Tab 7 Speed Estimates. Skid Marks Sliding Objects Scuff Marks Linear Momentum Airborne Kinetic Energy.

Tab 7

Speed Estimates

Speed Estimates

Skid MarksSliding ObjectsScuff MarksLinear MomentumAirborneKinetic Energy

Energy of Motion

(Kinetic Energy)

= Energy to stop

(Work Energy)

½ M V2 Fd=

Determining Slide to Stop

Speed

Through algebraic substitution from one side of the equals sign to the

other, the above formula becomes:

FdMV 22

1

dfSmph 30

dfSkph 254

Three basic speed formulae

dfS 30

f

Sd

30

2

d

Sf

30

2

Three basic speed formulae

dfS 30

f

Sd

30

2

d

Sf

30

2

Speed from skid marks

Three basic speed formulae

dfS 30

f

Sd

30

2

d

Sf

30

2

Distance from speed and factor

Three basic speed formulae

dfS 30

f

Sd

30

2

d

Sf

30

2

Factor from speed and

distance (test skids)

Traffic Template 101

On a clean sheet of paper, draw a vertical

line from top to bottom in the middle of the

sheet

This page will be needed many times in the next couple days

Time for another test

Distance (feet)

Feet per Second

(Velocity)

Miles per Hour

(Speed)

Drag Factor or

Acceleration Factor

Acceleration Rate

(FPS2)

How many feet per second will a vehicle travel at 30 mph?

FPS = Speed (1.47)

FPS = 30 (1.47)

FPS = 44.1

Calculate using the Template

Metric: 1 Km/hr = 1000 meters in 3600 sec.

1000 m/3600 sec = 0.278 conversion factor

First, refer to

the “SPEED” side of

the center column

Second, using

the page you

made with the

line, place the

line horiz. across

the center column at 30 MPH

Third, on the FPS side of

the center

column, read the

cor-responding Feet per

Second opposite 30 MPH

30MPH = 44 FPS

If a vehicle travels 110 feet in one second, how many miles will it

travel in one hour?

Use your template to calculate the answer.

110 FPS = 75 MPH

What would be the safe following distance at 60 MPH, assuming a 2-

second following distance?

Use your template to calculate.

60 MPH = 88 FPS X 2 seconds

= 176 feet

A car left 100 feet of skid marks on a dry, traveled, asphalt surface. How fast was it going when it

began to brake?

Use your template to calculate.

First, decide what you’ll use

as the drag factor,

let’s say .70

Second, place your line

(horiz.) on the drag factor

column at .70

Third, place the other end of

the line on the 100-ft mark on

the distance (left) column

Fourth, read the SPEED in

the center column

~46 MPH

(Remember, MPH, not FPS)

A pickup skidded for 75 feet on a wet, traveled, asphalt road surface. How fast was it going when it began

to brake?

Use your template to calculate.

Drag factor = 0.60

Speed = ~36 MPH

How far will a car traveling 30 MPH slide to a stop on a surface with a

drag factor of .70?

Use your template to calculate.

Drag factor = 0.70

Speed = 30 MPH

Distance = 43 feet

How far will a car traveling 60 MPH slide to a stop on a surface with a

drag factor of .70? (If we double the speed, will the distance double,

too?)

Use your template to calculate.

Drag factor = 0.70

Speed = 60 MPH

Distance = 172 feet

If we double the speed, we quadruple the distance (so long as the factor remains

equal)

Similarly, if we double the speed, the Kinetic energy quadruples. This is due to the term “v2” in the formula KE = ½ Mv2

If you know two out of three, you can calculate the third:

If you know the factor and the speed, you can calculate the distance.

If you know the distance and the factor, you can calculate the speed.

If you know the distance and the speed, you can calculate the factor.