When is something in equilibrium? When is it not? Bob Kibble looks through forces spectacles
Everyone knows about forces. Unlike some areas of science which struggle to find a place in everyday parlance, such as galaxies and electrons, forces happen on a human scale and in everyday life. It is this familiarity which makes the scientific understanding of forces so difficult for learners.
Force is an everyday word: "I was forced to do my homework." "She used a forceful argument." Intuition adds to the confusion. Isn't it obvious that heavy objects fall faster than lighter ones and that a moving object will stop once it has run out of its force? Intuitive perhaps, but incorrect.
Primary school and daily play will have given children experiences of forces used to make things move or to stop them moving. Push or pull is a tangible experience and one with an immediate visual effect.
Much less obvious are the forces acting on the book on a teacher's desk - an example of forces acting in balance or in equilibrium. Look around, most objects you see are in equilibrium. For stationary objects, all the forces are in balance. A key conceptual challenge is that books, apples and cups of tea don't come with force arrows attached.
Our challenge as teachers is to help students take a fresh look at their world through the eyes of a scientist and learn to recognise situations which are in equilibrium and those where unbalanced forces act. I call this learning to look through forces spectacles.
Gravity can be explored by dropping objects of different mass. Consider air resistance by dropping sheets of paper crushed into a ball, or as a leaf, then investigate parachutes, sycamore seeds or balloons. Does an inflated balloon fall faster or slower than an empty balloon? Is there a relationship between size of balloon and fall time? A wooden block will slide down an inclined table top because of gravity. Yet its motion will be slower than in free fall. Frictional forces are at work.
Investigate these: which trainer is grippiest? Do heavier trolleys roll faster than lighter ones? Does water make shoes slide more easily? How can we make things slide faster? It is helpful if no one knows the answer in advance. Discussion, using the language of forces, will extend understanding: how were the stones for Stonehenge moved?
Such explorations must be accompanied by the process skills of observation, measurement and recording. You need standard equipment too. Newton meters (force meters) will allow students to take measurements and report on quantitative results. Data loggers are available to record forces. But it need not break the bank: a small teddy bear tethered to an elastic cord becomes a model bungee jumper to be investigated using a force sensor and data logger.
Bob Kibble lectures in science education at the University of EdinburghFor CD-Roms on teaching forces and key stage 3 material: www.teachingphysics.iop.org; www.physics.ucla.eduK-6connectionQuestions to stimulate discussion: www.york.ac.ukdeptseducprojsDiagQuestionsIntro_Dec02.htm
FAVOURITES FOR KEY STAGE 3
* The role of the skeleton, joints and movement Waiting for a bus while carrying a backpack full of books results in compressive forces acting on our spine. Carrying the pack by hand will result in tension forces in your arm. The rucksack might be in equilibrium under the action of balanced forces: weight acting down and the tension in the strap acting upwards, for example.
* Classifying materials, pressure With a kinetic particle model of matter, the pressure in a balloon can be related to collisions between air particles and the balloon wall. The net outward force being balanced by the inward force due to outside air and tension in the balloon.
* Changing materials Springs and elastic bands will change shape when acted upon by stretching forces.
* Electricity and magnetism Here we experience forces which act without touching. Such non-contact forces may attract or repel and are responsible for everything in our world remaining in shape, and stop our bodies falling apart.
* Force and motion Speeding up and slowing down of vehicles requires unbalanced forces. Frictional forces act against motion.
* Earth and beyond Gravitational forces act between all pairs of masses and are another example of non-contact forces. Our weight is the effect of a gravitational force between us and the mass of the Earth