The Association for Science Education

K4.2 Forces and motion

Abstract

The concept of 'force' and its applications in both science and everyday life are problematic for most pupils and many adults. This article explores the idea in various contexts and in relation to motion, momentum, energy, gravity. This is supported by the provision of a sample 'Forces' presentation and a number of other practical suggestions that could be adapted for use both with trainee teachers and for them to use with students in school.

This is one of 17 articles whose main aim is to support the processes of teaching/learning between the science education tutor and the trainee science teachers with a focus on “teachers’ knowledge and understanding”. During a primary or secondary BEd, PGCE or GTP we hope that those learning to become science teachers will be able to challenge their own understanding of science and scientific concepts. Unit K0 specifically explores general issues relating to all the knowledge units - to the learning of science.

Standards: This unit specifically addresses Q14 but, appropriately used can contribute to and provide evidence of competence for many others of the standards especially Q4,6,7,8,18, 22 and 25.

Keywords: Force,motion, momentum, energy, gravity, trainee teachers.

Contents

1.0 Introduction
2.0 The barriers to understanding forces
2.1  Force and energy
2.2  Force and movement
2.3  Force and momentum
2.4  Force and gravity
2.5  Floating sinking and convection
3.0 Giving Practical experiences
3.1  Primary
3.2  Secondary

1.0 Introduction

The word ‘Force’, in common everyday, use has a much broader meaning than the way we use it in physics. This point is explored in download 1 ‘Lead lecture’ which is a PowerPoint used to help Primary ITE students reflect on their own understanding of ‘force’ and introduce them to some of the conceptual barriers which students encounter as they begin to form a scientific (Newtonian) understanding of ‘Force’

Download K4.2_1.0a 'Lead Lecture - Force'

Driver R, Guesne E & Tiberghien A (Eds) 1985 Children's Ideas in Science Buckingham: Open UP has two chapters which explore problems children have in coming to terms with forces and gravity: Gunstone and Watts “Force and Motion” and Nussbaum “The Earth as a cosmic body” are also useful sources.

This brief article from Physics Education followed an ASE Annual Meeting talk, suggesting that y12 students should try out the elicitation questions about force and motion on their family and younger pupils in order to clarify their own misconceptions - a message we must pass on to our own ITE students.

But more influential than just sharing research findings with year 12 students might be to get students to actually carry out their own mini research project using similar simple free body diagrams. They might gather data from friends and family but also from younger pupils in lower years and so start to recognise and reflect on the origin of their own misunderstandings.

Kibble, R. and Parker, B. (1997) 'Forces and Motion through the Key Stages', Phys. Educ. 32, accessed from http://www.iop.org/EJ/abstract/0031-9120/32/2/002 in Feb 2005. This is also available as download K4.2_1.0b 'Forces and Motion through the Key Stages'

2.0 The barriers to understanding forces

Type "misconceptions about forces" into a search engine and you will come up with close on half a million hits. Of course most of these are using the word ‘force’ as an everyday way. Some of these hits, however, will link to pages where the ideas are laid out in a teaching structure, and student teachers who themselves are unclear about force and motion should work through some of these. A good example is at http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l3e.html

'Force' was one of the 5 ‘key ideas’ of the original KS3 strategy for science.  The rest of this section is a brief summary of the field. Many of the issues below are addressed in the PowerPoint in download 1.1. You will also find the discussions in unit Professional Issues/Teaching: teaching/Misconceptions, esp. Download p4.1_6.0b

2.1 Force and energy

When you give a toy car a ‘push’ you are giving it kinetic energy. Many children and adults think of this as giving the car a 'force' which travels with the car and 'keeps it moving', so it is not surprising that people say things have a 'force travelling with them', and that the object ‘runs out of force’. If ‘force’ means ‘energy’ or 'momentum' they are right - but rather than ‘runs out of energy’ we’d prefer to say ‘the kinetic energy is dissipated, through friction etc. as waste heat.’

2.2 Force and movement

Children think of forces in terms of movement, not staying still. Children are likely to believe that if something is not moving there are no forces acting on it. Think of a person trying to push a car with the handbrake on. The car does not move so to children no force is evident. Yet scientifically we know there are several (balanced) forces at work.

2.3 Force and momentum

The legacy from Galileo and Newton is that forces change motion. Apply a force and the object will change speed or direction. But apply no force and the object stays moving at constant speed (which might be zero) and direction. No matter how many times we refer to Newton’s Second Law, pupils will insist that moving objects have a force driving them and they stop when this ‘runs out’. They cannot see that the object slows due to the force of friction and that without this friction the object would continue forever. The problem is that there are no common examples relevant to them where there is no friction. When we push a trolley we need a driving force to keep going, only because of friction. So the word force becomes used by most of us to mean something like momentum. Momentum is a measure of how difficult an object is to change direction or speed. The higher its speed and the greater its mass the greater its momentum. Apply a force and the momentum will change.

