How to play Spring Shop
- Choose a test and read the springs and the load on the card.
- Decide how the springs share the load: side by side they share it, end to end each carries it all.
- Type your prediction and press Start. The load is lowered gently beside a ruler.
- Compare where it settles with your orange line, or the bounce with your period.
How do springs combine?
A spring stretches in proportion to the force on it, F = k·x, and the stiffness k says how many newtons stretch it one metre. When two springs hang side by side and hold one load together, they both stretch the same amount and share the force, so the pair is stiffer: k = k₁ + k₂. When they hang end to end, each spring carries the whole load, both stretch, and the stretches add, so the pair is softer: 1/k = 1/k₁ + 1/k₂, and two equal springs make one half as stiff. Cutting a spring works the other way round: each piece has fewer coils to share the stretch, so half a spring is twice as stiff. A car's four springs share its weight side by side, a mattress uses hundreds of springs side by side, and a long chain of springs is soft and stretchy. The stiffness also sets how fast a mass bounces: T = 2π·√(m ÷ k).
A focused learning model
The springs are ideal: massless, with stretch in proportion to force and no friction. Each spring is drawn 18 cm long before it stretches, and the load is lowered gently over 2 s to where the springs hold its weight. For the bounce, the mass starts 3 cm below rest and bounces for 2 s. Stretches are to scale on the ruler. Answers within 2% are right. g = 9.8 m/s².