Day 9 Lab
AA210
Free Body Diagrams
Reminders:
• All members of a lab group are expected to participate and help each other to solve and
understand the solution to the lab. It’s OK if you don’t know how to solve the problem.
Make sure you are at least asking your other group members or the TA questions so you can
learn how to solve the lab too. If you think any group member(s) are not actively participating
in the lab, please let a TA know.
• Your submissions should be able to clearly communicate to the reader without having to refer
to external material (i.e. this file) three main things:
1. what problem you are solving,
2. how you are solving it,
3. and the key results.
• Only submit one copy per group. Make sure everyone’s name in your group is on the submitted
copy.
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Reminders when drawing free body diagrams:
• When drawing free body diagrams, only include forces which are externally touching/interacting
with the body. Don’t forget about reactions with walls/floors/etc.
Figure 1: Body being considered is drawn with solid outlines. Forces you should include are green.
Forces you should not include are red.
• Don’t double draw forces; if you draw it once as an “odd direction” pointed vector, don’t also
draw it again as the x and y components. Choose one and only one.
• Be sure to label all of your forces on your free body diagram, as well as your coordinate
system.
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• Make sure you are consistent with your free body diagrams. You are usually free to choose
if you want to assume two bodies are either pulling or pushing on each other. However, one
body can’t be pulled on while the other one is pushing.
• Once you’ve drawn your free body diagrams, make sure your equations of equilibrium match
your free body diagrams.
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Drawing 2D free body diagrams and writing equilibrium equations
1. In figure 2 below,
(a) Draw an FBD for the weight W , then write the equilibrium equations for
(b) Draw an FBD for beam AB. Write the equilibrium equations for
the moments about point A.
P
Fx ,
P
P
Fy .
Fy , and sum
2. For figure 3 below,
(a) Draw an FBD for the entire assembly.
P
(b) Draw an FBD for the pulley at A. Write equilibrium equations for Fx and
can assume that the pulley is frictionless and has a negligible radius).
(c) Draw an FBD for beam BC. Write the equilibrium equations for
sum of the moments about point C.
2
P
Fx ,
P
P
Fy (you
Fy , and the
A
L
θ
y
B
x
W
Figure 2: Problem 1
3. For figure 4 below,
(a) Draw an FBD for member AC. Write the equilibrium equations for
sum of the moments about point A.
P
Fx ,
P
Fy , and the
(b) Draw an FBD for the upper member ABE. Write the equilibrium equations for
P
Fy , and the sum of the moments about point A.
(c) Draw an FBD for the lower handle GCD. Write the equilibrium equations for
P
Fy , and the sum of the moments about point C.
(d) Draw an FBD for the lower jaw BDE. Write the equilibrium equations for
and the sum of the moments about point D.
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P
Fx ,
P
Fx ,
P
Fx ,
P
Fy ,
y
A
α
x
B
L
W
C
θ
L
F = −wj
Figure 3: Problem 2
Figure 4: Problem 3
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