NQT Conference The Royal School of Mines 14 th July 2014 Teaching forces - common misconceptions and ideas for teaching Liz Hutchins.

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NQT Conference The Royal School of Mines 14 th July 2014 Teaching forces - common misconceptions and ideas for teaching Liz Hutchins

Dealing with misconceptions /alternative conceptions 1 in 7 people believe that the Periodic table is arranged alphabetically

Some features of misconceptions : linked to everyday use of language; constructed from everyday experience and are usually adequate for everyday life; explain how the world works in simple terms; often similar to earlier scientific models; may be inconsistent with science taught in schools; can be resistant to change; may inhibit further conceptual development. “A misconception can be defined as a view that does not fully coincide with the scientific view.”

You may have heard….? ‘I am going to weigh myself’ (everyday language) ‘Forces make things move.’ (similar to earlier scientific model) ‘Astronauts float because there is no gravity in space.’ (simple explanation) Others? Barriers to learning

Identifying pupil misconceptions Use true /false sheet or card sort activity Use a questionnaire/diagnostic questions e.g. (SPT) Through experiment

Question What do you think about science? For each of the statements below, show whether you Agree Disagree Not sure 1.To keep an object moving a force must be kept on it. 2.Objects stop moving when their force runs out.

Using sort cards The cards show statements made by pupils. Decide if the statement is “on the right lines” or “on the wrong track” For those you identify as “on the wrong track” is a misconception is involved?

1. If an object is stationary there are no forces acting on it 2. If a force acts on an object it must move 3. The weight of an object is always equal to its mass 4. distance travelled = average speed x time 5. An object moving at a steady speed has equal and opposite forces acting on it 6. If the average speed is 20 m/s a car can have gone at lots of different speeds or even stopped 7. If an object is accelerating it must have a force acting on it 8. Forces can change the direction of an object while its speed stays the same 9. On a distance time graph an object travelling at a steady speed is shown as a straight horizontal line 10. An object falling at a steady speed will slow down if the resultant force on it is zero 11. All objects have the same acceleration due to gravity on Earth 12. As an object falls its speed remains the same 13. When an object runs out of force it stops moving 14 Work = force x distance moved in the direction of the force 15. The steady speed reached by a falling skydiver is called the terminal velocity 16. According to Hooke’s Law doubling the force doubles the extension

1. If an object is stationary there are no forces acting on it 2. If a force acts on an object it must move 3. The weight of an object is always equal to its mass 4. distance travelled = average speed x time 5. An object moving at a steady speed has equal and opposite forces acting on it 6. If the average speed is 20 m/s a car can have gone at lots of different speeds or even stopped 7. If an object is accelerating it must have a force acting on it 8. Forces can change the direction of an object while its speed stays the same 9. On a distance time graph an object travelling at a steady speed is shown as a straight horizontal line 10. An object falling at a steady speed will slow down if the resultant force on it is zero 11. All objects have the same acceleration due to gravity on Earth 12. As an object falls its speed remains the same 13. When an object runs out of force it stops moving 14 Work = force x distance moved in the direction of the force 15. The steady speed reached by a falling skydiver is called the terminal velocity 16. According to Hooke’s Law doubling the force doubles the extension

Diagnostic questions Which misconceptions are being addressed by these questions? ForcesAWayOfLookingAtTheWorld.pdf

http://www.physicsclassroom.com

Some very common misconceptions Only animate objects can exert a force (a table can’t push on a book) Mass and weight are the same thing Force is a property of an object If an object is moving there must be a force acting on it If an object is stationary there are no forces acting on it Heavy things fall faster than light things

Changing ideas Early idea “all objects have a natural tendency to stop moving” If friction were eliminated a ball on a curved track would continue to rise to the same height - Galileo Forces do not cause motion they cause acceleration – Newton

Using models to help reduce misconceptions Forces – cardboard arrows Why? Making forces explicit Can be applied at all levels (year 7 to A Level) Allows an “at a glance view” of pupil understanding

How we get free body diagrams - Representing forces - A butterfly

..and simplified butterfly Support force from leaf Gravitational force from Earth Support force from leaf Gravitational force from Earth

Free body diagram butterfly Support force from leaf Gravitational force from Earth

http://www.physicsclassroom.com/

Confusion between resultant forces acting on an object and Newton’s 3 rd law pairs Pairs of forces act between different objects They are the same type of force They act for the same time. They act in opposite directions They are the same size (magnitude) E.g The gravitational pull of the earth on you is the same size as the gravitational pull of you on the earth

Confusion between resultant forces acting on an object and Newton’s 3 rd law pairs Only use the forces that are acting on one object when you work out the resultant force. Here the resultant force is zero because the weight of the girl is equal to the support force from the springboard. The forces are balanced.

