Post on 27-Jun-2015
description
Definition of Friction
“Friction is a force that opposes motion”
Hardly!!!
Definition of Friction“Friction is a force that opposes two surfaces sliding
past each other”
FRICTION
Easy Force Diagrams
SUPPORT
GRAVITY
FRICTION
PUSH
• The problem arises for people or vehicles propelling themselves.
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 the directions of ALL FOUR forces acting on Toni and the bike. Label the forces.
In assessment schedule:
FRICTION PUSH
SUPPORT
WEIGHT
Alan Van Heuvelen's 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 can't identify a thing outside the circle then you're drawing a force vector that doesn't exist. E.g. Driving force/engine force/push force….
Real forces:
AIR RESISTANCE AND ROLLING RESISTANCE
STATIC FRICTION
SUPPORT
WEIGHT
KINETIC FRICTION (on box due to ground)
STATIC FRICTION (on box due to ground)
FRICTION OPPOSES SLIDING
sliding
Kinetic friction
Direction object WOULD slide if there was no grip
Static friction
Direction bottom of wheel WOULD slide if there was no grip
STATIC FRICTION
RESISTANCE – (Rolling resistance and air resistance) STATIC FRICTION
SUPPORT or REACTION
WEIGHT or GRAVITY
Accelerating
braking
Constant speed
AIR RESISTANCESTATIC FRICTION
SUPPORT
WEIGHT /GRAVITY
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
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:
RESISTANCE
SUPPORT or REACTION
WEIGHT or GRAVITY
THRUST
“Weight”• Definition that is pretty universal: “Weight is
the gravitational force on an object” (+/- effect of rotation of earth)
“the astronauts experience apparent weightlessness because they have no support force acting on them”
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)