I only studied Physics in high school.4. I am a Chemistry, Systems, or Mathematics teacher
who got roped into doing physics.5. I’m a Physical Education teacher, am I in the wrong
place?
How many objects does
it take to make a force?
Naming Forces with 3 word names:
“Object pushes/pulls object”
1 2 3
What are three forces acting on you right now?
*Don’t forget to use three word names!
Benefits to teaching 3rd Law with 3 word
names:The equal and opposite force can be found by switching the two objects in the name.
“Cat pushes Magpie”
“Magpie pushes Cat”
Names are consistent, but 3 words are all that is needed.
This and all other images of textbooks are from Heinemann 3rd edition Physics Units 1-4
Names are consistent, but 3 words are all that is needed.
The labels on the forces and arrow style in this example portray a lack of consistency that leads to confusion.
Here is another style of labeling forces that can easily be muddled up in students’ heads when everytihing else is represented by a letter.
Possible Confusion from textbooks.Styles of arrows:
Or?
Benefit of “object pushes/pulls object” for dealing with misconceptions around 1st law and objects remaining in motion:
If you can’t come up with a three word name, there is no force.
So a bowling ball that rolls at a constant velocity does not have something pushing it.
Can an object push itself? “Ball pushes Ball” or “Momentum pushes Ball”
Try to push yourself over.
Shifting your centre of mass away from your base does not count because something else is then doing the pulling.
What are all of the forces acting on this box on a table? What is the sum of all the forces?
Draw the object in question all by itself. So just draw a box.
Drawing the object all by itself helps people remember to only include forces on that object, whereas the tendency in a picture with a table is to include the forces on the table.
Gravity is not an object!
How do those forces on the box change with a stack of books resting on top of the box?
Physics
Physics
Physics
Be systematic about identifying all of the forces acting on the object in question.
Start with non – touching forces, or forces at a distance.
Then go through all of the objects touching the object in question. Only one book is actually touching the box in the last example.
And our box with books on it that is being pushed from the side, by a vertically challenged student, but is still not moving.
Physics
Physics
Physics
Friction is not an object!
To differentiate between forces that have the same names, like the friction and normal forces on the box in the previous example, use Ff and FN.
“Ff – Table pushes box” “FN – Table pushes box”
Consistent with the text sometimes.What is “T” representing? This may be construed as a force of Tension, not a Normal force.
How about a plain box on a table sitting on a hill?
Problem from the exam, Atwood’s machine. Without friction, 2kg mass and 3kg box. Acceleration of the system and tension in the string?
What is the source of the forward force here? Ask a room full of students and you might hear:TyresPedalsFeetPersonRoadChainGearsWhich one is correct?
Do you see some ways to make these forces easier to understand?
Other areas of study where
FBD’s can enhance
understanding.
Year 12
Voltage, including Photoelectric effect stopping
voltage exerting forces on charged particles.
Circular Motion – no force pushing out because
no object to push!
Material and their use in structures – Torque
with struts and ties.
What are some areas you can
think of?
Amos FreemanChristian College Geelonga.freeman@ccg.vic.edu.au
Please email me for electronic copies of the worksheets, this power
point, related lesson plans or related assessments.
Questions are always welcome. Happy
diagraming!
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