Transcript Misconceptions in teaching forces

NQT Conference
The Royal School of Mines 14th 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
3. The weight of an 4. distance travelled
2. If a force acts on an
stationary there are
object is always equal = average speed x
object it must move
no forces acting on it
to its mass
time
6. If the average speed
5. An object moving
7. If an object is
8. Forces can change
is 20 m/s a car can
at a steady speed has
accelerating it must the direction of an
have gone at lots of
equal and opposite
have a force acting on object while its
different speeds or
forces acting on it
it
speed stays the same
even stopped
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
13. When an object
runs out of force it
stops moving
16. According to
15. The steady speed
14 Work = force x
Hooke’s Law
reached by a falling
distance moved in the
doubling the force
skydiver is called the
direction of the force
doubles the
terminal velocity
extension
11. All objects have
the same
acceleration due to
gravity on Earth
12. As an object falls
its speed remains the
same
1. If an object is
3. The weight of an 4. distance travelled
2. If a force acts on an
stationary there are
object is always equal = average speed x
object it must move
no forces acting on it
to its mass
time
6. If the average speed
5. An object moving
7. If an object is
8. Forces can change
is 20 m/s a car can
at a steady speed has
accelerating it must the direction of an
have gone at lots of
equal and opposite
have a force acting on object while its
different speeds or
forces acting on it
it
speed stays the same
even stopped
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
13. When an object
runs out of force it
stops moving
16. According to
15. The steady speed
14 Work = force x
Hooke’s Law
reached by a falling
distance moved in the
doubling the force
skydiver is called the
direction of the force
doubles the
terminal velocity
extension
11. All objects have
the same
acceleration due to
gravity on Earth
12. As an object falls
its speed remains the
same
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
Support force from leaf
butterfly
Gravitational force from Earth
Free body diagram
Support force from leaf
butterfly
Gravitational force from Earth
http://www.physicsclassroom.com/
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
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.
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 – 3rd 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
Using Marvin and
milo Cartoons
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
References
• http://www.nationalstemcentre.org.uk/elibrar
y/resource/7415/understandingmisconceptions
• http://listverse.com/2009/04/08/top-10ridiculously-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
Using force arrows
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.
2.
3.
4.
5.
It is stationary on Level 1
It is accelerating upwards
It is travels at a steady speed between Levels 1 and 2
It decelerates as it reaches Level 2
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.
2.
3.
4.
5.
It is stationary at the top of the ramp
It travels at constant speed down the ramp
It is at the bottom of the loop
It is at the side of the loop
It is at the top of the loop
Using force arrows
Use the arrows to illustrate the Newton’s third law pairs
when ….
1.
2.
3.
4.
A book rests on the table
You carry a shopping bag
You drop some keys
You deflate a balloon
Short demos/
experiments
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?
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