Student Practical

Student practical

Name Class Date

Stretch tests – Target 8

Specification references

· P8.2.6 Required practical activity 6; P.5.3 Forces and elasticity

· WS 2.6, 3.1, 3.2, 3.3, 3.5, 3.7

· AT 1, 2

· MS 2a, 2b, 3b, 3c, 4a, 4c, 4d.

Background

By measuring how much a spring extends when different forces act on it, we can describe its behaviour and make predictions about how it will behave in future. Springs have many uses, for example in newtonmeters, toys, pens, mattresses, and car and bike suspension.

Learning objectives

After completing the practical you should be able to:

· make measurements of force and extension for a spring

· plot a graph of the results

· use a graph of extension against force to find the spring constant of a spring.

Safety

· Eye protection must be worn.

· The clamp stand should be securely fastened to the bench.

· Do not make the masses bounce up and down on the spring.

· Make sure masses do not fall onto the floor or onto people’s feet.

Equipment and materials

· Eye Protection

· Spring

· Set Of 50 g (0.5 N) Masses

· Mass Holder

· 1 m ruler

· Clamp stand with three clamps

· G-clamp

Method

1 Attach the spring to the clamp stand by hanging it off a clamp, and allow the spring to hang freely over the side of the bench.

2 Use the G-clamp to fasten the clamp stand to the bench.

3 Use the other two clamps to hold the ruler vertically, close to but not touching the spring. You will use this to measure the length of the spring.

4 Measure the length of the spring with no force acting on it.

5 Hang the mass holder from the spring. Check the mass of the holder, and measure the new length of the spring. Record the length of the spring and the mass suspended from it.

6 Add a 50 g (0.5 N) mass and measure the length of the spring.

7 Repeat step 6 until a total of 250 g (including the mass holder) is hanging from the spring. Each time, record the length of the spring and the total mass suspended from the spring.

8 Remove 50 g.

9 Measure the length of the spring.

10 Repeat steps 8 and 9 until there is no mass hanging from the spring.

11 Record all your results in a table.

Results

Complete the table below, or copy it into your book and complete it.

Stretched length / cm
Mass
in g / Weight
in N / When adding masses / When removing masses / Mean / Original length in cm / Average extension (average stretched length – original length) in cm

Questions

1 Plot a graph of your results. Put the independent variable on the x-axis. Draw a best fit line. (8 marks)

2 Describe the relationship between force and spring extension shown by your graph.

(2 marks)

3 Comment on:

a the repeatability of your results

(2 marks)

b the reproducibility of your results.

(2 marks)

4 The spring constant of the spring is given by the equation

spring constant = force ¸ extension. This can also be found from your graph. The gradient of the graph is extension ¸ force, so the spring constant is

1 ¸ gradient (the inverse of the gradient).

a Using pencil and a ruler, make a large triangle under your graph as you would for finding the gradient. (1 mark)

b Use this to work out the spring constant of your spring. Write your answer to two significant figures, with the correct unit.

(4 marks)

Follow-up

For each numerical answer, include a suitable number of significant figures and write a unit.

1 A student carries out an experiment similar to the one you did. His results graph is shown below.

a The student has made a mistake throughout his experiment.

i What mistake he has made?

(1 mark)

ii Explain how you can see this from the graph.

(1 mark)

b The student has stretched the string beyond its limit of proportionality.

i Mark the limit of proportionality with a P on the graph.

(1 mark)

ii Explain why you chose that position for P.

(1 mark)

c What was the unstretched length of the spring in this experiment?

(1 mark)

2 In this question you will need the formula force = spring constant ´ extension (which can also be written as F = k e, where k is the spring constant).

a A student exerts a force of 2.0 N on a spring of spring constant 10 N/m. Calculate the extension of the spring.

(3 marks)

b A spring whose original length is 7 mm (to the nearest mm) lengthens to 10 mm (to the nearest mm) when a force of 0.4 N pulls on it. Calculate the spring constant of the spring.

(4 marks)

3 a Estimate the spring constant of the spring in a school newtonmeter with a measurement range of 0–10 N. Show your reasoning.

(3 marks)

b Use your answer to a to estimate the spring constant for a newtonmeter with a range of 0–50 N.

(1 mark)

4 A particular type of spring has an extension of 6 cm when hung with a mass of 100 g.

a A mass of 100 g is hung from two of these springs in parallel (side by side). Suggest whether the extension will be more or less than for one spring, and predict what the value of the extension might be.

(2 marks)

b A mass of 100 g is hung from two of these springs in series (end to end, making one long spring). Suggest whether the extension will be more or less than for one spring, and predict what the value of the extension might be.

(2 marks)

c What property of springs allows newtonmeters to have a linear (uniform) force scale?

(1 mark)

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