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# 15.30 o12.1 c low and f bryden

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Physikos 4: C Low and F Bryden

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### 15.30 o12.1 c low and f bryden

1. 1. Definition of Friction“Friction is a force that opposes motion” Hardly!!!
2. 2. Definition of Friction“Friction is a force that opposes two surfaces sliding past each other” FRICTION
3. 3. Easy Force Diagrams SUPPORTPUSH FRICTIONGRAVITY
4. 4. • The problem arises for people or vehicles propelling themselves.
5. 5. NZQA examFour forces act on Toni and the bike while she rides:weight, support, friction and push.On the diagram below, draw in arrows to show thedirections of ALL FOUR forces acting on Toni and thebike. Label the forces.
6. 6. SUPPORTIn assessment schedule:FRICTION PUSH WEIGHT
7. 7. Alan Van Heuvelens ALPS kits(a) draw the situation including all interacting objects(b) draw a circle around the specific object you wish to analyze(c) include on your free body diagram only those forces exerted on a thing inside the circle by a thing outside the circle. Common notation FA→B If you cant identify a thing outside the circle then youre drawing a force vector that doesnt exist. E.g. Driving force/engine force/push force….
8. 8. SUPPORT Real forces:AIR STATICRESISTANCE FRICTIONANDROLLINGRESISTANCE WEIGHT
9. 9. KINETIC FRICTION (on box due to ground) sliding Kinetic frictionSTATIC FRICTION (on box due to ground) Direction object WOULD slide if there was no grip Static frictionFRICTION OPPOSES SLIDING
10. 10. Direction bottom ofwheel WOULD slide if STATIC FRICTIONthere was no grip
11. 11. SUPPORT or REACTIONRESISTANCE – (Rollingresistance and air resistance) STATIC FRICTION WEIGHT or GRAVITY
12. 12. Accelerating Constant speed braking
13. 13. SUPPORT STATIC FRICTIONAIR RESISTANCE WEIGHT /GRAVITY
14. 14. Why then is friction drawn backwards?• People are actually trying to draw an energy flow diagram• The phrase “friction opposes motion”• The lumping together of several forces
15. 15. Problems with FRICTION FORWARD• Not with the students!• With the teachers who have taught friction backwards for years...A minimal change suggested by Francis Bryden: SUPPORT or REACTION RESISTANCE THRUST WEIGHT or GRAVITY
16. 16. “Weight”• Definition that is pretty universal: “Weight is the gravitational force on an object” (+/- effect of rotation of earth)“the astronautsexperience apparentweightlessnessbecause they have nosupport force acting onthem”
17. 17. My hope• Submit to MoE a guide to forces with – Definitions – How to work out whether forces are acting or not – Directions – Common diagrams – (Could have a couple of alternative names)