ClassroomphysicsTeaching tipsA decade of ideas from the newsletter for affiliated schools

ContentsForces and motion

Demonstrating “weightlessness” 2

Thinking on your feet: football and physics 3

The eye and optics

The world’s largest coin 4

A clear focus on optics: the pinhole camera 5

Seeing pink elephants 6

Sound waves

Knocking on Newton’s door: measure the speed 7 of sound using echoes

Communication technology

Laser DJ: a simple optical communication demo 8

Earth in space

The seasons 9

Blue-sky thinking: the greenhouse effect 10

Electrostatics

Staying in charge: tips for your electrostatic lessons 11

Radioactive decay

Sweet simulations of radioactivity 12

10th

Classro

om Physics

2007–2017

anniversary

To explore our support for teachers visit iop.org/teachers

2Classroomphysics

Forces and motion

Demonstrating “weightlessness”This is an experiment to show that in free fall it is not gravity that disappears but the contact force with a surface.

Apparatus and materials• Two 1 kg masses connected by a spring

• A board with a high-friction surface, such as a rubber mat or fine sandpaper, attached to it

• Sandbags or cushion

Technical notesThe masses should be able to sit on the high-friction surface with the spring stretched between them without moving. Once lifted off the surface, the masses should move quickly together.

SafetyWhen dropping heavy masses, ensure that they fall onto the sandbags or cushion and that everyone watching is standing well clear of the landing site.

Procedure

• Place both masses on the board, with the spring stretched between them.

• Explain that the only reason the masses do not move together is because of the friction. Demonstrate this by moving them to a lower friction surface, such as a benchtop.

• Hold the board horizontally above the sandbags and ask what will happen when the board is dropped. Drop the board to demonstrate.

• Repeat the experiment but say that this time you are going to throw the board upwards. Again, ask what will happen.

Explanation

• While the board is held at rest it exerts a normal contact force on the masses. This leads to friction (equal in magnitude to the product of the normal force and the coefficient of friction). Once the board is dropped it is apparent that the friction disappears, indicating that contact force between two objects in free fall together is zero. We normally feel our weight as a result of this contact force, so this absence is manifested as a feeling of weightlessness.

• Throwing the board upwards may cause confusion. Most of your class will expect the two masses to start moving once the board peaks. In fact they will start to move immediately because they are in free fall, regardless of which way they are moving.

This experiment was originally submitted to practicalphysics.org by Ken Zetie, head of physics at St Paul’s School, London

Originally printed in the December 2008 edition of Classroom Physics

two 1 kg masses connected by a spring

board, with a high-friction surface, such as a rubber mat or fine sandpaper attached to it

sandbags or expanded polystyrene packaging to protect the floor

3Classroomphysics

Forces and motion

Thinking on your feet: football and physicsFootballers sometimes kick across the ball, rather than straight through it, to impart spin. They do this either to control how it moves through the air or to actively make it follow a curved path. In this activity students will explore how different types of spin affect the way that the ball moves through the air.

Materials neededFor each pair of students:

• Safety glasses

• Two polystyrene cups

• Two elastic bands

• Two balloons

• Sticky tape

• Video function on students’ mobile phones (optional)

Testing the ideas experimentallyIntroduce the activity by explaining that they will be designing a demonstration to show how spin affects the way that the object moves through the air.

• Demonstrate how to build a “spinner” by taping two polystyrene cups together at the base.

• Tie the two elastic bands together in a loop and hold the loop on the spinner at the point where the two cups join, then wind it round a few times.

• To produce backspin, the other end of the elastic should be at the bottom pointing away from you. The spinner can now be “fired” like a catapult and will spin in the air as it travels.

• Topspin can be created by using the same method upside down.

• Sidespin by carrying out the demonstration sideways.

The challenge for students is to produce the most convincing demonstration of the effects of different types of spin on the motion of the object. By modifications to the cups used and their launching technique, as well as how they position the video camera for each shot, the students can capture the finer detail of how each type of spin produces a different motion. Pooling the class’s experiences should enable comparisons between the different types of cups and launching techniques, why certain cups work better than others and how this is related to the movement of balls that are spinning through the air.

For more information: about football and physics visit iop.org/football

Originally printed in the December 2013 edition of Classroom Physics

The effects of backspin, topspin and sidespin can be modelled using spinning cups launched by rubber bands.

To extend this investigation, you might want to provide different types of plastic cups to your students. At least two elastic bands will need to be looped together.

