1. Perception and Reaction Friction Evaluation
Tab 6
Perception and Reaction
Friction Evaluation

2. Perception and Reaction
Necessary for most collision evaluations
Function of human factors and limitations
Variable – no fixed or exact time for an individual or situation

4. Friction
When discussing friction, we’re simply talking about how much resistance there is for one object to move against another
We do that by quantifying the relationship of what something weighs vs how much force it takes to slide it

5. Friction Normal Force Weight Weight:
how much something presses toward the center of the earth
Newton’s 3rd law: For every action there is …..
Normal Force:
how much something presses away from the center of the earth
Assume that this projector weighs 10#
Weight

6. Friction Normal Force Weight 10#
The projector presses on the table with a 10# force directed toward the center of the earth
Weight
10#

7. Friction 10# Normal Force Weight 10#
Table pushes up against the projector with 10# of force
if the force were greater than 10#, what would happen to the projector
if the force were less than 10#, what would happen?
Weight
10#

8. Friction (Opposing Force)
Normal Force
Motion
Force
Friction (Opposing Force)
Motion with applied sufficient horizontal force, the body will be put in motion
Newton’s 1st law: A body at rest will remain at rest, and a body in motion will remain in motion until affected by an outside force.
AKA: Inertia
Friction: how much something resists motion upon/across a surface
In order for motion to occur, the horizontal force must exceed friction
if the sliding surface is horizontal, the horizontal force required is proportional to the object’s weight
dynamic friction is less than static friction:
after an object is sliding, it takes less force to keep it sliding
the friction force does not depend on how much area of the sliding object is in contact with the surface
bigger tires don’t mean more force is necessary for the object to slide
the friction force doesn’t change with velocity change
the friction force doesn’t change with temperature (unless radical)
Weight

9. Friction Normal Force Force 3# Weight 10# Friction
Assume that there is a 3# force needed to move the object
Weight
10#

10. Sets up a Mathematical relationship:3# 3 = =
0.30
Friction or Drag Factor = 10
10#

11. Instead of sliding the projector on the table, let’s say we move it to the carpet and measure the force needed to move it, again.
Force needed: 7#. What is the drag factor?

12. Friction or Drag Factor7# 7 = =
0.70
Friction or Drag Factor = 10
10#

13. Friction or Drag Factor7# 7 = =
0.70
Friction or Drag Factor = 10
10#
X #
Pounds or Kg = =
No units
Y #
Pounds or Kg

14. Change the weight of the projector to 1000#
How much force will be needed to move it on the table?

15. Friction or Drag Factor
300# 3 = =
0.30
Friction or Drag Factor = 10
1000#

16. The mathematical relationship still exists.

17. Bottom line: It takes more force to move the object UP the slope than it does to move it DOWN the slope.
Normal Ø
Motion
Weight = W (cosine ø) Ø
Weight = W (sin ø)
Weight

18. This is the only interface of the vehicle with the pavement surface.
Pickup weighs slightly over 6000#.
attach a scale to the front bumper and pull, you MIGHT be able to move it
if the transmission is in neutral.
Lock the brakes and the drag factor goes up dramatically and quickly
HANDOUTS: Friction tables
This is the only interface of the vehicle with the pavement surface.

19. Drag factor (f) = the force required for acceleration (or deceleration) in the direction of the acceleration (or deceleration) divided by the object’s (vehicle’s) weight
Coefficient of friction (µ) = the force required when an object is sliding across the surface.
Drag factor and Coefficient of friction are the same only when all wheels are locked and sliding on a level surface.

20. Generally more appropriate values
Draw a line through both “1.20” figures
Utilize the factors for “More than 30 mph” as they are more realistic
Discuss differences between “Travelled” and “Traffic Polished”
Generally more appropriate values
Source: How to use the Traffic Template, Page 19
The Traffic Institute, Northwestern University
Copyright 1984