# Factors affecting the stretching of springs and rubber bands

Topics: Hooke's law, Elasticity, Linear elasticity Pages: 7 (1762 words) Published: July 7, 2004
Aim:

To investigate the factors affecting the stretching of springs and rubber bands

Scientific Knowledge:

Before doing the experiment I came to the conclusion that this experiment relates to Hooke's Law which states that extension is proportional to the load, meaning that if you stretch something with a steadily increasing force, then the length will increase steadily too. By looking at various sources I have also found out that if a mass m on a spring is displaced from the equilibrium position (x0 = 0) to a new position x, Hooke's law states that the spring will exert a restoring force on the mass Fr = -kx. The "-" sign indicates that the direction of the spring force is in the direction opposite to the direction of the displacement. The value "k" is a constant for a given spring, but different springs have different "k-values." Thus, the force exerted by a spring is variable, specifically the greater distance it is stretched from equilibrium; the greater is the spring force attempting to restore the spring to its equilibrium position. This relationship holds up to a point called the elastic limit. Each spring has its own value of this limit. If you stretch a spring beyond its limit, then the spring will not return to its original shape, but will remain stretched out.

Not all "springy" things obey Hooke's Law. When a rubber band is stretched, the rubber will exert a restoring force. The amount of this force depends on the amount that the rubber is stretched, but perhaps not in the same simple way as the spring.

The F=k*x expression used to calculate the spring constant leads to other uses of the spring constant. Once k is known, we can use the displacement of the spring to determine the force applied to it. Then the spring constant becomes useful as a force measurement. As such, springs are used to measure the weight of objects in common household scales.

Regarding a rubber band, the spring constant will depend on the nature of the rubber; some varieties are stiffer than others. It will also depend on the thickness of the band. Thicker bands will tend to have higher spring constants. Also the length of the band will have an effect. For a given thickness, the longer the band, the less force it will produce for a given displacement. This is because there are a greater number of inter-molecular bonds to participate in the stretching so each bond suffers less strain. Rubber is sensitive to temperature changes so its spring constant will change with temperature, probably increasing as temperature decreases up to a point where it becomes inelastic at very low temperatures.

Prediction

In my experiment I predict that only the spring will obey Hooke's Law and the extension will increase directly proportional to the force applied to it, as from my scientific knowledge I have learnt that if you stretch something with a steadily increasing force then the length will increase steadily too. I also predict that the results will give me straight-line graph. This is how I think the graph for the extension of spring will look: -

Apparatus

§ Clamp

§ Ring stand

§ Table

§ 3 Springs

§ 3 Rubber bands

§ Meter stick

§ Various masses

§ Notebook

§ Pen

Diagram

Method

1) Firstly set up apparatus as shown. The clamp should be resting on the table and the ring stand must be attached to the clamp, there should be another clamp connected to the ring stand where the meter stick can hang down. The meter stick should be parallel to the ring stand. To ensure the results are accurate everything must be precise and levelled out.

2) Start by attaching a spring onto the clamp next to the meter stick, the spring must be new and one that has not reached its elastic limit. Using the meter stick measure the bottom of the spring without any weights added and record your results on a clear table.

3) Then gradually start adding weights to the spring going up in 50g e.g. 50g, 100g, etc. the weights...

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