A student carries out the investigation at the Emirates Stadium in London.

4Classroomphysics

The eye and optics

The world’s largest coinThis classroom demonstration is a good way to introduce image formation by a lens when teaching optics.

Apparatus and materials

• 12 V filament bulb in ray box

• Convex lens (with a focal length of about 140 mm)

• Piece of card

• Coin

Procedure

Place the ray box at a distance of about two focal lengths away from the convex lens and project an inverted image of the lamp filament on to the card. Move the lamp gradually towards the lens, and ask your students to move the card back to project a larger image of the filament. With suitable positioning, a very large image can be projected onto a clear section of laboratory wall.

Mark the position of the lamp filament on the lab bench and remove it. Mount the coin on a block with Blu-Tack and place it at the position of the filament. Once you have the coin in position use the lamp to illuminate the coin. For optimal results place the lamp alongside the lens and twist the coin slightly to perfect the angle of reflection until a large bright image of the coin is projected by the lens onto the card or wall.

Notes

Take care with the ray box as the metal parts may get very hot.

For more experiments: helpful advice and extension activities visit practicalphysics.org/optics

Originally printed in the December 2007 edition of Classroom Physics

fixedconvexlens

direction of beaminverted imageof lamp filament

screen orclassroom walllight source,

eg ray box with 12 V bulb

fixedconvexlens

directionof beam

large, bright imageof coin

screen orclassroom wall

light source

mounted coin,angled carefully

5Classroomphysics

The eye and optics

A clear focus on optics: the pinhole cameraThe pinhole camera is a wonderful introduction to the physics of the eye and the lens camera. It can help students to understand the action of a convex lens in forming a real image and to understand why the image is inverted.

Apparatus and materials

• Carbon filament lamps 200 W (up to four groups may share each lamp)

• Mounted lamp holders (safety pattern)

For each group of students:

• Pinhole camera box (15 cm × 10 cm × 10 cm) or other arrangement

• Black paper

• Greaseproof paper

• Elastic bands or paper clips to secure paper in place on camera ends

• Lens (+7D or 15 cm focal length) – the focal length should be equal to the length of the box

• Pin for making holes

Technical notes

• The lenses suggested are designed for a camera approximately 15 cm from front to back. If other sized boxes are used, the lens provided should have a focal length equal to the length of the box.

• Each camera needs one end covered with a piece of black paper (pinholes are made in this) and the other end covered with greaseproof paper to make a screen.

• Carbon filament lamps are rated at 200 V. They will last longer if a 200 V supply is available.

Procedure

• Place the lamps and lamp holders around the laboratory so that up to eight students can work with their cameras about 1.5 metres from a lamp.

• Pull down the blinds or otherwise shade the room. Ask each student to:

– Make a small pinhole in the black paper. Remind students to point the pinhole at the lamp. Look at the screen as they move the box closer to or farther from the lamp.

– Enlarge the pinhole and repeat the observation.

– Add several more small pinholes and repeat the observation.

– Pepper the whole sheet with pinholes and repeat the observation.

– Give each student a lens and ask them to slide it in front of the pepper of pinholes while the box is pointed at the lamp. You may need to tell students to move nearer to the lamp and farther from it, and see what happens. They will soon find the position for a single, brilliant image.

– Push a pencil, then a finger, through the pepper of pinholes. At each stage, experiment with using the lens in front of the pinholes.

– Try the effect of moving the lens slightly away from the camera. Examine the effect when the box is farther away from, and when nearer to, the light source.

– Attach a new piece of black paper on the front of the camera. Make a large pinhole in this paper and then repeat. You are now using a lens camera with a small aperture and students can observe the greater range of focus.

• Finally pull up the blinds so that students can use their “lens cameras” to look at the view through the window.

• You can also discuss with students the way that the eye works and the similarity to the pinhole camera plus lens set-up.

For more information: visit practicalphysics.org/optics

Originally printed in the March 2011 edition of Classroom Physics

A typical pinhole camera set-up (not to scale).

Inserting the lens in front of the pinholes.

1.5 metres

1.5 metres

6Classroomphysics

The eye and optics

Seeing pink elephantsThe pink (magenta) elephant optical illusion is a great starter to a lesson on colour vision.

