The effects of centripetal forces on a rotating object can be shown impressively by making a circular jelly about 3 cm deep in a crystallising dish. When it is set, empty it out carefully onto the centre of a polystyrene plate which is securely fixed to the centre of the plywood platform.
Slowly increase the rate of spin of the table. The jelly will flatten.
Further increasing the rate of spin will eventually make the jelly break up - the cohesive forces within it being less than the centripetal forces needed. It can be used to demonstrate why car tyres fly apart when they are spun too fast. I was told that if you used some of the old forms of tyre remoulds that you should not travel at more than 60 mph to reduce the risk of the tyres breaking up!
The jelly experiment also shows the shape of the liquid surface while rotating. It is useful to photograph it or take a video for later analysis.
Age range: 16-18 for real benefit
Apparatus required:
Crystallising dish Jelly Polystyrene plate Variable speed electric drill Circular plywood platform with central bolt for fitting to the drill chuck
An interesting and amusing experiment on the balancing of the human body that can be done with your older students is to get first a male student and secondly a female to stand facing a wall, three foot lengths away from it and with their hands behind their backs. Now ask them to bend forwards so that their nose just touches the wall. Female students are supposed to be able to do this without overbalancing while male students are not. Why is this?
An alternative method is to kneel down with your forearms on the floor, your elbows touching your knees and with a small object between your fingers. Now sit up, clasp your hands behind your back and see if you can tip the object over with your nose. Women can do it - men can't. Why?
Theory
Men have relatively heavy upper bodies compared with the hips and so tip forward. Women are relatively heavier around their hips. This distribution of weight acts like a counterbalance and so allows them to bend forward further so that they can touch the match box before tipping over.
Note: It may not always work - all people are different. I suggest that you try it out with two of the staff before a lesson. Better to use two adults and do not embarrass either the adults or the pupils! If it works it makes a really fun introduction to moments.
Age range: 16-18
Apparatus required:
One male and one female
A jar about half full of water has a block of wood floating in the water and is suspended from a helical spring. Initially the jar is supported. If the jar is released the water level stays at the same place in the jar and the block floats at the same level as it falls.
Theory:
The depth at which the block floats depends on its weight and the upthrust on it. The upthrust depends on the weight of water displaced and so as the acceleration of the jar and block change BOTH the weight of the block and the upthrust change in the same way - the block floats at the same depth as it falls.
Objects in accelerated frames of reference behave in the same way as they would in gravitational fields. The falling on the spring is subject to a varying acceleration just like it would be if it were taken to the Moon where the gravitational acceleration is less. This is a very useful demonstration of one of the ideas of General Relativity!
Age range: 11-18 depending on the treatment of the theory
Apparatus required: Jar Water Wooden block Helical spring
See also Waves. A further experiment about gravity involves a falling candle and is described in the section on convection (number six).
(My thanks to Martin Freer for this idea)
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 10o with your spine!) The tension in the string represents the huge tension in your back muscles as you bend over. Bending at 45o produces a tension of over double your own body weight!
Theory:
The tension in string (back muscles) T is given by the equation:
Tsin A = mg cos Θ
where A is the angle of the string with the mop (back muscles with the spine) and Θ is the angle that the mop handle (spine) makes with the horizontal
Age range: 11-18 depending on treatment and theory used
Apparatus required:
Mop, String, Newton meter, Metal rod for pivot, Retort stand and clamp, G clamp
Both of these experiments are vivid demonstrations of the pressure of the atmosphere
(a) Get a tin can and put a little water in it. Heat the can vigorously over a bunsen until the water has been boiling for some time. Although there will be some liquid left, the can will be full of steam. Remove the can from the heat and quickly insert the stopper or screw on the lid.
Safety - don't heat a closed can!
As the can cools the steam inside the can condenses and therefore the pressure drops. After a few moments the pressure difference between the atmosphere outside and the small amount of residual air inside is enough to crush the can flat! I usually stand on the top of the can first before starting the experiment to demonstrate how strong it is and then get a pupil to try and straighten out the can at the end after it has cooled.
A variation of this is to use an empty drinks can, put a little water in it and boil the water as before. Then pick it up with a pair of tongs and rapidly invert it in a bowl of water. The steam inside the can condenses, the small hole in the can prevents water from being sucked in too rapidly and the can collapses.
(b) This next demonstration of atmospheric pressure is very simple and direct and avoids heating cans of air! Completely fill a plastic squash bottle with water - bigger bottles are more impressive. Put a bung in it with a glass tube in the centre and attach a 2 m length of rubber tubing to the tube - more if the height of your lab will allow it. Get someone to hold the end of the tube closed while you climb on a bench and upend the bottle with the rubber tube dangling vertically downwards. Now open the lower end of the tube. As the water runs out the bottle will be squashed flat by the pressure of the air on the outside! The long tube gives a bigger pressure difference between the top and bottom of the water column and also prevents air leaking in.
Theory
Pressure difference between the two ends of the water column of height h = ρgh where ρ is the density of the water.
Age range: 11-13
Apparatus required:
(a) Tin can Bunsen Heat proof mat Drinks can Tongs
(b) Plastic bottle with rubber tube fitted to a bung in its neck Water Bucket
A very simple and yet impressive example of friction can be seen using two paper backed books. It is best done with ones made from rougher poor quality paper.
Hold the books with their open sides facing each other and carefully interleave the pages so that about half of one book extends into half the other as shown by the diagram.
Then get two people to hold the books by their spines and try and pull them apart. There is so much friction between the pages that it is likely that the books will be damaged before they can be pulled apart - so be careful! It makes a stunning introduction to work on friction!
The experiment also works well with two magazines
Age range: 5-14
Apparatus required: Two paper backed books