1. Forces and simple machines

2. Learning outcomes
identify forces acting on an object in static equilibrium (Newton 1)
identify the two forces acting in a variety of interaction pairs (Newton 3)
distinguish between weight and mass and recall that W = mg
explain buoyancy in terms of Archimedes’ principle
explain levers and gears in terms of forces, distances and work done
calculate efficiency in these energy transfers and recognise dissipation
introduce an abstract concept by giving suitable examples
establish concepts qualitatively (using proportional reasoning) before introducing quantitative relationships (equations)
recognise that forces are vector quantities, with magnitude & direction
correctly use units kilogram and Newton
convert between units of g/cm3 and kg/m3

3. Starting points Prior learning: Misconceptions:
What do pupils learn about forces at primary school?
Misconceptions:
What are some common misconceptions about forces?

4. The idea of forces
Learning at Key Stages 1 and 2 underpins learning at Key Stage 3 4E
Friction 2E
Forces and
movement 5E
Earth, Sun and
Moon 1E
Pushes and pulls 3E
Magnets and
springs 6E
Forces in
action

5. Teaching challenges
You cannot see forces; they are an abstract construction, especially forces that act at-a-distance.
The laws of motion are mostly counter-intuitive. Newton himself struggled for many years to produce the consistent account given in Principia.
Newton’s 3rd law: students have difficulty identifying force pairs. This is not helped by popular shorthand phrases for Newton 3 (e.g. ‘every action has an equal and opposite reaction’) which do not make clear what the forces are acting on.

6. Axiomata sive Leges Motus
Philosophiæ Naturalis Principia Mathematica
Axiomata sive Leges Motus
Lex I
Corpus omne perseverare in statu suo quiscendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.
Lex II
Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur.
Lex III
Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi.
A quick glimpse at a slide to show what Isaac actually wrote.... ‘Axioms, or laws of motion’ 6

7. Law I
Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed upon it.
Does ‘mtion’ mean ‘velocity’ or ‘momentum’? See Law II... 7

8. Law II
The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.
Illustrating that acceleration is not actually anywhere present in the original. ‘Motion’ is still undefined, but clearly ‘momentum’ is a better match than ‘mass × acceleration’. 8

9. Law III
To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
Underlying the idea of interaction pairs: note how most parroting of this statement stops at the semi-colon, and.... 9

10. Two categories of forces
Forces acting at a distance
Gravity
Electrostatic
Magnetic
Contact forces
Pushes and pulls
Springs
Weights
Friction
Drag
Upthrust

11. The four forces of nature
gravitation
electromagnetism (electricity, magnetism)
and inside the nucleus…
strong force
weak force

12. Objects in static equilibrium
In pairs:
1. For several objects in the circus:
identify forces acting (recall W = mg)
sketch a diagram using arrows to represent these forces
2. Look carefully through SPT Forces episode 1
(physics narrative, teaching and learning issues, teaching approaches)
3. Discuss and complete the diagnostic questions.

13. Forces come in pairs Law 3: Same kind of force
Same magnitude, but opposite direction
Forces acting on two different objects
Participants in pairs work on diagnostic questions, and try 3 PP experiments:
Action and reaction: trolleys Action and reaction with a metre rule Skateboard forces

14. Law III, Newton’s examples
Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other.
...Newton’s examples add immeasurably to the axiomatic statement preceding. 14

15. Why don’t you fall through the floor?
Forces can change the shape of things.

16. Density definition Units: g/cm3, kg/m3 In pairs:
Choose one experiment or more to carry out.
Discuss the set of questions.
Try these conversions: 10 kg/m3 = g/cm3
1000 g/cm3 = kg/m3

17. Floating and sinking
Archimedes’ principle: the upthrust acting on a body immersed in a fluid is equal to the weight of the fluid displaced.
[Note that, conversely, the immersed body exerts a downward force on the liquid.]
Demos: push inflated balloon into a bucket of water.

18. Buoyancy explained www.youtube.com/watch?v=QmOP3O1KGz4
Buoyancy arises from the fact that fluid pressure increases with depth and from the fact that the increased pressure is exerted in all directions so that there is an unbalanced upward force on the bottom of a submerged object.
Since the "water ball" at left is exactly supported by the difference in pressure and the solid object at right experiences exactly the same pressure environment, it follows that the buoyant force on the solid object is equal to the weight of the water displaced.

19. Simple gadgets and tools
Work done = force x distance moved in direction of the force
Try measuring input & output forces and distances, then work out their efficiency.
You might also consider forces involved in human muscular-skeletal systems.

20. Endpoints In small groups: Review the main ideas.
Sort out anything that is not clear.
Make connections: How might these activities affect your classroom teaching?