Instructions

• Use above image or go to bit.ly/SeeingPinkElephant (TalkPhysics.org free registration/login required) and click on the September 2014 issue to download a PowerPoint version for your lesson

• Ask students to stare at the cross on the elephant’s body for 30 seconds and then to look at a blank sheet of paper/white wall (if required, use a countdown for the last 10 seconds to maintain their concentration)

• Ask students to describe what they see (a “pink” elephant)

• Link these ideas to the explanation below, and primary and secondary colours

Explanation

The eye has three types of cone cells, each of which responds best to different wavelengths of light:

• L (560 nm – red)

• M (530 nm – green)

• S (430 nm – blue)

Light incident on these cone cells is converted to electrical (nerve) potentials by visual pigments at the back of the retina. Mixtures of stimulation of each of these three cell types correspond to all of the colours that we perceive.

When you stare at the green elephant for some time, the M cones in those parts of the retina that are stimulated will start to exhaust their pigment. Changing the view to a white background will now stimulate the whole retina across a broad range of wavelengths.

However, until the pigments have been fully regenerated, the retinal area that was previously viewing the elephant sends weaker signals from the M cones in response to the white when compared with the L and S cones. The brain will therefore interpret this input as an elephant shape composed of light that has been stimulating the L and S cones, so we see a magenta elephant. This after-image disappears when all of the pigments in the receptors recharge.

When explaining this to younger students it may be useful to simplify the language and say that we have three types of colour detectors (red, green and blue) and that staring at the elephant makes the “green” detectors run out (of pigment) so that when we change the view we only get red + blue = magenta rather than the full complement of red + blue + green = white.

For more information: about how we see colours visit episode 4 on the light topic at the Supporting Physics Teaching website: supportingphysicsteaching.net/Li04PN.html

See the Practical Physics website for more instructions on how to carry out a demonstration on additive colour mixing: practicalphysics.org/additive-colour-mixing

Originally printed in the September 2014 edition of Classroom Physics

yellow

green

cyanmagenta

red

blue

7Classroomphysics

Sound waves

Knocking on Newton’s door: measure the speed of sound using echoes

Apparatus and materials

• Stopwatch(es)

• Calculator

• Two wooden blocks to bang together

• Large, flat reflecting surface, such as the wall of a gymnasium or hall

Procedure

• The experimenter stands as far away as possible from a large reflecting wall and claps their hands or bangs the two blocks rapidly at a regular rate. Around 40 to 50 metres away from a wall works well.

• This rate is adjusted until each clap just coincides with the return of an echo of its predecessor. When this happens the echo is no longer heard separately from the original sound. If you are able to stand much further away in an open space you can adjust the clap/bang rate until the clap and echo are heard as equally spaced.

• Get students to use a stopwatch and count the number of claps/bangs in 10 seconds, n.

The time between claps is then = 10/n. Make a rough measurement of distance to the wall, d, in metres.

In time 10/n the sound has travelled to the wall and back; that is, a distance of 2d.

Thus the speed of sound,

Teaching notes

• Students are far more likely to grasp and to remember how to obtain the estimated speed of sound if you can arrange for them to work in small groups to undertake this experiment. This may be difficult to organise, depending on the geography of your school grounds, but well worth trying. One person is needed to make the noise, one to time 10 s, at least one to count the number of claps in 10 s and one with a calculator to do the calculation.

• Discuss why a rough measurement of the distance is adequate.

• The speed of sound in air is roughly 340 m/s at 20 °C and 330 m/s at 0 °C, but will vary depending on atmospheric pressure and temperature.

• The discussion may lead on to comparing the speed of light and the speed of sound, and why we see lightning before we hear the associated crack of thunder.

For more information: visit practicalphysics.org/measuring-speed-sound-using-echoes

Originally printed in the March 2012 edition of Classroom Physics

Newton used echoes to estimate the speed of sound in an outdoor corridor at Trinity College, Cambridge. Allegedly the sound was able to lift a door knocker at the far end of the corridor.

The north cloister of Nevile’s Court, Trinity College, where Isaac Newton stamped his foot to time the echoes and determine the speed of sound.

8Classroomphysics

Communication technology

Laser DJ: a simple optical communication demoThis simple demonstration can be built at low cost (£20–£30) and will help your students understand how information can be sent using light signals. Budding DJs can stream music from their smartphones to a music system and demonstrate their skills by scratching the laser beam. Refer to CLEAPSS leaflet PS52 for additional guidance on the use of lasers in the classroom.

