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blargh4fun
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Homework Statement
A body (let's call it a rod for simplicity) is in frictionless space, and is composed of 4 smaller sub-rods fused (cannot break) end to end. Each sub-rod has a unique mass (m1, m2, m3 and m4) and length (l1, l2, l3, and l4), but they all have the same diameter d. A force F is applied laterally to the very end of the rod (the end of the m1 rod). The center of mass happens to fall in the third rod due to the values of m1-m4 (which are not necessary).
Calculate the bending moments in each segment of the rod.
I know four blocks is kind of a headache. 3 blocks would be more than enough to help explain the problem.
Homework Equations
Eq1. Torque: [itex]\tau = r F = I \alpha [/itex]
Eq2. Mom. Inertia of rod about z: [itex]I_{rod} = \frac{mL^2}{12} [/itex]
Eq3. Parallel axis theorem: [itex]I=I_{cm} + mr^2 [/itex]
Eq4. Center of mass: [itex]R=\frac{1}{M}\sum\limits_{i=1}^n m_i r_i [/itex] where [itex]M=\sum m_i[/itex]
The Attempt at a Solution
Step1: Center of mass (com) of entire body.
Use Eq4.
Step2: I of entire body about com
Use Eq. 2 and 3
Step3: Calculate alpha of entire body.
Use Eq. 1 using the results from Step1 and Step2:
[itex]r F = I \alpha [/itex]
gives [itex]\alpha =\frac{r F}{I} [/itex]
Step4: Draw free body diagram for each sub-rod and solve for all forces
note: a, b, c, and d are the vector distances from an intersection to the com.
Body1: [itex]\tau = Fa - F_{2/1}b=I_1 \alpha[/itex]
[itex]F_{2/1} = \frac{1}{b} (Fa -I_1 \alpha)[/itex]
Body2: [itex]\tau = F_{1/2}b - F_{3/2}c=I_2 \alpha[/itex]
[itex]F_{3/2} = \frac{1}{c} (F_{1/2}b -I_2 \alpha)[/itex]
Body3: [itex]\tau = F_{2/3}c - F_{4/3}d=I_3 \alpha[/itex]
[itex]F_{4/3} = \frac{1}{d} (F_{2/3}b -I_3 \alpha)[/itex] note d<0
Body4: Same as above but no new information is gained.
4. And now...
And this is where I'm stuck. I feel confident about steps 1-3, and 4 seems reasonable. Where do I go from here to get the bending moments? Thanks.