Linking misconceptions to possible solutions Mass/weight – experiments with Kg and Newton scales, reference to language Force needed to continue motion- low friction experiments, hovercraft, air track Effects of air resistance – experiment with dropping objects Surfaces cannot exert a force – microscopic view, experiments with compressing foam and masses Forces can change direction – experiment with marbles and straws Forces cause acceleration – experiment with trolley and string, tug of war

Experiments and demos summary Using arrows – free body diagrams Using arrows – 3 rd Law pairs Balanced and unbalanced forces – cutting the string Marbles and straws to change motion Stretching and squashing – foam, board and springs Falling objects – paper, paper and book (Marvin and Milo), identical balls Reducing friction - hovercraft Card, weight and cup Marvin and Milo experiments

Gripping rice– balanced forces, pupils may think there are no forces acting on an object, when it is not moving On a roll – forces can act at a distance, pupils may think that objects have to be touching to exert a force. Dare devil egg (object) – inertia, pupils may think the card and the cylinder will have the same acceleration Book launch– the effect of air resistance, pupils may think the paper falls more slowly in air because it is lighter than the book. Slinky Drop– forces on a falling object, pupils may think the coils at the top and bottom of the slinky have the same forces acting on them Using Marvin and milo Cartoons

References http://www.nationalstemcentre.org.uk/elibrar y/resource/7415/understanding- misconceptions http://www.nationalstemcentre.org.uk/elibrar y/resource/7415/understanding- misconceptions http://listverse.com/2009/04/08/top-10- ridiculously-common-science-myths/ http://listverse.com/2009/04/08/top-10- ridiculously-common-science-myths/

Teaching and learning challenges from SPT (supporting physics teaching) PN physics narrative TL teaching and learning challenges TA teaching approaches http://supportingphysicsteaching.net/FoHome.html http://supportingphysicsteaching.net/FmHome.html

Finding Resources www.talkphysics.org www.iop.org www.physics.org www.talkphysics.org www.iop.org www.physics.org

Use the arrows to illustrate the forces acting on a cup when …. 1.It rests on a table 2.It is suspended from a rubber band 3.It is suspended from a rubber band and pulled down (stationary) 4.It is lifted up (accelerating) 5.Is suspended from the centre of a string held at two different angles Using force arrows

Use the arrows to illustrate the forces acting on a person in a lift when …. 1.It is stationary on Level 1 2.It is accelerating upwards 3.It is travels at a steady speed between Levels 1 and 2 4.It decelerates as it reaches Level 2 5.The doors open on Level 2 Using force arrows

Use the arrows to illustrate the forces acting on a marble travelling at constant speed on a vertical circular track when …. 1.It is stationary at the top of the ramp 2.It travels at constant speed down the ramp 3.It is at the bottom of the loop 4.It is at the side of the loop 5.It is at the top of the loop Using force arrows

Use the arrows to illustrate the Newton’s third law pairs when …. 1.A book rests on the table 2.You carry a shopping bag 3.You drop some keys 4.You deflate a balloon Using force arrows

1.Drop two balls of identical size but different masses from the same height (record the sound or film it) 2.Change the speed or direction of marbles by blowing through straws. 3.Two people pull with the same force on a toy car, one person lets go, what happens? Short demos/ experiments

Ideas for short experiments and demos What happens when you drop the spring? a)The whole spring falls at once b)The bottom coil does not start to fall initially What happens when you drop the “groan tube”? a)You only hear the sound when the tube stops falling b)You hear the sound all continuously as it falls What happens when the spoon is balanced? a)There is the same weight on both sides at the balance point b)The weight on either side will be different

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