Forces, Movement, Shape and Momentum » Practical: Stretching Materials

What this practical is about

This is a practical named in the specification, so the exam can ask you to describe it, improve it or use its results. You do not need a school laboratory to answer those questions. You need to know what is done, in what order, and why.

The aim is to investigate how extension varies with applied force for three things: a helical spring, a metal wire and a rubber band. You hang masses on each one, measure how far it stretches, and plot a force-extension graph. Remember, extension is the stretched length minus the original length.

Key terms:

• Helical spring: a spring made from wire wound into a coil, like the springs inside a mattress.
• Slotted masses: metal discs, usually 100 g each, with a slot so they slide onto a hanger.
• Parallax error: a wrong reading caused by looking at a scale from above or below instead of straight on.

The apparatus

There are no drawings here, so picture each set-up as you read. In the exam you may be asked to describe or label them.

Why is the wire so long? A metal wire stretches very little, so a short one would give an extension too small to measure with a ruler. A wire two or three metres long stretches enough for the marker to move a distance you can read.

The method

These steps are for the spring.

1. Set up the clamp stand, spring, ruler and pointer. Check that the ruler is vertical.
2. With nothing on the spring, read the position of the pointer on the ruler. This is the starting reading.
3. Hang a 100 g mass on the spring. Wait until it stops bouncing.
4. With your eye level with the pointer, read its new position. Record the reading.
5. Add another 100 g mass and read the pointer again. Carry on, one mass at a time, up to 500 g. This is loading.
6. Now take the masses off one at a time, reading the pointer each time. This is unloading.
7. Work out each extension: the loading reading with the masses on, minus the starting reading.
8. Repeat the whole experiment and calculate a mean extension for each force.
9. Do it all again with the rubber band, and then with the wire on the bench.

Why unload? It tells you whether the material went back to where it started. If the pointer returns to the starting reading, the spring behaved elastically, as you saw in Hooke's Law and Elastic Behaviour. If it does not, the spring was stretched too far and has been permanently deformed.

From mass to force

The masses are marked in grams, but the graph needs force in newtons. The force stretching the spring is the weight of the masses, so use W = m × g with g = 10 N/kg. Change grams to kilograms first.

A full results table for the same spring looks like this.

Mass (g)	Force (N)	Reading, loading (cm)	Reading, unloading (cm)	Extension, loading (cm)
0	0	10.0	10.0	0.0
100	1	11.6	11.6	1.6
200	2	13.2	13.2	3.2
300	3	14.8	14.8	4.8
400	4	16.4	16.4	6.4
500	5	18.0	18.0	8.0

Every column has a heading with a unit. The readings are all written to the same number of decimal places. The unloading readings match the loading ones and the pointer is back at 10.0 cm, so this spring was not damaged. For the rubber band the unloading readings may differ a little from the loading ones. What matters is whether the pointer returns to the starting reading.

When you repeat the experiment, add a second set of reading columns and a final column headed 'Mean extension (cm)'.

Plotting the graph

Draw one force-extension graph for each material. Put force in N on the vertical axis and extension in cm on the horizontal axis, as in the last lesson. Choose scales so the points fill more than half the grid. Mark each point with a small, sharp cross. If the points lie along a line, draw a straight line of best fit with a ruler. If they follow a bend, draw one smooth curve. Never join the points dot to dot.

What the shape of each graph means was covered in Hooke's Law and Elastic Behaviour. Use that lesson to describe your three graphs.

Variables and accurate readings

The main sources of error, and how to cut them down:

• Parallax error. If you look at the pointer from above or below, it lines up with the wrong mark on the scale. Put your eye level with the pointer. A pointer that almost touches the ruler helps as well.
• Ruler not vertical. A sloping ruler measures a longer distance than the spring really stretched. Check it against a set square standing on the bench.
• Reading too soon. A spring that is still bouncing gives a different reading each time. Wait for it to stop.
• Measuring the length of the spring instead of its extension. Always subtract the starting reading.

Repeating the experiment and taking a mean reduces the effect of small reading errors. It also shows up any odd result, which you should measure again.

Safety

• Wear eye protection. A wire, spring or rubber band that snaps can fly back towards your face.
• Clamp the stand to the bench, or put a heavy weight on its base. Otherwise the masses can pull the stand over.
• Keep your feet clear of the space under the masses. Put a box or a tray of sand on the floor below to catch them if they fall.

Exam-style question

A student investigates how the extension of a rubber band varies with the force applied. The rubber band hangs from a clamp stand. The student adds slotted masses to it and measures with a ruler.

(a) Describe how the student should find the extension of the rubber band for one mass, and how to make the reading accurate. [3 marks]

(b) The student hangs a total mass of 350 g on the rubber band. Calculate the force stretching the rubber band. Use g = 10 N/kg. [2 marks]

(c) State one safety precaution the student should take and give the reason for it. [2 marks]

(d) Explain why the student also takes readings while removing the masses. [2 marks]