Elasticity is a familiar concept to most school students, which makes it an excellent topic for physics experiments. If you apply force to an object such as a spring, you essentially transfer energy to it, which is held as “strain potential energy” in the object, causing its shape or size to become deformed. When you let go, the energy is released in the form of kinetic (or movement) energy. Numerous experiments can be done to investigate the phenomenon of elasticity, and many can be completed at home or in a classroom with everyday materials.
Strain potential energy
This simple experiment demonstrates how the “work” you do to deform an elastic band is transferred into movement energy. Stretch a rubber band around the two legs of a chair, keeping the band close to the ground. Clear space in front of the chair and move it close to a wall in order to maximise the room you have. Lay a ruler beside the elastic band so that the “0 cm” mark is matched up with the starting position of the elastic band. Rest the rear of a toy car against the elastic band (where your measurement starts) and pull it back by 5 cm (2 inches), stretching the elastic band back and bringing the car with it. Let go and watch the strain potential energy you sorted be converted into kinetic energy in the car. Measure the distance the car travelled and note of both the distance you stretched the band and how far the car went. Repeat the experiment, pulling the band back further each time. Is there a relationship between how far you pull the band back and how far the car travels? Why?
The elastic limit
Hooke’s Law shows that there is a clear relationship between the extension of an elastic object and the amount of force imparted on it, but this degrades at a point known as the elastic limit. If the force applies exceeds the elastic limit, then it won’t return to its original shape again. Get a couple of tables or chairs (of equal height), an elastic band, scissors, a ruler, a skewer, re-usable adhesive and a series of weights with hooks or rings on the top. Cut the elastic band into a long, straight ribbon, and measure its length. Tie one end to the centre of the skewer. Rest the ends of skewer on the surfaces of the two tables or chairs, so the elastic band dangles in the space between, and apply re-usable adhesive to keep it in place. Attach a small weight to the other end of the elastic band (it’s important to know how much weight you’re using) and allow it to stretch downwards. Remove the weight and measure the length of the elastic band. Repeat the experiment with gradually increasing weight and measure the band after each. When the band’s length increases, you’ve exceeded its elastic limit. Using the fact that 1 kg (2.2 pounds) is 9.8 Newtons, make an estimate of the band’s elastic limit in Newtons.
Temperature and elasticity
Replicate the setup of the elastic limit experiment, except with skewer, elastic band and weight (below the elastic limit) suspended over a cylindrical measuring jug so it hangs down on the inside. Carefully pour some boiling water into the jug (getting help from an adult if needed to increase the temperature, and measure how hot it is with a thermometer. Measure the length of the rubber band from the skewer to the weight and record it alongside the temperature. Wait five minutes and repeat the measurements, continuing in this fashion until the water temperature is around 20 degrees Celsius (68 Fahrenheit). You can remove some water and replace it with ice water to lower the temperature further. Is there a relationship between the elasticity of the band and the temperature?
Finding the spring constant
Hooke’s Law can be stated as, “the force a spring exerts on an object is equal to the extension of the spring multiplied by the spring constant.” To test this, replicate the experimental setup of the elastic limit experiment. Measure the starting length of the elastic band, from the skewer to the tip. Add mass to the elastic band, increasing by 0.1 kg (3.5 ounces) at a time. Measure the band and record how far it stretches under each amount of weight. When you’ve reached 1 kg (2.2 pounds), plot your results as a graph of force versus extension, with force on the Y axis and extension on the X axis. Draw a line of best fit and find its gradient to work out the value of the “spring constant.”