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Activity

Balance point on a stick or ruler

Summary: 
Balance a stick or ruler on your finger, then experiment with adding mass to one or both sides of the stick or ruler, to see how it changes the balance point.
Science content (2016 curriculum): 
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Lessons activity is in: 
Materials: 
  • long wooden stick, up to 1/2" diameter
  • play dough
  • optional: ruler for smaller scale activity
Procedure: 

Using a long stick
More dramatic, but more chaotic with many long sticks in use.

Balance the stick on one finger.
The balance point should be half way along.
This is because the mass of the stick is the same on each side, so gravity pulls down on each side with equal force. As the stick is an even width, the balance point will be in the centre.

A trick to find the balance point of any stick:
Start with the stick resting on a finger at each end, then slowly slide the fingers together along the underside of the stick. The rod will slide over the fingers, then stop moving over them, in turn, as the weight on each finger changes. The fingers will end up meeting at the balance point in the middle of the stick. See http://www.exploratorium.edu/snacks/center-gravity

Now add some play dough to one end of the stick, and find the new balance point.
It should be nearer the weight, as this end now has more mass, and so must be balanced by a longer length of rod.

Now experiment with adding different sizes of play dough to different places along the stick.
The balance point will move depending on the relative weights of each end of the stick.
Students can draw what they find in each case (see photo and attached worksheet).

Using a ruler
Less fun with the smaller scale, but easier to manage and easily made quantitative with the ruler measurements.

The same activity can be conducted with a ruler, and the markings used to record how far a mass is from the centre and how this affects the balance point. Graph the results and the points should make a straight line (though data will be more messy than this). To graph the data from multiple students on one graph, make sure they have the same sized play dough.

Students can also use a pencil to make a see saw with a ruler, and balance different sized balls of play dough on each end. Encourage investigation with questions: How does moving the fulcrum change how they balance? Can a small ball lift a large ball? How small can a small ball be to do this? Where does the fulcrum have to be to make them balance each other?

More free exploration
Students can also use these materials and be given additional materials to try lever free experimentation, for some less structured exploration of levers and balancing, probably best done after these more structured activities, to ensure some focus.

Notes: 

Construct a balancing stick with defined positions that weights can be added to see the effect of position and mass on the balance of the stick: https://www.youtube.com/watch?v=MAINjKTu1As

Balancing and torque: http://www.discovery.com/tv-shows/mythbusters/about-this-show/physics-of...

Grades tested: 
Gr 4
Gr 5
Teacher: 
Ingrid
Teaching site: 
ingridscience afterschool