Rotational Inertia

Corla Jean Wilson-Hawkins      Bethune Elementary School
8122 South Avalon              3030 West Arthington
Chicago, Illinois 60619        Chicago, Illinois 60612
312-933-0527                   312-534-6890                 

Objective:

To demonstrate how the resistance of an object to rotation is rotational 
inertia. 

Materials Needed:

This list is for a class size of 32 students divided into groups.

4 ramps                                      4 meter sticks
4 cans of broth soup covered with paper      8 100 gm weights
4 cans of tomato soup covered with paper     masking tape
pendulum                                     balance beam and long pole
hollow wheel                                 solid wheel
turn table for turning (if possible - optional)

Strategy:

To begin the lesson, have the students give you several brainstorming 
definitions of what they think inertia is.  Next discuss what they think 
rotational inertia is and write that on the board.  After discussing the 
vocabulary, have one student from each group sit on the turn table.  Spin them 
slowly.  Using equal weights or books in their hands have them pull their hands 
close to their bodies and then extend them away from their body.  As they pull 
their hands closer to their bodies they will find that they spin faster than 
they do when their hands are extended outwards.  Use this opportunity to discuss 
how the distribution of mass makes a difference in the rotational inertia. 

Next, using your groups, take the 4 meter sticks and tape the 100 gram weights 
to the bottom of the stick.  Put one on each side and tape around it.  Have 
students collect data on who could balance it vertically the longest with the 
weights on the bottom and again with the weights on the top.  Have the students 
explain why they thought it was easier to balance the weight on the top of the 
stick versus the bottom.  Some will be good both ways but the norm tends to be 
with the weights on the top. 

Using the ramps, have your groups take the soup cans of different contents that 
are covered with paper and time them as they roll down the ramp.  Have them make 
a graph charting the time it took and the can that won.  Have them to try to 
figure out why one can won over the other.  After a thorough discussion, uncover 
the cans and talk about the contents in the cans and the distribution of the 
mass. 

Now take the balance beam and have it properly mounted with the wooden stands on 
each end.  Have a student walk on the beam with their hands in their pockets, 
and again with their hands extended holding a long pole or long stick.  Talk 
about which way was easier to walk.  Again reinforce that it is easier to 
balance when the rotational inertia is farther away from the axis. 

For some additional fun, make a pendulum and push it back and forth on a long 
string.  Ask the children to tell you how it is moving.  Then shorten the string 
and let them tell you how it is going.  Of course the shorter one moves faster 
because it is closer to the axis.  At this point the children can make a 
pendulum for a hands on activity. 

For a final activity take a ring and a solid disk and roll them down a ramp. 
They do not have to be the same weight or size because the theory will still 
prove itself.  Have the children guess which one will come down first.  Explain 
to them that since the wooden disk is solid, it's mass is closer to the axis 
than the hollow ring which has all it's mass on the outer rim. 

Conclusion:

The conclusion of this lesson would be to have students explain what we did 
today in all our demonstrations.  Ask them to tell you what they liked best and 
what they liked the least.  See if any of your students can think of other 
examples of rotational inertia that were not used today that can be added to the 
lesson. 
  
Evaluation:

Often times when we do a lot of hands on activities, we fail to reinforce the 
comprehension version in writing.  My evaluation of this process will be a short 
quiz that would include all the examples that we had today and to have the 
children explain them to me.  I would also have them draw some of the demos for 
me on paper so that I know who really was aware of what was happening in the 
demonstration.  I would be looking for 90% accuracy on the quiz. 

Summary:

     Event                    Results        
       
     bottles         empty-easy to move             full-hard to move
     turntable       mass at center-easy to rotate  mass at edge-harder to do
     meter stick     wts on bottom hard to balance  wts on top easy to balance
     balance beam    mass at center-easy to fall    mass out-easier to stay
     pendulums       short-swings faster            long-swings slower
     soup cans       tomato solid-rolls fast        chicken-liquid rolls slower
     disk and ring   disk is faster-mass spread     ring is slower-mass all at  
                       uniformly throughout           the edge (further out)
  
Reference:

 Conceptual Physics -  Addison Wesley  Paul G. Hewitt   1986 pg 192-204