Elastic deformation
A force can change the shape of an object. If you pull a spring, it gets longer. If you press a sponge, it gets shorter. This change of shape is called deformation.
A material is elastic if it goes back to its old shape when the force is removed. Springs, rubber bands and steel strips are elastic. Clay and dough are not: they keep the new shape. That is called plastic behaviour.
Elastic force
A stretched spring pulls back on whatever is stretching it. A squeezed spring pushes back. This force is the elastic force. It always tries to bring the spring back to its normal length.
The bigger the stretch, the bigger the elastic force. When a weight hangs still on a spring, the elastic force upward is equal to the weight downward. So the two forces are balanced.
Other examples: the push of a trampoline, a bent bow pushing the arrow, the springy mattress.
Stretch and load, and the elastic limit
Hang 1 unit of load: the spring stretches by some amount. Hang 2 units: it stretches twice as much. So the stretch is proportional to the load. This is the idea behind Hooke's law: F = k × x, where x is the stretch and k tells how stiff the spring is.
This works only up to a point called the elastic limit. If you pull beyond it, the spring does not return fully. It stays longer or even breaks. A spring balance has a maximum reading to keep you inside this limit.
The spring balance
A spring balance has a spring, a hook and a scale. The pointer moves along the scale as the spring stretches. The marks are made at equal gaps, because equal loads give equal stretches. It measures force in newtons (N), so it measures weight when you hang something on it.
- Hold it straight up, with the pointer at zero. If not at zero, adjust it first (zero error).
- Hang the object gently. Do not jerk it.
- Wait until the pointer is still. Read the scale at eye level.
Try it
Take a rubber band and a few identical coins in a small bag. Hang the bag from the band, hold a ruler next to it, and note the stretch with 1, 2 and 3 bags of coins. Predict the stretch for 4 bags before you check. In the 3D above, do the same, then pull too far and see what happens when you remove the load.
Key formulas and definitions
- Elastic force F = k × x (inside the elastic limit)
- x = stretch (extension), k = stiffness of the spring (N per m)
- Hanging at rest: elastic force up = weight down
- Stretch ∝ load (double load, double stretch)
- Unit of force: newton (N)
Worked examples
1. A spring stretches 4 cm when a 1 N load hangs on it. How much will it stretch for 3 N? (inside the limit)
Stretch is proportional to load. 3 N is 3 times 1 N, so stretch = 3 × 4 = 12 cm.
2. A spring stretches 6 cm with a 2 N weight. Find the load that gives a 15 cm stretch.
Each 1 N gives 3 cm. For 15 cm: load = 15 / 3 = 5 N.
3. A bag hangs still on a spring balance reading 12 N. What is the elastic force of the spring on the bag, and its direction?
The bag is still, so forces balance. Elastic force = 12 N, pointing upward.
Common mistakes
- Thinking a spring always returns to its old length. It does so only inside the elastic limit.
- Measuring the length of the spring instead of the stretch. Stretch = new length − old length.
- Reading the spring balance without checking zero first.
- Mixing up mass and force. A spring balance reads force in newtons, even if its scale shows kg marks for convenience.