# What torque would have to be applied to a rod with a length of 3 m and a mass of 4 kg to change its horizontal spin by a frequency 7 Hz over 1 s?

Feb 26, 2017

The torque (for the rod rotating about the center) is $= 131.95 N m$
The torque (for the rod rotating about one end) is $= 527.8 N m$

#### Explanation:

The torque is the rate of change of angular momentum

$\tau = \frac{\mathrm{dL}}{\mathrm{dt}} = \frac{d \left(I \omega\right)}{\mathrm{dt}} = I \frac{\mathrm{do} m e g a}{\mathrm{dt}}$

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

$I = \frac{1}{12} \cdot m {L}^{2}$

$= \frac{1}{12} \cdot 4 \cdot {3}^{2} = 3 k g {m}^{2}$

The rate of change of angular velocity is

$\frac{\mathrm{do} m e g a}{\mathrm{dt}} = \frac{7}{1} \cdot 2 \pi$

$= \left(14 \pi\right) r a {\mathrm{ds}}^{- 2}$

So the torque is $\tau = 3 \cdot \left(14 \pi\right) N m = 42 \pi N m = 131.95 N m$

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

$I = \frac{1}{3} \cdot m {L}^{2}$

$= \frac{1}{3} \cdot 4 \cdot {3}^{2} = 12 k g {m}^{2}$

So,

The torque is $\tau = 12 \cdot \left(14 \pi\right) = 527.8 N m$