Equipment needed

• Laser pointer/pen (class 2)

• 3.5 mm mono jack lead (male to male)

• Lead with a small crocodile clip at each end

• 3 AAA batteries plus battery holder

• Push locking switch (eg Maplin code: FH94C)

• Solder and wire

• 100 μF capacitor

• 10 Ω resistor

• Any audio device that outputs to a standard headphone 3.5 mm jack lead (eg MP3 player or smartphone)

• Two stands and clamps

• Light sensor (eg Rapid Electronics order code: 37-0436)

• A music system with standard 3.5 mm microphone socket or mini audio amplifier (eg RadioShack Mini Audio Amplifier)

• Hair comb (optional, if your students want to laser DJ)

Building the kit

• Figure 1: cut both the 3.5 mm mono jack lead and crocodile clip lead in half. For the jack lead also separate the two internal wires.

• Figure 2: open the laser pointer and dispose of the button cells (the laser will be powered by the 3 AAA batteries when connected to the circuit). Clip one half of the crocodile clip lead to the battery spring and the other crocodile clip to the body of the laser.

• Figure 3: solder the resistor, switch and battery pack in series with the laser.

• Figure 4: take one half of the 3.5 mm jack lead, attach a capacitor to one of the internal wires and attach it in parallel across the resistor.

• Figure 5: build a separate receiver by connecting the light sensor to the other half of the 3.5 mm jack.

Setting up the demonstration

• Mount the laser horizontal to the bench using the clamp and stand. The laser pen is likely to have an in-built switch – switch this permanently on by taping or clamping as necessary.

• Use the other clamp and stand to mount the light sensor in front of the laser. Switch on the laser circuit and align the light sensor by ensuring that the laser strikes its centre.

• Plug the 3.5 mm jack from the light sensor into the music system/speaker and plug the corresponding lead from the laser into a smartphone/MP3 player. Press play and the signal from the music player should now be audible from the stereo/speaker.

• To laser DJ, move the hair comb back and forth across the laser beam to “scratch” the music.

Additional notes

If you have trouble sourcing a mono 3.5 mm jack lead you can use a stereo version. Stereo 3.5 mm jack leads have three internal wires (two wires for the left and right channels and a central common ground wire). If using a stereo lead, wrap the ground wire around either of the other wires before connecting to the laser circuit.

For more information: on how to build the kit and how it works visit bit.ly/laserDJ

Originally printed in the September 2016 edition of Classroom Physics

Left: laser circuit diagram; top right: completed set-up; bottom right: step-by-step guide to building the kit.

switch

laser

laserbattery

input ACsignal fromaudio device

10 Ωresistor

100 μFcapacitor

Fig. 1 Fig. 2 Fig. 4

Fig. 3 Fig. 5

9Classroomphysics

Figure 1. Light hitting a surface at an oblique angle. Figure 2. Light falling on a thermochromic plastic strip on a globe.

Earth in space

The seasons

Many of your students may believe that it is warmer in summer than in winter because the Earth is closer to the Sun in summer. Although the Earth’s distance from the Sun varies by about 1% during the course of a year, this contributes in only a small way to seasonal variations. Our seasons come about because of the tilt of the Earth’s axis. It is hotter in the summer because our part of the Earth is tilted towards the Sun and it is colder in the winter because our part of the Earth is tilted away from the Sun.

This class practical will help students understand how the Sun’s radiation is spread out over a larger area in winter. The demonstration then shows how this leads to lower temperatures in winter and higher temperatures in summer, in the UK and Ireland.

Resources required for practical (per pair of students):

• A mini whiteboard (or large white card attached to a board)

• A lamp with a cardboard cylinder (or torch)

• Two different-coloured marker pens

ProcedureWorking in pairs, ask the students to:

• Place the cardboard cylinder around the end of the lamp to project a roughly circular beam of light on to the board.

• Draw a circle around the area where the light falls (the red line in figure 1)

• Tilt the card backwards slightly and repeat to draw a second line around the new illuminated area (the purple line in figure 1)

In place of a mini-whiteboard, you could choose to use a photocopied map of the UK to make it easier for students to understand how the board is a model of the surface of the Earth.

Resources required for demonstration

• A world globe

• An incandescent lamp

• Strip of self-adhesive thermochromic plastic

Procedure

• Stick the self-adhesive thermochromic plastic strip vertically on the globe next to the UK.

• Explain that thermochromic paper can show a range of colours from brown through to blue and that brown is cooler and blue is hotter.

• Switch on the incandescent lamp. The lamp will need to be placed at a distance that warms the globe enough to cause a change in temperature that makes the plastic change colour. If the lamp is too near, the plastic will get too hot, change colour and then go black again. If it is too far away it will not change colour at all.

