Answer to Question #156169 in Physics for Marie

Question #156169

A wheel of mass 50kg has a radius of 0.4m. It is making 480rpm. What a the torque necessary to bring it to rest in 40 seconds.


1
Expert's answer
2021-01-19T07:09:47-0500

Let's first find the angular acceleration of the wheel:


α=ωfωit,\alpha=\dfrac{\omega_f-\omega_i}{t},α=0480 revmin1 min60 s2π rad1 rev40 s=50.3 rads2.\alpha=\dfrac{0-480\ \dfrac{rev}{min}\cdot\dfrac{1\ min}{60\ s}\cdot\dfrac{2\pi\ rad}{1\ rev}}{40\ s}=-50.3\ \dfrac{rad}{s^2}.

The sign minus means that the wheel decelerates.

Let's find the moment of inertia of the wheel:


I=mr2=50 kg(0.4 m)2=8 kgm2.I=mr^2=50\ kg\cdot(0.4\ m)^2=8\ kg\cdot m^2.

Finally, we can find the torque necessary to bring the wheel to rest:


τ=αI=50.3 rads28 kgm2=402.4 Nm.\tau=\alpha I=-50.3\ \dfrac{rad}{s^2}\cdot 8\ kg\cdot m^2=-402.4\ N\cdot m.


The sign minus means that the torque directed in the opposite direction to the rotation of the wheel.

Answer:

τ=402.4 Nm.\tau=-402.4\ N\cdot m.


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