Cars, football and skidding Ask a pupil to run and stop or run and change direction. Talk about the need to get a grip on the floor or grass in order to stop or change direction. Talk about icy roads, wet floors and football boot studs. If a footballer, or anyone else, needs to change their motion they need to get a good grip. It is only when you attempt to change your motion and can’t get that grip, that skidding happens. People often associate skidding, or slipping, with turning corners, and it is true that it is during cornering (or trying to start or stop) that cars and footballers skid. But a skid happens when there is reduced frictional force from the ground, so your attempt to slow down or turn a corner fails, and you carry on in the same direction that you were going. With no frictional force (or grip) there can be no change in motion.

Force Arrows. Children find the force arrows in diagrams difficult and think they do not make sense. This is because scientists use force arrows from the centre of gravity not the top or bottom of an object. So there is no distinction between pushes and pulls.

2.4 Force and gravity

The PowerPoint in download 1.1 discusses the confusions between free fall and terminal velocity in slides 10-13. ‘Weightlessness’ is another confusing idea. Most media reports talk of ‘zero gravity’ to describe the effect of weightlessness in orbit. But gravity is certainly acting - it is there to keep all the objects, including the space vehicle, in orbit round the Earth. We only feel gravity when we oppose it - such as standing in the ground. If we allow the gravitational force between us and the earth to accelerate us, we will be in free fall, we will feel weightless, and we call it, ironically ‘zero gravity’.
At KS5 we will need to look at Einstein’s theory of gravity but Newton works well enough to KS4.

2.5 Floating sinking and convection

Children think that heavy objects sink and light ones float. There is a grain of truth here, because, in everyday language, ‘heavy’ can mean ‘dense’, and light can mean low density - as in “Polystyrene is light, stone is heavy” or 'it is as heavy as lead'.
Air is made of atoms, which have mass, so our atmosphere is ‘heavy’ and is attracted to the Earth by gravity. Why is it, then, that hot air and helium move away from the centre of the Earth - i.e. why do they rise? Accounts of the weather frequently describe convection currents as being caused by ‘hot air rising allowing cold air to rush in’. What they should say is that the cold, dense air moves in, buoying (pushing) the less dense hot air away from the earth against gravity. Gases are real, massive forms of matter and we need to appreciate that if they rise, something denser must be pushing them upwards, just like water buoys up ice. Often no connection is made between the idea of floating in water and floating in air. Children need to place material half way down in water to test them - if they move upwards they ‘float’ and if they move downwards they ‘sink’. (When a piece of wood floats on the surface of water we understand that the water has 'pushed' it up until the upthrust of the water displaced is exactly equal to the weight of the piece of wood. However, we rarely consider that the wood has also 'sunk' in the air. These ideas are explored further in the download below.)

Download: K4.2_2.5a_Some notes for teachers on Forces and Floating and Sinking.

3.0 Giving Practical experiences

3.1 Primary

Most work on forces should be done by the children moving their bodies, or using their muscles - there are wonderful opportunities to link the effects of forces on movement when children are playing games in the hall or outside. We need to provide ITE workshop sessions where student teachers can try out these activities.
Water play can be just that - but if pupils are to begin to gain some understanding of floating and sinking their teachers need to see how this water play in the classroom can be linked to ideas about why things happen as they do. Controlling the variables is the key: release objects halfway down under water and change only one variable at a time:

  • same shape (ball, cube) and size, but made of different material.
  • same material and shape but different size (and mass),
  • and finally using film canisters which have a fixed volume and shape but can be made more or less massive by adding air, sand and water.

Ice balloons make wonderful ‘playthings’ too. These are made by filling small balloons with cold water and leaving them in a freezer until frozen solid. The balloon 'skin' can then be peeled off and the 'ice-balloon' floated in a tank of cold (or tepid) water. (Beware, the ice may be very cold and could cause skin damage - wear gloves when handling them.)

3.2 Secondary

These are some of the experiences trainee teachers need to have in their ITE programme. Many of these are best carried out in a school setting, often during the ‘practical try-out time’ when trainees can try things out ahead of the lesson.

  • An air track or puck table is essential for showing frictionless motion, backed up by computer simulation and video clips of space flight.
  • Evacuating a tube containing a feather to show free fall when no air is present.

There are plenty of more complex apparatus for supporting student’s work on forces, such as ticker timers and dynamics trolleys, but until the pupils are able to think in Newtonian ways, much of this later teaching will fail.

This Section prepared by:
Keith Ross, University of Gloucestershire

Published: 23 Jun 2006, Last Updated: 19 Mar 2008