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A wheel made of a hoop has a mass of 1.00 kg and a radius of 50.0 cm, and spokes with a mass of 20.0 g each. The wheel should have a total moment of inertia 0.310 kg · m2.1. how many spokes are necessary to construct the wheel? 2. What is the mass of the wheel?

User Moh Tarvirdi
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1 Answer

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22 votes

We will have the following:

First, we write down the values given:


\begin{cases}M_1=1.0\operatorname{kg} \\ r=50.0cm=0.5m \\ M_2=20.0g=0.02\operatorname{kg} \\ I=0.310\operatorname{kg}\cdot m^2\end{cases}

Then, from definition of intertia, we will have that:


I=I_{\text{rim}}+n\cdot I_{\text{spoke}}

Here "n" is the number of spokes the wheel has, so:


I_{\text{rim}}=M_1\cdot r^2\Rightarrow I_{\text{rim}}=(1.0\operatorname{kg})(0.5m)^2\Rightarrow I_{\text{rim}}=0.25\operatorname{kg}\cdot m^2

&


I_{\text{spoke}}=(1)/(3)\cdot M_2\cdot r^2\Rightarrow I_{\text{spoke}}=(1)/(3)(0.02\operatorname{kg})(0.5m)^2\Rightarrow I_{\text{spoke}}=\frac{_{}1}{600}kg\cdot m^2

Now, replacing the values, we will have that:


0.310\operatorname{kg}\cdot m^2=0.25\operatorname{kg}\cdot m^2+n((1)/(600)kg\cdot m^2)\Rightarrow0.06\operatorname{kg}\cdot m^2=n((1)/(600)kg\cdot m^2)
\Rightarrow n=36

So, the number of spokes is 36.

Now, we calculate the mass of the wheel:

Here, we will have that:


M_w=M_1+n\cdot M_2

Where "Mw" is the mass of the wheel. So, we replace the values:


M_w=(1.00\operatorname{kg})+36(0.02\operatorname{kg})\Rightarrow M_w=(43)/(25)kg
\Rightarrow M_w=1.72\operatorname{kg}

So, the mass of the wheel is 1.72 kg.

User Refactorthis
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