In a world where the laws of motion are being constantly demonstrated naturally, it seems rather incredible that dynamics still remains to be perceived as one of the driest areas of physics. However, the concepts of force and energy can be made very real by way of experiment, and to provide an educational experience to school children and the public requires only some surprisingly simple equipment.
Many of the concepts defined in Newton's Laws of motion were anticipated by Galileo, who had active in many areas of science including dynamics. When trying to convey the importance of Newton's three laws, we make reference to visual demonstrations for each.
The first law can be demonstrated with the use of a child sitting on a trolley. By moving the trolley, and adjusting its speed or direction sharply, the principle of the first law can be seen by the effect on the child. Perhaps a little more abstractly, a bucket containing water can be swung in a complete circle, and the water remains in the bucket. Again, the first law is responsible for this effect, while the bucket is constrained to move in a circle, the water tries to continue moving in a straight line.
The second law is rather more tricky to explain, because it contains the unavoidable comparison of inertial and gravitational mass. Indeed, even this very observation is something that would be lost on a primary school audience. And so we must be careful, to avoid terms that are dry to our audience. We can kick a ball with a given force, and show that it accelerates to some speed (ignoring the path under gravity it may take and the fact that it eventually slows down). If we take a ball twice as massive, and give it roughly the same amount of kick, then hopefully it may accelerate more sluggishly, and one may insist half as much as before. If this demonstration is visually convincing, then perhaps it is sufficient for our younger audiences.
The idea of kinetic energy can be impressed on people by nothing more complicated than the throwing of a ball. And, if one is rich enough, the potential energy contained in a cup or saucer can be released by dropping it to the ground! Again, simple demonstrations often provide the best examples of the concepts. We have a giant pendulum, which is used to illustrate the principle of conservation of energy. By releasing the bob from somebody's nose, and examining the proximity between the bob and nose on the return swing, the idea is got across. At this point, we can introduce friction as a dynamic loss, and the idea that energy can be converted from one form to another, not just kinetic to potential and vice versa, but then to heat also. The superball bounce demonstration, where two or three superballs of ascending size are released one on top of the other, is always popular with children and adults alike. With five balls, the top one should theoretically hit the moon. With two, and all conditions ideal, the top one should travel nine times the height of release. With three, you'd be surprised how many children will guess the answer eighty-one for the height jumped by the top ball!
The concept of pressure can be illustrated again with reference to simple examples. Imagine a woman with high heel shoes walking on a sandy beach. How do snow shows work? Why do cross-terrain cars have huge tyres? We demonstrate pressure very simply using plastic cups and a wooden board. Put one cup down, upside down, and try and stand on it - crunch! However, put twenty down in a square, and then stand on a board placed on them, and it will withstand the weight of even a heavy adult. We extend this idea to lying on a bed of nails - that well known pseudo-mystical object of the far-east street entertainers.
Our dynamics show contains many spinning objects, like the famous bicycle wheel demonstration of angular momentum. We are lucky enough to have the large gyroscopes that Lord Kelvin used in his lecture demonstrations, and we speed these up using an electric motor, and show the precession effect when they are set on a point. We have dumbbells which we give to someone on a swivel chair, set it rotating and ask them to bring their arms in and out, demonstrating again the idea of conservation of angular momentum. These are effects that anyone who has watched ballet or ice-skating has seen many times, but without perhaps relating it to such laws of motion. The whirlpool in a coke bottle is another simple but effective demonstration.
Separately, the props we've described don't form a spectacular experience for the audience, but together, and with just the right presentation and delivery, they have made our dynamics show one of the most popular science events we've staged. People enjoy watching physics being demonstrated using other people! And this is what our dynamics show is really all about. For more educated audiences, such as latter higher school years, we have models of the four important dynamic curves - the parabola, the catenary, the circle and the cycloid. We have a cycloid ramp, and other mechanical curiosities which demonstrate the important properties of these curves. The ideas of structural resonance are demonstrated using vibrating Cladni plates, bells and tuning forks and of course, by playing the famous Tacoma Narrows bridge collapse sequence. Together all these exhibits form a show that can last a frantic twenty minutes, or a more relaxed, thought provoking hour or so, depending on our audience. However the one real ace we have with S&R is the fact that it is largely a show that involves the audience at every junction. At no time does our science become too formal that we lose the vital element of audience participation. If that happens, then we lose their interest too. And as long as we have their interest, then we are managing to pass on at least some of the wonderment of the physics that is, after all, the heart of our show.
Credits
Created and maintained by: Ken Skeldon
Photography:
Text by: Rebecca Crawford & Ken Skeldon
