Balancing stick - rotational Inertia

"Sorry, for coming late " said the kid who was panting .
When I asked him "What happened, that got you delayed?".
He said "I was running back from the shop and couldn't stop myself from running even after coming closer home".
" Wonderful! you  just experienced  something called inertia in physics, dear" I replied.
And that's how the class started this week.


What is Inertia?.
Inertia  is the tendency of  a body to resist any change in motion or at rest. This can be explained with a simple example. When a vehicle stops,  you tend to jerk forward before coming to a stop. Also, when a vehicle starts you tend to jerk backward.

What is rotational Inertia?
Rotational inertia measures how much an object resists the change in rotation. Rotational inertia depends on the distribution of the mass in the stick. For example, when a sportsman is holding a bat
(baseball) and swings it, he can swing quickly if he chokes the bat by reducing the rotational inertia.


To demonstrate and understand it, we did the following simple activity.


Materials needed

1. Wooden dowel rod -  approximately 3 feet long and 1/2 an inch thick
2. Clay - small lump

Procedure

1. Give the clay and the wooden dowel to a group of students.
2. Tell them to attach the clay  to the dowel anywhere.
3. Challenge them to balance the stick on the tip of one finger.
4. Check who can balance it the longest duration.


Variation

1. Balance the stick with clay on the tip of your finger keeping the clay on the top.
2. Next turn it around with clay closer to your finger.
3. Now keep the clay somewhere in the middle and notice the difference.



Observations  & Reasoning

Did you notice that the stick is easier to balance, when the clay is on the top?.

Wonderful ...the dowel moves slowly and has bigger rotational inertia which gives you time to balance it when the clay is on the top. When the clay is closer to the finger, there is less rotational inertia and so it tips very quickly.

Though the mass is the same whether the clay is on the top or bottom, the distribution of the mass has only changed with respect to your finger. This is due to the reason that the rotational inertia depends on the distribution of the mass.












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