This document provides a definition and explanation of friction. It states that friction is a force that opposes the relative motion between two surfaces that are sliding past each other, not simply opposing all motion as the initial definition stated. It then provides examples of force diagrams to illustrate friction and other forces acting on objects in motion.

1. Definition of Friction
“Friction is a force that opposes motion”
Hardly!!!

2. Definition of Friction
“Friction is a force that opposes two surfaces sliding
past each other”
FRICTION

3. Easy Force Diagrams
SUPPORT
PUSH
FRICTION
GRAVITY

4. • The problem arises for people or vehicles
propelling themselves.

5. NZQA exam
Four 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.

6. SUPPORT
In assessment
schedule:
FRICTION PUSH
WEIGHT

7. 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….

8. SUPPORT
Real forces:
AIR STATIC
RESISTANCE FRICTION
AND
ROLLING
RESISTANCE WEIGHT

9. KINETIC FRICTION (on box due to ground)
sliding
Kinetic friction
STATIC FRICTION (on box due to ground)
Direction object WOULD slide
if there was no grip
Static friction
FRICTION OPPOSES SLIDING

10. Direction bottom of
wheel WOULD slide if STATIC FRICTION
there was no grip

11. SUPPORT or
REACTION
RESISTANCE – (Rolling
resistance and air resistance) STATIC FRICTION
WEIGHT or
GRAVITY

12. Accelerating Constant speed
braking

13. SUPPORT
STATIC FRICTION
AIR RESISTANCE
WEIGHT
/GRAVITY

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. 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. “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”

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)