Understanding

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Understanding Force
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Download 1 Forces lead lecture Authored by
Keith Ross, University of Gloucestershire
accessed from http//www.ase.org.uk/sci-tutors/kn
owledge/science_4/forces_motion.php date created
February 2005
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Forces

• The word force is used widely in everyday
  English
• police force,
• 'Dad forced me to do it',
• 'May the force be with you',
• 'Dont force it it might break',
• G-force.

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Aristotle and Newton

• 1600s Galileo and Newton gave us a new picture,
  revolutionising the older ideas derived from the
  Greek philosopher Aristotle.
• What is interesting is that this same revolution
  is needed in each one of us.
• The ideas of Aristotle are descriptive common
  sense, and we need to undergo a paradigm shift,
  just as Newton did, to come to our present
  scientific understanding.

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1) What can a force do?
makes something start moving makes something
stop moving makes something that's already moving
go faster makes something that's already moving
go slower causes a turning movement causes
something to change shape

• A push
• A pull
• A force at a distance

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You fire a pebble using a catapult and it hits a
tin can.

• Pull the elastic causes elastic to change
  shape.
• Let it go applying a push to pebble makes
  pebble start moving.
• Pebble make a curved flight path causes a
  change in direction
• Pebble hits can can pushes pebble makes it
  stop moving.
• Pebble pushes can makes it change shape.

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1. You take your loaded supermarket trolley round a
   corner and stop at the check-out.
2. You change direction whilst running on a football
   pitch by digging your boot studs into the ground.
3. You throw a ball in the air and let it fall to
   the ground where it bounces.
4. A Catherine wheel firework operates.
5. You kneed dough, or model plasticine.
6. A golf ball makes a curved flight through the air.

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2) Force-at-a-distance

• Is the wind (blow football, wind on leaves) a
  force-at-a-distance?
• Is there really any difference between 'contact'
  forces (eg pushes and pulls) and forces at a
  distance (eg gravity and magnetism)?

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3) The natural state/direction of motion (if no
further human interference takes place)

• the air
• flames from a fire
• a stone that's thrown in the air
• water
• a cart when the horse stops pulling it
• the sun moon

Air
Fire
Earth
Water
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4) A car travels at a constant 60 mph, along a
flat road.

• There is a forward force from the engine, and a
  backward force from wind resistance, friction
  etc.
• Is the forward force greater than, equal to or
  less than the combined backward forces?

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A ball has been thrown up mark any forces
(neglect air resistance).
Science Issues - Atmosphere
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6a) When you release objects from a high building
on Earth, do

•  do the objects drop at a constant speed, with
  heavy objects (like a stone or person) dropping
  faster than lighter objects (like a spider or
  feather)?
• Or do the objects gradually pick up speed and
  fall faster and faster, but with everything,
  light or heavy, falling together?

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6b) What happens if you are on the moon and
release the same objects?

• do the objects drop at a constant speed, with
  heavy objects (like a stone or person) dropping
  faster than lighter objects (like a spider or
  feather)?
• Or do the objects gradually pick up speed and
  fall faster and faster, but with everything,
  light or heavy, falling together?

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Terminal velocity
(not to scale)
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• Childrens notion of up and down
• From Childrens Ideas in Science Ch 9

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The children are standing on the Earth. They are
all holding stones. They let go of them. Draw
lines to show where they all go.
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• The Earth is round
• From Childrens Ideas in Science p182

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Concept cartoons

• http//www.conceptcartoons.com/index_flash.html

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Foundation/Key stage one

• Children may say things move because of the
  wheels, engine or legs, eg 'Its wheels make it
  roll down the slope'.
• They may say that things fall because you dropped
  them.
• They may say moving objects get tired, or run out
  of petrol, to explain why they stop.
• They may say things float because the water is
  shallow, or the object is light (but this might
  be correct, if light means low density).
• They may say that the faster something moves the
  more force it has (here the word force is more
  like the scientific word momentum).

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Key stage 2

• gravity as a force at a distance.
• the idea of friction to explain why toy cars slow
  down
• The material is important in deciding if
  something will float things that are light for
  their size will float
• objects at rest are subjected to balanced forces
• unbalanced forces will lead to objects slowing
  down, speeding up or changing direction.
• An object moving at constant speed in a constant
  direction also has balanced forces (Key Stage 3).
  This problem avoided at KS2 where it says that
  forces acting on an object can balance, and that
  when this happens, an object at rest stays
  still'.