Alternative conceptions about gravity
Educational research has shown that students develop intuitive ideas and beliefs about natural phenomena. As students learn more about their physical environment, they tend to interpret any new information from the viewpoint of these existing ideas and beliefs. Students’ existing ideas and beliefs may be significantly different from accepted scientific viewpoints. (Palmer, 2001).
Osborne (1981) writes, “We believe that children’s non-scientific ideas are not loosely held, isolated misconceptions but are part of a firm and self-consistent viewpoint. If we wish to modify children’s views to make them more scientific then it would appear to us that we need to focus children’s attention very clearly on the differences between their views and the scientific viewpoint.”
Earth-Moon system and gravity
Any two masses are attracted towards each other by gravity. This force of gravity causes the Moon to change direction to make it orbit around the Earth.
This resource introduces common student alternative conceptions about gravity. It is important that teachers are aware of these during a sequence of lessons so students have a chance to change their thinking as the sequence progresses.
Simply telling the student the correct answer will not lead to lasting change. Unless they go through the stages of identification of existing conceptions, are challenged on these through activities, discussion and reflection and have the chance to form their new conceptions, they will often revert to what they have always thought.
The Atlas of Scientific Literacy (AAAS, 2001) lists five conceptual steps students may take as their scientific understandings about gravity develop:
Things near the Earth fall to the ground unless something holds them up.
The Earth’s gravity pulls objects on or near the Earth towards it.
Everything on or near the Earth is pulled towards the Earth’s centre by gravitational force.
Every object exerts gravitational force on every other object. The force depends on how much mass the objects have and on how far apart they are. The force is hard to detect unless at least one of the objects has a lot of mass.
Gravitational force is an attraction between masses. The strength of the force is proportional to the masses and weakens rapidly with increasing distance between them.
Following are some alternative conceptions students may hold about the concept of gravity along with suggestions on how teachers can provide opportunities for students to develop their understanding of gravitational force.
Student’s view
Scientist’s view
Teaching points
Objects fall ‘down’ because it’s natural for them to do this. Students observe objects falling when dropped and think that things simply fall ‘down’. Students may use the term ‘gravity’ without recognising it as a force that pulls objects to the centre of the Earth.
Objects experience gravitational force that pulls them towards the centre of the Earth. forceObjects experience gravitational force that pulls them towards the centre of the Earth.
Use a globe to demonstrate that the Earth is a sphere and objects that appear to be falling ‘down’ are actually falling towards the centre of the Earth. Point out that even things on the ‘bottom of the world’ (southern hemisphere) also fall towards the Earth’s centre.
Gravity is not a force because it does not physically touch the objects it acts upon. forceGravity is not a force because it does not physically touch the objects it acts upon. The concept that the Earth pulls on objects may be difficult for students to grasp because they usually associate forces with physical pushes and pulls.
The Earth pulls any object towards the centre of the Earth without touching it.
Demonstrate a physical pull – one student tugs on a piece of string being held by a second student. Then demonstrate the pull of gravity – one student lets go of their end of the string. Gravity pulls this end ‘downwards’ to the centre of the Earth; a physical pull was not involved.
Students may consider gravity to be a ‘thing’ rather than a force. gravityStudents may consider gravity to be a ‘thing’ rather than a force. forceStudents may consider gravity to be a ‘thing’ rather than a force. Again, students are probably aware of and use the term ‘gravity’ in appropriate contexts without recognising its scientific meaning.
Gravity is a type of interactive force between objects. forceGravity is a type of interactive force between objects.
Use the term ‘gravity force’ instead of ‘gravity’ when explaining the pull of the Earth on an object. Use a magnet to demonstrate one force interacting on another. (Ensure students are aware that they are watching magnetic force – the magnet is an example of an interacting force.)
Gravity only acts on falling objects. Students may be unaware that gravity acts on objects continuously. They may think that gravity acts on an object while it is falling but then stops when it reaches the ground.
Gravity acts whether an object is moving or not. Gravity is the force of attraction between all masses. Objects stay where they are (hanging on the wall or sitting on the shelf) because of balanced forces
Place an object on a table. Discuss forces acting on the object. The supporting force of the table balances the pull of gravity to the centre of the Earth. When the supporting force is removed (i.e. the object is pushed off the table), gravitational force pulls the book downwards.
Gravity only exists on the Earth or its surface . Students may think that there is no gravity away from the Earth – weightlessness occurs in all of outer space. Students may also think that gravity exists due to the presence of air or the Earth’s atmosphere. If there is no air or atmosphere, there is no gravity, for example, there is no gravity on the Moon or in space because there is no air.
Gravitational force is an attraction between objects with a force proportional to their masses – the greater the mass, the greater the gravitational force forceGravitational force is an attraction between objects with a force proportional to their masses – the greater the mass, the greater the gravitational force forceGravitational force is an attraction between objects with a force proportional to their masses – the greater the mass, the greater the gravitational force massGravitational force is an attraction between objects with a force proportional to their masses – the greater the mass, the greater the gravitational force force. Other planets have mass so they have gravitational force as well. However, the farther an object travels from the centre of a planet, the weaker the gravitational force.
Show students photos or videos of astronauts walking and driving on the moon. If the Moon had no gravity, they could not have done this. The Moon has mass, therefore it has gravity. Its mass is less than the Earth so its gravitational pull is also less.
Astronauts in space may feel weightless, but this is caused by them ‘falling’ towards the Earth at the same rate as the spacecraft. The force of gravity for an astronaut on the International Space Station is only slightly less than on Earth, but there is no force from the walls or floor pushing back against the astronaut, so they don’t feel this weight.
Students may like to find out their weight on other worlds (see Useful links). Discuss the concepts of weight and mass. The mass of an object (our body) does not change whether in space or on the Moon. Weight is a measure of the pull of gravity.
Tower of Pisa
Galileo Galilei is said to have dropped balls of different masses from the Tower of Pisa to show that their speed of descent was independent of their mass. Nigel Latta points out, this is considered an apocryphal story, and was likely a ‘thought experiment’.
Exploring students’ current thinking about gravity
To gauge your students’ conceptions about gravity, consider discussing the four line drawn images and questions from the State of Victoria’s Department of Education and Early Childhood Development Science Continuum website (see Useful links). These images could form part of a pre- and post-test assessment.
Activity idea
Use this interactive or paper-based true or false graphic organiser in this activity to find out what students think about gravity and satellites.
Useful links
To gauge your students’ conceptions about gravity, consider discussing the images and questions from the State of Victoria’s Department of Education and Early Childhood Development Science Continuum website. These images could form part of a pre- and post-test assessment.
Find out how much you would weigh on other worlds .
References
American Association for the Advancement of Science. (2001). Atlas of Science Literacy Volume 1
Osborne, R., Schollum, B. and Hill, G. (1981). Learning in Science Project. Force, Friction, Gravity: Notes for teachers . Working paper no. 33. University of Waikato.
Osborne, R. & Freyberg, P. (1985). Learning in science – the implications of children’s science . Hong Kong: Heinemann.
Palmer, D. (2001). Students’ alternative conceptions and scientifically acceptable conceptions about gravity. International Journal of Science Education , 23(7) pp 691-706.
State of Victoria Department of Education and Early Childhood Development. (2009). Science Continuum: Gravity . Retrieved from www.education.vic.gov.au...teachingresources/discipline...gravity.aspx
