Forces and simple machines

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: What do pupils learn about forces at primary school? Misconceptions: What are some common misconceptions about forces?

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

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. Philosophiæ NaturalisPrincipia 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.

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.

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.

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.

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

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.

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.]

18. Buoyancy explained www.youtube.com/watch?v=QmOP3O1KGz4

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?