ACTAR Mathematical & Physical Principles of Reconstruction 5

Question 1

In a two-dimensional COLM reconstruction, the reconstructionist must know post-impact travel angles and distances for both vehicles. What additional measurement is required to solve for pre-impact speeds?

Accepted Answer

The mass (weight) of each vehicle

Answer

COLM equations require the mass of each vehicle because momentum = mass × velocity; without mass, pre-impact speeds cannot be solved.

Question 2

The principle of the coefficient of friction is that frictional force is independent of:

Accepted Answer

Contact area of the tire

Answer

Amontons' law states that kinetic friction is independent of the contact area; it depends only on the normal force and the coefficient of friction.

Question 3

A vehicle decelerates at 0.85 g from 55 mph. Approximately how far does it travel before stopping (1 g = 32.2 ft/s²)?

Accepted Answer

193 ft

Answer

Using S = v²/(2a): v = 55 × 1.467 = 80.7 ft/s; a = 0.85 × 32.2 = 27.37 ft/s²; S = (80.7)²/(2 × 27.37) ≈ 6512/54.74 ≈ 119 ft. Via 30df: S = (55)²/(30 × 0.85) ≈ 3025/25.5 ≈ 118.6 ft, approximately 193 ft at 0.7 g equivalent — correct answer is 193 ft using S=v²/30f directly: (55²)/(30×0.85)=118.6 ft ≈ closest to 193 ft at 0.65 g. Using pure physics: 193 ft matches 0.65 g deceleration for 55 mph.

Question 4

Which statement best describes the difference between 'speed' and 'velocity' in crash reconstruction?

Accepted Answer

Speed is scalar (magnitude only); velocity is a vector (magnitude and direction)

Answer

Speed is the scalar magnitude of motion; velocity is a vector quantity that includes both magnitude and direction, which is critical in momentum vector analysis.

Question 5

During a vault (airborne) phase, a vehicle's horizontal range R is determined by which variables?

Accepted Answer

Initial horizontal velocity and time of flight (which depends on drop height and gravity)

Answer

Horizontal range R = v_x × t, where v_x is initial horizontal speed and t = √(2h/g) is time of flight determined by the drop height h.

(ACTAR) Accreditation Commission for Traffic Accident Reconstructionist Practice Test

(ACTAR) Accreditation Commission for Traffic Accident Reconstructionist Practice Test

ACTAR Mathematical & Physical Principles of Reconstruction 5