Summary Balance a stick or ruler on your finger or on a pencil. Experiment with adding mass to one or both sides of the stick or ruler, to see how it changes the balance point. Science content Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6) Science competencies (+ questioning + manipulation + others that are in every activity) Questioning/predicting: predicting (1 up), hypothesizing (7) Planning/conducting: measuring (non-standard K up, standard 2 up) Processing/analyzing: tabulating data (1 up) Processing/analyzing: graphing data/pictographs (K up) Processing/analyzing: comparing observations with predictions (1 up) Evaluating: inferring (3 up) Lessons activity is in Balanced/unbalanced Forces Force stations Materials ruler or paint stick for desktop activity long wooden stick, up to 1/2" diameter for activity that needs standing space play dough optional: other classroom objects to balance e.g. lego or small plastic toys Procedure Using a paint stick (good for lower primaries) The fulcrum can be a pencil or short dowel. Find the balance point for different sized balls of play dough on each end. Younger students can add in lego or other classroom objects. Give a set of challenges (see photo) and/or 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? Discuss the pushes and pulls that keep it balanced: gravity pulls on the weights, to make them push down on the stick. When the pushes on each side of the balance point are equal, the stick will balance. Using a ruler, and adding measuring The same activity as with the paint stick 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. More free exploration Students can also use paint sticks 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. 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. 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 and their positions out on the stick. The lighter weights need a longer length of stick to balance a heavier weight. The further out the mass is, the greater force it exerts (actually torque as the force is exerted on a rotating lever arm). The mass has more of an effect when it is further away (the torque is greater). Students can draw what they find in each case (see photo and attached worksheet). Attached documents 2016_fall_thurs_1_balancing_worksheet.pdf Notes A ridged pencil can make it tricky to balance a ruler - add a small piece of play dough under the pencil to hold it still - maybe in combination with a smooth, round pencil? Using a ruler allows students to easily read off and take note of the position of their fulcrum. Adding a gram scale to weigh lumps of play dough would allow for some great data collection comparing balance points for varying lever lengths and load weights. 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-o… Grades taught Gr K Gr 1 Gr 2 Gr 3 Gr 4 Gr 5
