What torque would have to be applied to a rod with a length of $4 m$ and a mass of $5 kg$ to change its horizontal spin by a frequency of $9 Hz$ over $7 s$ ?

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Answer 1 · 2017-10-15T07:41:42

The torque for the rod rotating about the center is $=53.86Nm$
The torque for the rod rotating about one end is $=215.42Nm$

The torque is the rate of change of angular momentum

$tau=(dL)/dt=(d(Iomega))/dt=I(domega)/dt$

The moment of inertia of a rod, rotating about the center is

$I=1/12*mL^2$

$=1/12*5*4^2= 6.67 kgm^2$

The rate of change of angular velocity is

$(domega)/dt=(9)/7*2pi$

$=(18/7pi) rads^(-2)$

So the torque is $tau=6.67*(18/7pi) Nm=53.86Nm$

The moment of inertia of a rod, rotating about one end is

$I=1/3*mL^2$

$=1/3*5*4^2=36kgm^2$

So,

The torque is $tau=36*(18/7pi)=215.42Nm$

What torque would have to be applied to a rod with a length of 4 m4 m and a mass of 5 kg5 kg to change its horizontal spin by a frequency of 9 Hz9 Hz over 7 s7 s?