Middle school students know about bicycles. Use bicycles to demonstrate principles of mechanical advantage and efficiency. Explain the old high-wheeler bicycles had pedals attached directly to the wheels; one turn of the pedal meant one turn of the wheel. Gear ratios allow the amount of turns of the pedals to differ from that of the wheels. Explain friction decreases efficiency in all machines. Discuss ways bicyclists decrease friction -- special helmets, thin tires and tight clothing. Have a bike and helmet ready for experimental riding over a variety of surfaces -- bumpy, muddy, etc.
The Science Museum of Minnesota suggests using a regular can of food like one for soup or pet food, a pencil, a lid designed to cover opened cans, marbles and clay to experiment with friction. Children attach the pencil to the lid using clay. Knobs of clay are put on the ends of the pencils to increase weight. Students try to spin the lid on the top of the can. After putting marbles between the lid and can, the lid will spin well. Students can discuss why this occurs.
Inclined planes are one of the simplest machines. Skis use this principle. To go down an incline quickly, skiers often wax the skis. Students can experiment with ice cubes and planks waxed with several types of ski wax to see which lowers friction the most. Students will measure how high the boards must be lifted before ice cubes move without a push.
Fiziks.net suggests an experiment using different pulley configurations each with different distance ratios. Teachers will need string, pulleys and weighted blocks. The distance ratio is varied by adding more pulleys and string to lift the same amount of weight. Students can calculate ratio M/m, the weight moved (M) by the weight needed to move it (m). The distance ratio is equal to the number of strings supporting the lower, movable, pulley block. Students will note that the additional pulleys make the movement easier, but that efficiency is lost.