Let's first find the angular acceleration of the wheel:
α=tωf−ωi,α=40 s0−480 minrev⋅60 s1 min⋅1 rev2π rad=−50.3 s2rad.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 kg⋅m2.Finally, we can find the torque necessary to bring the wheel to rest:
τ=αI=−50.3 s2rad⋅8 kg⋅m2=−402.4 N⋅m.
The sign minus means that the torque directed in the opposite direction to the rotation of the wheel.
Answer:
τ=−402.4 N⋅m.
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