Diffraction through tights

This is a very good example of diffraction from an irregular obstacle giving circular diffraction rings. Take a section of the tights, stretch it between your hands and then view a small torch bulb through them. Lovely coloured diffraction rings can be observed due to the diffraction from the irregular weave of the tights of the light from a white light torch bulb. Moving the tights towards and away from your eyes helps to make the coloured rings more easily visible.

Compare the effect with the regular diffraction pattern that you get from a clean finely woven handkerchief. You can produce your own obstacles for this experiment by photographing a set of irregular dots (drawing shading film is ideal) and a set of regular dots. The negatives are then used with the light. The initial size of the pattern can be altered using a computer or a photocopier to scale the image.

Age range: 14-18 depending on treatment

Apparatus required:
Small torch bulb Tights Handkerchief

Silly putty

This is wonderful stuff and is available in many toyshops but if you see some - buy it! Stocks may not last. There are two different types:-
(a) this type glows in the dark or
(b) this type changes colour when you heat it.
Both types have the same elastic properties. If a ball of it is held by one side it will creep (slowly sag), if dropped it will bounce, and if hit with a hammer it will shatter. Another example of the application of a large impulse is if you hold a piece in your hands and then pull it sharply. It will snap – reminiscent of a severe muscle pull!

Age range: 11-18

Apparatus required:
Silly putty Hammer Ultra violet lamp Safety spectacles Ultra violet goggles

Eddy currents and the linear air track

An interesting variation on the theory of eddy currents can be carried out using the linear air track. Mount a large U shaped magnet over the track between two light gates so that a rider can pass beneath it. Mount one of the aluminium foil absorbers from the radioactivity kit on the rider so that the foil can pass between the poles of the magnet. Now accelerate the rider along the track with a constant force (by using a weight over a pulley). As the foil passes between the poles of the magnet eddy currents will be induced in it and electromagnetic braking will result. Investigate the size of the eddy currents produced for different thicknesses of foil.

Theory:
Since the eddy currents act to oppose the motion they reduce the acceleration of the rider.
The effect of different resistances, and therefore different eddy currents can be found changing the foil absorbers and measuring the resulting change in the velocity of the rider using the light gates.

Age range: 16-18

Apparatus required:
Linear air track Absorbers set Pulley, weight and thread Blue tac Light gate Large U shaped magnet

Barbie doll on a Van de Graaff

If you are worried about the effects of static charge on your students and do not want to make their hair stand on end using a Van de Graaff then try this experiment. Stand a Barbie doll (or any other doll with long hair) on top of the Van de Graaff dome (you may need to fix it down). Turn on the machine and watch the doll's hair rising!

Age range: 11-14

Apparatus required: Van de Graaff generator Barbie doll (or alternative)

Smiley pop ups and projectile motion

In his book 'Experiments in Physics' Colin Siddons suggested these small 'pop-up' toys to study projectile motion. This is a really good idea and can form the basis of an investigation at GCSE or A level. The toys, called Smiley Pop Ups, are very cheap (about 35p in 2005) and introduce a little bit of fun into the experiment.

You squash the toy onto the bench and then the rubber sucker slowly comes off and the toy launches itself into the air. Since the same spring is used each time the launching force should be the same. This means that both vertical motion and motion at an angle to the vertical can be investigated. For the angles motion I have used tilted runways or even tilted the lab tables where this has been possible.
Fix a pin through a piece of sellotape stuck to the ramp on put a piece of rough paper on the slope will stop the toys slipping down the slope.

Age range: 14- 19

Apparatus required:
Smiley pop up (or similar), ruler, ramp, rough paper or pin and sellotape

Mop and back muscles

The following demonstration is a simulation of the enormous tension produced in the muscles of your back when you lean over. Tie a piece of string to the handle of a mop about a quarter of the way from the mop head. The head of the mop represents your head and the handle of the mop represents your spine. Drill a hole through the end of the handle furthest from the head and pivot it here, the head of the mop being at the top.

Now try and support the mop as it tilts by holding the string at a small angle to the handle of the mop (I am told that they make an angle of only 10^o with your spine!) The tension in the string represents the huge tension in your back muscles as you bend over. Bending at 45^o produces a tension of over double your own body weight!

Holding something in your hands while you bend over into a car boot will increase the tension even further.

Theory:
The tension in string (back muscles) T is given by the equation:
Tsin A = mg cos q
where A is the angle of the string with the mop (back muscles with the spine) and q is the angle that the mop handle (spine) makes with the horizontal

Age range: 16- 18

Apparatus required:
Mop String Newton meter Metal rod for pivot Retort stand and clamp G clamp

Resonating plank

A man walks across a field carrying a long plank on his shoulder. At each step the plank flexes a little (a) and the ends move up and down. He then starts to trot and as a result bounces up and down (b). At one particular speed resonance will occur between the motion of the man and the plank and the ends of the plank then oscillate with large amplitude.



Age range: 14-19

Apparatus: Person Plank