Stretching and Elastic Forces

Things that pull back

Pick up a rubber band and gently stretch it between your fingers. Feel that? It pulls back, trying to get smaller again. Let go, and it springs back to its old shape in an instant. A rubber band is elastic — and the pull you felt is an elastic force.

An elastic object is one that stretches (or squashes) when you push or pull it, and then springs back to its original shape when you let go. Rubber bands, springs, bungee cords, sponges, and trampolines are all elastic. This lesson is all about what happens when you stretch elastic things, and the surprising force they push and pull back with.

The elastic force always pulls back

When you stretch a rubber band, you are pulling it longer than it wants to be. The band does not like that. It always tries to return to its original, comfy shape. So while it is stretched, it pulls back in the opposite direction to your pull. That pull-back is the elastic force.

Here is the important pattern:

• Stretch the band a little, and it pulls back gently.
• Stretch it a lot, and it pulls back much harder.

The further you stretch, the stronger the elastic force becomes. You can feel this yourself — a rubber band that is only slightly stretched is easy to hold, but one stretched right out tugs hard at your fingers. Springs behave in exactly the same way, which is why we can use them to measure forces. You can read more about that in measuring forces with springs.

Stored-up energy, ready to spring

When you stretch a rubber band, you are not just bending it — you are storing energy inside it. We call this elastic energy (or stored elastic potential energy). The band is like a tiny battery, holding the energy until it is released.

Let go of one end and snap! — all that stored energy is released at once. The band flies forward. This is how a catapult, a slingshot, or a wind-up toy works: you store elastic energy by stretching or winding something up, then release it to make something move. A bouncing trampoline does the same: it stores energy as it squashes down under your weight, then springs back and throws you up into the air.

Elastic or permanent? Every stretchy thing has a limit

Elastic objects are amazing, but they are not magic. Every one of them has a limit. Stretch a rubber band gently and it always springs back perfectly. But pull it too far, past its limit, and one of two things happens:

• It may stay stretched, loose and floppy, never returning to its old shape. We say it has been permanently changed.
• Or it may snap completely.

You have probably seen an old, tired rubber band that has gone loose and saggy — it was stretched past its limit too many times. Some materials, like modelling clay or wet sand, are not really elastic at all: squash or stretch them and they just stay in their new shape. They have almost no elastic force. So whether something springs back depends on what it is made of.

Elastic forces all around you

Elastic forces are at work in more places than you might think:

• A trampoline stretches downward and springs you back up.
• A bungee cord stretches to slow a jumper, then pulls them back.
• The springs in a bed, a car, or a clicky pen push and pull to do their jobs.
• A bow and arrow stores elastic energy in the bent bow, then fires the arrow.
• Even your own skin and muscles are a bit elastic, stretching and springing back as you move.

In every one of these, an object stretches or squashes, stores energy, and pushes or pulls back with an elastic force. Elastic forces are just one kind of push or pull, and you can learn about forces more generally in pushes and pulls.

Why elastic forces matter

Elastic forces let us store energy and release it whenever we want — to launch, to bounce, to cushion, and to spring. Engineers use elastic materials to make car suspensions smooth, to absorb shocks, and to build everything from mattresses to mountain-bike forks. Once you understand that some materials pull back when stretched, you start to see clever springs and elastic tricks hidden in machines all around you.

Try it yourself! 🧪

Feel the elastic force grow — safe and easy.

You will need a few rubber bands and some small identical objects, like coins or washers, plus a ruler.

1. Hang a rubber band from a hook, a door handle, or your finger so it dangles freely. Notice how long it is with nothing on it.
2. Carefully hang one small weight on the bottom (loop it on, or use a little bag of coins). Measure how far the band has stretched.
3. Add a second weight. Measure again. Then add a third. Each time, write down the stretch.
4. Look at your numbers. Each extra weight pulls the band a bit further down — because gravity pulls harder with more weight, and the elastic force has to grow to match it.

Safety note: never stretch a rubber band toward anyone's face, and do not over-stretch it until it snaps. Stop while it still springs back. Take off the weights and watch the band return to its starting length — proof that the elastic force was pulling back the whole time, ready to restore its shape.