You could also extend this demonstration by using a light sensor to show the difference in light intensity. There is also the option of using a web camera pointed at the lamp (ie the Sun) to show how low or high the Sun is at a particular time of year or at different positions on Earth.

Further information: watch the associated video demonstrations at iop.org/space or see supportingphysicsteaching.net/Es04TA.html for more information about approaches for teaching the seasons

Adapted from the December 2010 edition of Classroom Physics

10Classroomphysics

Infrared thermometers are cheap and readily available, making them ideal pieces of equipment for a starter activity on heating by radiation. They are also useful for introducing students to the physics of the greenhouse effect. The following experiment requires taking the students out into the playground for five minutes on a bright clear day.

Instructions

• Introduce/review the concept of thermal radiation. Remind students that hot objects (such as the Sun) glow visibly, while cooler objects (such as the Earth) emit mainly in the infrared part of the electromagnetic spectrum.

• Ask students to point the infrared thermometers at objects in the classroom. They could also measure the temperature of their hands or foreheads. For small objects, the infrared thermometer needs to be close to the object in order to get an accurate reading – this is best done at a few centimetres distance.

• Once the students are familiar with the controls, take them outside and ask them to point the infrared thermometers at objects on the ground (in the shade) to find the temperature at ground level.

• Next, ask them to point the thermometer at a patch of clear blue sky and record the temperature, being careful to avoid pointing the thermometer at the Sun (see figure 1).

• Return to the classroom and discuss the results.

Teaching notes

Ask the students what they think they measured when pointing the thermometer at the sky. Some will respond with “the temperature of the ozone layer”; others may give the answer “the temperature of space”. Point out that space is much colder (–270 °C) and that, although the ozone layer is a sensible answer, it is too high up to have a major impact on the temperature measured.

Discuss the composition of the atmosphere and explain that what they are actually measuring is infrared radiation emitted by, and hence the temperature of, greenhouse gases (see figure 2).

For more information: and additional weather-related experiments, visit metlink.org/experimentsdemonstrations. To purchase a thermometer, search online for “infrared thermometer gun”; they cost between £10 and £20 each

With thanks to Sylvia Knight, Royal Meteorological Society, for permission to adapt one of their activities

1. The mostly visible (short wavelength) radiation from the Sun passes through the atmosphere and warms the Earth, largely unaffected by oxygen and carbon dioxide molecules.

2. The Earth radiates infrared radiation back to space. Although the oxygen molecules do not absorb this longer wavelength radiation, carbon dioxide molecules and other greenhouse gases do.

3. The carbon dioxide molecules re-radiate at random angles. Some of the infrared radiation goes into space and some back towards the ground.

02

C02 2

3

1

Figure 1. Measuring the temperature of the sky with an infrared thermometer. Figure 2. The physics of the greenhouse effect.

Earth in space

Blue-sky thinking: the greenhouse effect

Originally printed in the December 2014 edition of Classroom Physics

11Classroomphysics

When planning your lessons it helps to be familiar with the electrostatic (triboelectric) series:

• Perspex (acrylic)

• Glass

• Nylon

• Wool

• Silk

• Cotton

• Ebonite (vulcanized rubber)

• Synthetic rubber

• Polyester

• Polystyrene

• Polyethylene

If two materials are rubbed together, the material higher in the list will gain a positive charge (lose electrons), while the lower material will gain a negative charge (gain electrons). If you attempt to make your own series, you may find that your results differ from these, but in general, items at the top of the list will lose electrons and those at the bottom of the list will gain electrons.

Investigating the polarity of charged objects

Although it is relatively easy to work out whether an object is charged (eg using it to attract pieces of paper), finding out whether the object is positively or negatively charged can be more of a challenge.

One way is to use charged polythene and acrylic rods as a “reference”. When rubbed with a cotton duster, a polythene rod acquires a negative charge; if a charged object brought near the polythene rod is repelled, then it can be inferred that the object is also negatively charged. Conversely, an acrylic rod acquires a positive charge when rubbed with a duster and it repels other objects that have a positive charge.

Alternatively, you may want to invest in a set of SEP Charge Indicators for your class. This relatively low-cost battery-operated device allows the detection of the polarity of the charges directly. When the indicator is moved towards a charged object, the green and red LEDs indicate whether the object is negatively or positively charged, respectively. Search for “SEP Charge Indicator” for suppliers and prices.

