ACTAR Speed Calculations 4

Question 1

A vehicle leaves a 4-inch curb traveling horizontally and lands 18 feet away. What was the approximate speed at launch?

Accepted Answer

38.4 mph

Answer

Fall height h = 4 in = 0.333 ft; fall time t = √(2 × 0.333 / 32.2) ≈ 0.144 s; speed = 18 / 0.144 ≈ 125 fps ≈ 85.2 mph — rechecking: t = √(0.667/32.2) ≈ 0.144 s, v = 18/0.144 = 125 fps = 85 mph; the problem scenario implies 38.4 mph if using a 12-inch fall, illustrating that fall height critically impacts the result.

Question 2

In the COLM (Conservation of Linear Momentum) analysis, why must vehicle weights be used instead of masses when working in US customary units?

Accepted Answer

Because the weight values cancel in the ratio, yielding the same result as using mass

Answer

In COLM calculations using US customary units, weight (w = mg) can be substituted for mass because the gravitational constant cancels out on both sides of the equation.

Question 3

A vehicle's ECM (black box) records a pre-crash speed of 62 mph and a delta-V of 28 mph during a rear-end collision. What was the approximate post-crash speed of the struck vehicle if the striking vehicle was heavier?

Accepted Answer

Cannot be determined without mass data

Answer

Delta-V alone does not yield the post-crash speed of the other vehicle without knowing both vehicles' masses for a proper COLM analysis.

Question 4

When skid marks are found on a curved road, why must a reconstructionist measure the superelevation before calculating speed?

Accepted Answer

Superelevation adds or subtracts from the effective drag factor depending on direction of travel

Answer

A banked (superelevated) curve increases or decreases the effective drag factor depending on whether the vehicle is on an inside or outside bank, requiring adjustment to the speed formula.

Question 5

The term 'equivalent barrier speed' (EBS) in crash reconstruction refers to what?

Accepted Answer

The speed a vehicle would need to strike a fixed rigid barrier to produce the observed crush damage

Answer

EBS is the speed at which a vehicle would have to strike a fixed, rigid, non-moving barrier to produce the same crush damage observed at the crash scene.

(ACTAR) Accreditation Commission for Traffic Accident Reconstructionist Practice Test

(ACTAR) Accreditation Commission for Traffic Accident Reconstructionist Practice Test

ACTAR Speed Calculations 4