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)