1. Why a Pen Spring and a Rubber Band Pull Back for Different Reasons
The spring inside a ballpoint pen and a rubber band both return to their original length after being pulled. While they look similar, research from Osaka University explains that solid elasticity has two different origins[1]. One is "energy elasticity," which is the restoring force you imagine in a metal spring. The other is "entropy elasticity," which is the elasticity of rubber. Here, think of entropy as a measure of how easily things can become disordered or scattered[1]. Metal springs and rubber sit at opposite extremes, with various materials arranged between them[1]. So, what is trying to push a spring back into place?
2. How a Metal Spring Pushes Its Atoms Back into Position
When you pull on metal, the distance between atoms shifts slightly from its most stable position. The spring’s returning force is the effort to resist this shift and return to the original state[1]. If you hang a weight on a coil spring, the stretch increases in proportion to the weight’s heaviness. This is Hooke’s Law: as long as stress does not exceed a certain value, strain (the ratio of stretching) is proportional to stress[6]. The limit where this proportion holds is called the proportional limit; beyond that, it no longer applies[6]. If you pull further, atoms move to new positions and do not return. This is permanent deformation, distinct from the reversible deformation that happens before this point[7].
Yet, metal itself only deforms a tiny amount within its reversible range[7]. Despite this, coil springs stretch and shrink easily. Industrial textbooks explain that stretching a coil spring applies twisting and shearing forces to the wire itself[5]. Because the wire twists, the metal only deforms slightly, but the entire coil moves a lot. Metal springs stretch well because of their shape. So, what allows rubber to stretch?
3. How Tangled Chains in Rubber Make It Shrink Back
Rubber consists of long, chain-like molecules tangled in a disordered state. When pulled, the chains stretch and align in the direction of the pull, creating tension that tries to return to the original tangled state[3]. Imagine pulling a ball of yarn: it straightens out, but wants to curl back up when released. However, real rubber chains are invisible and are not moved like yarn. Stretching reduces the number of shapes the chains can take. The force trying to return from this limited state to the original state with many possible shapes is rubber’s returning force[9]. While springs try to restore "displaced atomic positions," rubber tries to restore the "number of shapes the chains can take"[1].
Many rubber products are made through vulcanization, a process that links molecules with sulfur bridges[11]. These bridges create a net-like structure, so when pulled, the chains do not slip apart, allowing them to return[3]. If the source of the force is different, what changes when you touch it?
4. Steel vs. Rubber: Which Is Harder to Deform and Which Stretches Farther?
Young’s modulus is a measure of how hard it is to deform a material. Data shows steel is about 201–216 GPa, aluminum is about 70 GPa, and rubber (at small deformations) is 0.01–0.1 GPa[4]. Calculating this, steel is about 2,000 to 20,000 times harder than rubber (201 ÷ 0.1 and 216 ÷ 0.01). Another explanation states that rubber’s Young’s modulus at room temperature is about 1–10 MPa, which is 1/10,000 to 1/100,000 that of metals or glass[3]. Because rubber’s values vary by type and condition, read these ratios as rough estimates depending on conditions.
Rubber stretches to several times its length without breaking and returns quickly when force is removed[3]. In an Osaka University experiment, EP rubber was stretched to 3.8 times its length and was measured to return reversibly[9]. Metal’s reversible deformation range is very small[7]. Springs stretch because of their shape (coil), while rubber stretches because of the chains inside the material. If the "ease of becoming scattered" is the source of the force, temperature must be involved.
5. Why Stretching Warms Rubber and Heating a Stretched Band Makes It Shrink
Most materials see a slight drop in temperature when tension is applied. However, rubber is an exception: its temperature rises, and the change is very large, according to Osaka University research[8]. This happens because stretching reduces the number of shapes the chains can take, lowering entropy. Because entropy decreases, the temperature rises[9]. The temperature changes during stretching and shrinking matched within measurement error and were reversible[9].
Rubber has another unusual property. If you raise the temperature while tension is applied, rubber shrinks. This is the opposite of most materials, which expand when heated[10]. It is also said that the force at a constant length is proportional to absolute temperature[10]. Force changes with temperature because rubber’s returning force comes from its tendency to move toward disorder. This contrasts with spring force, which comes mainly from atomic positions.
Materials between these extremes are still being studied. A 2021 study from the University of Tokyo showed that in polymer gels, which are relatives of rubber, the energy contribution to deformation can be largely negative. This suggests that the elasticity of rubber and gels is not the same[2].
6. How to Test the Difference Between a Rubber Band and a Spring Yourself
Take one rubber band that is not old or brittle. Hold it away from your face and slowly pull it with both hands. Gently touch the stretched part to the back of your hand. Do not touch your lips or face. Release the tension quickly, then stretch it again. You might feel a change in temperature. Because it can snap and fly back dangerously, do not stretch it to its limit directly in front of your face. Try pulling a ballpoint pen spring and comparing how it feels to the rubber band.
If you want to know more, research reports from the Osaka University Microcalorimetry Laboratory are available in Japanese[1][8][9]. They contain records of experiments measuring temperature changes when rubber is pulled.