The Penny Balance

A penny for your balance ~40 Minutes

Description: Different numbers of pennies are placed at different spots on a meter stick balance.

Objective: To investigate Forces and the lever.

-demonstrate how force can be applied to move an object (grade 1)
-demonstrate how various forces can affect the movement of objects (grade 5)
-demonstrate mechanical advantage (grade 5)

Materials: tape, large round marker (crayola), meter stick, ~20 pennies, hardcover book.

Doing it:
A.Tape the marker to the desk, this is what the meter stick will rest on. Place the meter stick on the marker and move it to a positions where it is balanced (it might not be exactly half).

B1. Put the book on the 100cm mark of the ruler and with the marker under the 50cm mark, push on the 0cm mark. How hard do you have to push to lift the book? Very hard, not very hard, or very lightly.
B2. Now move the ruler so the marker is under 20cm. How hard do you have to push on the 0cm mark now? Very hard, not very hard, or very lightly.
B3. Now move the ruler so the marker is under 80cm. How hard do you have to push on the 0cm mark now? Very hard, not very hard, or very lightly.

C1. Put ten pennies 10cm from where the ruler touches the marker (the fulcrum). Put pennies one by one onto the spot 10cm from the fulcrum in the other direction. How many does it take to be balanced?
C2. Leave the first stack in the same place, and remove the second stack. put pennies one by one on the spot 10cm further out from where the second stack was (20cm from the fulcrum). How many pennies do you think it will it take to balance the meter stick now? how many did it take it take?

Fig 1.1 - The ruler balanced with 10 pennies 10cm to the left, and 5 pennies 20cm to the right.

C3.Take the second stack off and put pennies onto the very end of the meter stick. How many do you think it should it take to balance it now? how many did it take it take?

Explanation: For the lever to be balanced, the torque (twisting force) on each side must be equal. The further a weight is from the fulcrum, the more torque it applies, i.e. something twice as far will apply twice as much torque. This is why you need half as many pennies to balance the meter stick when they are twice as far away, and one fifth of the pennies when they are five times further out. This is the mechanical advantage of a lever, placing the fulcrum very close to a heavy object results in very little force needed to lift it.


Experiment Feedback

Penny Balance Feedback
This activity was tried out on a group of 15 grade 5 students in June 2007. We divided them into groups of 4. We did not assign roles to each student for the experiment. Both parts of the experiment went well. I did have to work to keep each group on task and explain some steps, this may mean that the student instructions were not clear enough. I found that the students would do the experiment but not think about the results. When the experiment was complete, i drew some conclusions on the board about their results and they all seemed to understand more then. For example, one group was confused about the placement of the pennies and did that part backwards, so when they compared results with the other groups they thought they were 'wrong'. However when i put their results up on the board with the other, they noticed the fact that even though the numbers were different, the ratios were the same. In summary, i feel this is a good experiment for a class to do. I am going to make some changes to the student lab script to make it easier to understand, as well as a list of some important things for the facilitator to discuss between parts.

Updated in May 2007
Created by Brock Watson May 25 2007