- #1
jono90one
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I have a question but got two different answers by two different methods. The question:
"Two workers are holding an 80kg plank, one worker let's go. The weight is carried by 55% of the first worker. It is 2.5m long and no uniform. The angular acceleration is 5.5 rads/s^2, what is the moment of inertia of the plank about the axis perpendicular to the beam at the end held by the worker."
Method 1:
τ = F x r [1] (F=mg, r=2.5 x 0.45)
τ = Iα [2]
[1]=[2]
I = 161 kgm^2
Method 2:
I = ∑mx^2/l .δx between -0.45l and 0.55l
lim δx => 0
I = ∫mx^2/l .dx between -0.45l and 0.55l
I = 2060ml^2/8000
I = 128.75 kg/m^2
Using parallel axis theorem:
I = I1 +md^2
I = 128.75 + 80(0.45x2.5)^2
=230 kg/m^2
I do not know which method is the correct one, but unsure why the other would be wrong.
Can someone help me?
Thanks.
"Two workers are holding an 80kg plank, one worker let's go. The weight is carried by 55% of the first worker. It is 2.5m long and no uniform. The angular acceleration is 5.5 rads/s^2, what is the moment of inertia of the plank about the axis perpendicular to the beam at the end held by the worker."
Method 1:
τ = F x r [1] (F=mg, r=2.5 x 0.45)
τ = Iα [2]
[1]=[2]
I = 161 kgm^2
Method 2:
I = ∑mx^2/l .δx between -0.45l and 0.55l
lim δx => 0
I = ∫mx^2/l .dx between -0.45l and 0.55l
I = 2060ml^2/8000
I = 128.75 kg/m^2
Using parallel axis theorem:
I = I1 +md^2
I = 128.75 + 80(0.45x2.5)^2
=230 kg/m^2
I do not know which method is the correct one, but unsure why the other would be wrong.
Can someone help me?
Thanks.