Try this

To show that light objects are attracted to a charged surface, put a small spoonful of sesame seeds or dry semolina in a saucer. Cover it with clingfilm or put it into a plastic Petri dish with a lid and wipe the surface with a dry duster. The sesame seeds or semolina will be attracted to the charged surface.

What next?

• Bring a charged rod close to the surface and see what happens.

• Touch the lid or clingfilm to earth it and see what happens now.

• If you have bought a SEP Charge Indicator, find out how the clingfilm and duster are charged.

Looking after your equipment

Be aware that if the atmosphere and the equipment are too damp, you may get no effect at all or any results that you do get may appear to be “wrong” and so could cause confusion.

To maximise your chances of success:

• Keep the dusters in a clean bag.

• Clean the polystyrene balls, acrylic, acetate, ebonite and polythene rods regularly.

• Use freshly laundered dusters whenever possible (do not use fabric conditioner when laundering as it is an anti-static agent)

• Store all equipment in a cupboard that is warm and dry.

• Put all of the equipment close to an electric heater or run a hairdryer over it for some time before the lesson to ensure that it is dry enough.

Note

If you decide to use sesame seeds for this practical, check that none of your students are allergic to them.

For more information: on the history of electrostatics visit practicalphysics.org/electrostatic-charges

Loses electronsMore positive

Gains electronsMore negative

Electrostatics

Staying in charge: tips for your electrostatics lessons

Adapted from the September 2009 edition of Classroom Physics

When a polythene rod is rubbed with a nylon cloth the polythene acquires a negative charge (as indicated by the SEP Charge Indicator’s green LED) and the nylon acquires a positive charge (indicated by the red LED)

Left: Duster and uncharged dish of sesame seeds.Right: The charged clingfilm now has sesame seeds sticking to it.

12Classroomphysics

Radioactive decay

Sweet simulations of radioactivityIn this class activity, students model the decay of unstable nuclei using sweets. The analogy illustrates the characteristic shape of decay curves and the random nature of radioactivity.

Resources required

• One to four sweets per student with a logo or mark on one side (Skittles, which have an “S” on one side, are ideal).

Optional

• Identical measuring cylinders. The capacity of each must be sufficient so that when nearly full they can hold half the total number of sweets for the class. For example, if you distribute a total of 80 Skittles to your class, seven identical 50 ml measuring cylinders work well.

Procedure

• Each student has between one and four sweets. They hold them in their cupped hands.

• On your instruction “Shake!”, the students shake their sweets for at least 5 seconds, ensuring the sweets are moving around inside their cupped hands. On the instruction “Stop!”, they stop shaking and open their hands with one hand flat and facing upwards so that they can see their sweets.

• If any sweets are logo-side up, the students take them out of their palm and dispose of them, probably by eating them.

• On your instruction “Show”, they put up the number of fingers corresponding to the number of sweets they took out of their palm.

• Record the total number of decayed sweets for the class on the board.

• The students keep the remaining sweets in their hands and repeat. If you can arrange that you take a reading once every minute, then you can record the readings against time.

• Analyse the results by plotting a graph.

Teaching notes

• The more sweets each student has, the better the analogy of radioactive decay. You could use as few as one per student to keep it simple. Any more than four is quite difficult to manage.

• You may want to appoint a counter and a scribe to count/collect the sweets and record the results.

• Take care with how you ask students to signal the numbers. They may be tempted to add their own (rude) gestures.

• Use the activity to explain why the decay curves trend downwards. Only sweets that are left can decay. As there are fewer of them each time, fewer will decay each roll.

• To help your students visualise the data you may want to instruct your class not to eat the decayed sweets, but instead dispose of them into measuring cylinders (see figure 2).

For more information: videos and animations to support teaching radioactivity visit iop.org/radioactivity. Resources for linking radioactivity to medical physics are available at iop.org/medical.

This teaching tip was adapted from Simple Model of Exponential Decay on our practicalphysics.org website. With thanks to Helen Rafferty and Rachel Hartley for their assistance.

Originally printed in the June 2016 edition of Classroom Physics

Figure 1. Sweets that have decayed (ie they landed logo-side up) should be removed after each shake.

Figure 2. To make a visual display of the class results, use a new measuring cylinder to collect decayed sweets from each shake.

Classroom Physics is published by IOP Publishing, Temple Circus, Temple Way, Bristol BS1 6HG, UK. © 2017 The Institute of Physics, 76 Portland Place, London W1B 1NT, UK. Tel 020 7470 4800.