Forces SASP

Weight Time to get your forces glasses out 2

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. Time to get your forces glasses out 3

If there is motion there is a force acting If there is no motion, then there is no force acting There cannot be a force without motion When an object is moving, there is a force in the direction of its motion A moving object stops when its force is used up A moving object has force within it which keeps it going Motion is proportional to the force acting

A constant speed results from a constant force Force is a property of a single object rather than a feature of interaction between two objects The ‘passive’ partner in an interaction doesn’t exert a force The ‘push back’ is greater when an object that is pushed doesn’t move than it if the object does move An object which is being pushed where there is little friction does not exert any, or much, ‘push back’

The sizes of the forces in a pair depend on the masses of the objects, or their motion Friction only occurs between solids Friction depends upon movement Gravity only affects heavy things It is possible to have weight without gravity Heavier objects fall faster Where there is no air there is no gravity Gravity increases with height above the surface of the earth

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’ 7

Law I Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon. Does ‘mtion’ mean ‘velocity’ or ‘momentum’? See Law II... 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’. 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.... 10

Two categories of forces Contact forces Pushes and pulls Springs Weights Friction Drag Upthrust Forces acting at a distance Gravity Electrostatics Magnets Get teachers in pairs to answer these questions.

The forces of nature gravitation electricity magnetism electromagnetism and inside the nucleus… strong force weak force

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

Which Pair? 14

Which Pair? 15

Which Pair? 16

Which Pair? 17

Be careful with your pairs The box of misconception and confusion 18

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.

What happens to the water level when the ice melts?

Air bubble, water bubble, iron nail What happens to the water level when the ice melts? Air bubble, water bubble, iron nail 21

Can you float a battleship in a bath? 22

Which weighs more? A bath full to the brim with water A bathtub full to the brim with water but with a battleship floating in it Both weigh the same 23