1. Response Time Speed and Following Distance

2. WHAT IS SPEED?
Speed – the distance traveled per unit time Average speed – the total distance traveled divided by the time it took to travel that distance

3. International System of Units (SI)
Quantity
Unit
Symbol
Length
Meter m
Mass
Kilogram Kg
Time
Second s
Temperature
Kelvin K
Current
Ampere A

4. International System of Units (SI)
Prefix
Symbol
Multiple of ten by which base unit multiplied
Example
kilo k
103 = 1000
1 km = 1000m
1 m = km
centi c
10-2 = 0.01
1 cm = 0.01 m
1 m = 100 cm
milli m
10-3 = 0.001
1 mm = m
1 m = 1000 mm

5. Some Comparisons School Zone 25 mph 40 km/h 11 m/s Residential Street
Suburban Interstate
55 mph
90 km/h
25 m/s
Rural Interstate
75 mph
120 km/h
34 m/s

6. DESCRIBING MOTION Model 1: Strobe photos Model 2: Equations
Average speed = (distance traveled)/(time elapsed)
vav = Δd/Δt
Strobe photos are used to describe motion (each picture is the same car at different points as time passes)
Equations were used to describe how far a car moved after a certain period of time (see logbook 7e)

7. WHAT IS SPEED?
Speed – the distance traveled per unit time Average speed – the total distance traveled divided by the time it took to travel that distance Constant speed – speed that does not change over a period of time
Average speed how one would explain the equation (v=d/t)

8. SAMPLE PROBLEMS
If you drive a distance of 400 mi (640 km) in 8 h, what is your average speed?
V=400/8 or V=640/8

9. WHAT IS SPEED?
Speed – the distance traveled per unit time Average speed – the total distance traveled divided by the time it took to travel that distance Constant speed – speed that does not change over a period of time Instantaneous speed – the speed at a given moment

10. WHAT IS SPEED? Δd
vav Δt

11. SAMPLE PROBLEMS
You are traveling at 35 mph (~50 ft/s) and your reaction time is 0.2 s. Calculate the distance you travel during your reaction time.
“traveling at 35 mph (50 ft/s)”: this is speed
“your reaction time is 0.2 s.”: this is time
Cover up ∆d (see previous slide); speed (vav) and time (∆t) are beside each other, so you multiply: 35*0.2=7mi or 50*0.2=2.5km

12. SAMPLE PROBLEMS
In an automobile collision, it was determined that a car traveled 150 ft before the brakes were applied.
If the car had been traveling at the speed limit of 40 mph (60 ft/s), what was the driver’s reaction- time?
Witnesses say that the driver appeared to be under the influence of alcohol. Does your reaction-time data support the witnesses’ testimony?
“car traveled 150 ft”: distance
“car had been traveling at the speed limit of 40 mph (60 ft/s)”: speed
Cover up ∆t (see slide 8); distance (∆t) is above speed (vav), so you divide: 150(ft)/60(ft/s) (make sure that units are the same)= 2.5ft/s

13. WHAT IS SPEED?
Speed – the distance traveled per unit time Average speed – the total distance traveled divided by the time it took to travel that distance Constant speed – speed that does not change over a period of time Instantaneous speed – the speed at a given moment Velocity – the speed in a given direction
Velocity includes a direction!

14. DESCRIBING MOTION Model 1: Strobe photos Model 2: Equations
Average speed = (distance traveled)/(time elapsed)
vav = Δd/Δt
Model 3: Graphs
Distance-Time Graph
See logbook for different graphs

15. DESCRIBING MOTION
See logbook for different graphs

16. DESCRIBING MOTION
See logbook for different graphs

17. Coming Around Full Circle
Reaction Distance – the distance that a vehicle travels in the time it takes the driver to react
How is this related to Reaction Time?
While the message to react is getting sent to your leg, your car keeps moving forward at the same rate

18. Coming Around Full Circle
Reaction Distance
If a car is traveling at 40 mph (60 ft/s), How many car lengths does it travel in 1 second (1 car length = 15 ft)?
4 car lengths

19. What Do You Think Now?
What is a safe following distance between your automobile and the vehicle in front of you?
How do you decide what a safe following distance is?
While the message to react is getting sent to your leg, your car keeps moving forward at the same rate