Practical: stretching springs, wires and rubber bands revision notes

Edexcel International GCSE Physics (4PH1), chapter 2: Forces and shape, specification point 1.22. Free lesson.

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Measure how much a spring, wire or rubber band stretches as force is added. Accurate readings, safe handling and a graph of force against extension help you compare how each material behaves.

How to measure a spring's extension

Hang the spring from a clamp stand and place a vertical metre rule beside it. A pointer at the bottom of the spring makes its position easier to read. The reading with no load is the original length. After adding a load, subtract this original reading from the new one to find the extension. A 100 g mass has a weight of 1 N. Add masses one at a time, up to about 600 g, and read the pointer after each addition. Record the results as force (N) and extension (cm). Wear goggles in case the spring flies off, and keep your feet clear of falling masses.

How to calculate force and extension

The force from a hanging mass is its weight. Calculate weight by multiplying mass in kilograms by gravitational field strength, g = 10 N/kg. Extension is a change in length, so subtract the original reading from the reading with the load. Convert mass from grams to kilograms before calculating weight. Record the extension, not the spring's new length on its own.

How to make the readings reliable

Read the pointer with your eye level with it and look at right angles to the ruler. This avoids parallax error, where a reading appears to shift because you are looking from an angle. The independent variable is force. Change it by adding 100 g masses, each with a weight of 1 N. The dependent variable is extension. Find it from the metre rule readings. Keep the same spring throughout, because a different spring may stretch by a different amount. Add masses gently. A dropped mass can stretch the spring further than its weight alone would. When unloading, check whether the…

How to plot force against extension

Plot extension along the bottom axis, in centimetres, and force up the side, in newtons. Use the readings from your results table. The readings described here lie on a straight line through the origin, so draw one ruled line of best fit. Use a scale that increases in steps of 2 cm for extension. Plot each reading to within half a small square. Draw one ruled line or one smooth curve of best fit. Do not join the points dot to dot.

How the method changes for wires and rubber bands

The same idea applies to a metal wire and a rubber band, but the set-up changes to suit how much each stretches. Whatever you test, measure the extension rather than the length alone. A spring and a rubber band hang from a clamp. Read their positions with a pointer against a metre rule. Fix one end of a wire, lay it along the bench and run it over a pulley, with the masses hanging from it. Use a small marker, such as a tape flag, and a fixed ruler to read its position. A metal wire stretches very little, so use a long wire, often two metres or more. A rubber band stretches a lot. Wear…