Many sports use balls that bounce. Some are hollow and filled with air while others are solid. All of these can be used to demonstrate important physics concepts. For example, when a ball is held above the ground, it contains potential energy because it has the potential for movement, caused by gravity. After being dropped, the ball's potential energy decreases as its height decreases. Its kinetic energy -- the energy of movement -- increases as the ball picks up speed. This is an example of the law of conservation of energy, where energy is transferred from one type to another.
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Build a Bouncy Ball
Use adult supervision for this experiment. Mix 2 tablespoons of warm water and 1/2 teaspoon of borax powder in a cup. Place 1 tablespoon of white craft glue, 1/2 teaspoon of the borax solution and 1 tablespoon cornstarch in a second cup. Wait 15 seconds and then mix these ingredients until you cannot stir any more. Roll the mixture in your hands to form a ball. Drop the ball from different heights -- such as 1, 2 and 3 meters -- and measure how high the ball bounces. Compare the size, weight and bouncing ability of your ball to those of the other students.
Drop a small bouncy ball, tennis ball and basketball separately from a height of 3 meters and let the balls continue bouncing until they stop completely. Measure the height of each bounce by constructing a height diagram on the wall using strips of tape marked with the height. Using a digital stopwatch with a lap timer, record the time when the ball is dropped, each moment the ball hits the ground and when the ball reaches the highest point of each bounce. Repeat three times with each ball and graph the average measurements. Label the X-axis with time and the Y-axis with height.
Drop a small bouncy ball, tennis ball and basketball separately from a height of 1 meter. Observe that they bounce off the floor to almost the same height from which they were dropped. This demonstrates the law of conservation of energy. Next, drop the tennis ball and basketball at the same time with the tennis ball resting on top. The tennis ball bounces higher than its original position. When they hit the floor, the tennis ball bounces off the basketball, which is also moving upwards. This gives the tennis ball its increased speed and height. Try this with other combinations of balls.
Cold and Hot Bouncing
Obtain a "happy" and "unhappy" ball -- available from science education supply stores -- made of neoprene and norbornene, respectively. Drop these balls from a height of 1 meter and observe how high they bounce. Repeat this, but first place the balls in a beaker filled with water at 100 degrees Celsius for 5 minutes. Observe how high they bounce. Repeat several more times using water at 0, 10 and 50 degrees Celsius. Use a pair of tongs when handling the heated balls. The unhappy ball does not bounce much at room temperature, but will when heated or cooled. The happy ball, however, will bounce less when cooled.
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