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NQT Conference The Royal School of Mines 14th July
2014Teaching forces - common
misconceptions and ideas for teaching
Liz Hutchins
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Dealing with misconceptions /alternative conceptions
1 in 7 people believe that the Periodic table is arranged alphabetically
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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.”
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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
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Identifying pupil misconceptions
• Use true /false sheet or card sort activity• Use a questionnaire/diagnostic questions
e.g. (SPT)• Through experiment
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Question
What do you think about science? For each of the statements below, show whether you
Agree DisagreeNot sure
1. To keep an object moving a force must be kept on it.
2. Objects stop moving when their force runs out.
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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?
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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
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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
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Diagnostic questions
Which misconceptions are being addressed by these questions?
ForcesAWayOfLookingAtTheWorld.pdf
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http://www.physicsclassroom.com
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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
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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 - GalileoForces do not cause motion they cause acceleration – Newton
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Using models to help reduce misconceptions
• Forces – cardboard arrowsWhy?• Making forces explicit • Can be applied at all levels (year 7 to A Level)• Allows an “at a glance view” of pupil
understanding
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How we get free body diagrams - Representing forces - A butterfly
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..and simplified
butterfly
Support force from leaf
Gravitational force from Earth
Support force from leaf
Gravitational force from Earth
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Free body diagram
butterfly
Support force from leaf
Gravitational force from EarthGravitational force from Earth
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http://www.physicsclassroom.com/
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Confusion between resultant forces acting on an object and Newton’s 3rd 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
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Confusion between resultant forces acting on an object and Newton’s 3rd 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.
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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
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Experiments and demos summary
Using arrows – free body diagramsUsing arrows – 3rd Law pairsBalanced and unbalanced forces – cutting the stringMarbles and straws to change motionStretching and squashing – foam, board and springsFalling objects – paper, paper and book (Marvin and Milo), identical ballsReducing friction - hovercraftCard, weight and cup Marvin and Milo experiments
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Gripping rice– balanced forces, pupils may think there are no forces acting on an object, when it is not movingOn 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 accelerationBook 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
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References
• http://www.nationalstemcentre.org.uk/elibrary/resource/7415/understanding-misconceptions
• http://listverse.com/2009/04/08/top-10-ridiculously-common-science-myths/
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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
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Finding Resourceswww.talkphysics.org
www.iop.orgwww.physics.org
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Use the arrows to illustrate the forces acting on a cup when …. 1. It rests on a table2. It is suspended from a rubber band3. 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
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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 upwards3. It is travels at a steady speed between Levels 1 and 24. It decelerates as it reaches Level 25. The doors open on Level 2
Using force arrows
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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 ramp2. It travels at constant speed down the ramp3. It is at the bottom of the loop4. It is at the side of the loop5. It is at the top of the loop
Using force arrows
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Use the arrows to illustrate the Newton’s third law pairs when ….
1. A book rests on the table2. You carry a shopping bag3. You drop some keys4. You deflate a balloon
Using force arrows
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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
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Ideas for short experiments and demos
What happens when you drop the spring?a) The whole spring falls at onceb) The bottom coil does not start to fall initiallyWhat happens when you drop the “groan tube”?c) You only hear the sound when the tube stops fallingd) You hear the sound all continuously as it fallsWhat happens when the spoon is balanced?e) There is the same weight on both sides at the balance
pointf) The weight on either side will be different