# What torque would have to be applied to a rod with a length of #12 m# and a mass of #6 kg# to change its horizontal spin by a frequency #5 Hz# over #9 s#?

##### 1 Answer

The torque needed is

#### Explanation:

Assuming rotation around the center of mass and rotation only ever happens in one axis.

Moment of inertia of a rod is

The angular velocity has to change by

Thus angular acceleration is

Moment of inertia is rotational analogue of mass and torque is angular brother of force.

We could then use our intuition from Newtons second law to rotational motion. As the force

As you see

It is funny thou, that almost every mechanical quantity (lenght, velocity, acceleration, mass, momentum, force) has its own angular sibling.