Within its elastic limit, a spring obeys Hooke's law: the extension is directly proportional to the stretching force. Double the force and the spring stretches twice as far, so the amount of stretch is a reliable measure of the force applied.
A spring balance uses this. When you hang an object on it, the object's weight pulls the spring down, and the spring extends until its own pull balances the weight. A pointer attached to the spring moves along a scale that has been marked in newtons, so you read the weight directly from how far the spring has stretched.
It only works if you do not overload it. Beyond the elastic limit the spring is permanently deformed, no longer stretches proportionally, and the readings become wrong.
In SPM you should link the working of a spring balance to Hooke's law and to the idea of the elastic limit.
Common misconceptions
- A spring balance measures mass directly -> It measures weight, the force of gravity; the scale can be marked in newtons.
- A spring stretches proportionally no matter how much you load it -> Only within the elastic limit; beyond it the proportionality and the spring are ruined.
- A stiffer spring stretches more for the same force -> A stiffer spring has a larger spring constant, so it stretches less for the same force.
The physics behind it
A spring balance turns Hooke's law into a reading you can trust. Hooke's law says the stretching force is proportional to the extension, written F = kx, where F is the force in newtons, x is the extension in metres, and k is the spring constant in newtons per metre.
The spring constant measures stiffness: a large k means a stiff spring that stretches little for a given pull.
Suppose a balance uses a spring with k = 250 N m⁻¹ and the pointer shows an extension of x = 0.12 m. The force stretching it is F = kx = 250 N m⁻¹ × 0.12 m = 30 N, and since weight is what pulls on the spring, the load weighs 30 N. Dividing by the gravitational field strength gives the mass, m = 30 N ÷ 9.81 N kg⁻¹ = 3.1 kg.
On a force-extension graph this spring gives a straight line through the origin whose gradient is k. The maker simply prints the scale in newtons directly against the extension, so the pointer reads out the weight without any calculation.
At the wet market
A hanging spring balance is a familiar sight at a Malaysian wet market, where a trader hooks a bag of fish or vegetables onto it and reads the weight straight off the dial as the spring stretches. The heavier the catch, the further the spring pulls down and the higher the pointer climbs, exactly in step with the load.
Travellers use the same tool as a small luggage scale, clipped to a suitcase handle and lifted so the bag hangs clear, to check a case is under the airline limit before leaving home. A fishing enthusiast weighs a prize catch the same way.
Even a bathroom scale works on this principle, except its springs are squashed rather than stretched, and a lever system moves a dial. In all of them the design assumes the spring is kept within its elastic limit; a fish far too heavy for a small market balance would over-stretch the spring so it no longer springs back, and every reading after that would be wrong.
How this comes up in SPM
In Paper 2 this belongs to the Force and Motion II chapter, examined with command words such as state, explain, describe and determine. You might be asked to state Hooke's law, to explain why a spring balance stops reading correctly once overloaded, or to determine a spring constant from a force-extension graph.
Experiment-based questions may ask you to describe how to investigate the relationship between force and extension and to plot the results.
The idea is closely tied to its neighbours in the same chapter: elasticity and the elastic limit, the force-extension graph whose gradient gives k, and elastic potential energy stored in a stretched spring, given by E = ½ Fx or E = ½ kx². It also connects to arrangements of springs in series and in parallel, which change the effective stiffness.
When you meet a spring balance question, relate the reading directly back to F = kx and check whether the spring is still within its elastic limit before trusting the value.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)