ACTAR Crash Dynamics 3 — Questions and Answers
Question 1: Which physical principle explains why an unrestrained occupant continues moving forward at the vehicle's pre-crash speed when the vehicle suddenly decelerates?
- Newton's Third Law
- Newton's First Law (inertia) (Correct answer)
- Conservation of energy
- The impulse-momentum theorem
Correct answer: Newton's First Law (inertia)
Newton's First Law states that a body in motion continues at constant velocity unless acted upon by a net force; an unrestrained occupant lacks the restraining force the seatbelt would provide.
Question 2: When analyzing a low-speed rear-end collision, which factor most significantly affects the magnitude of delta-V experienced by the struck vehicle?
- Color of the striking vehicle
- Mass ratio between the striking and struck vehicles (Correct answer)
- Ambient temperature at time of collision
- Road grade perpendicular to travel
Correct answer: Mass ratio between the striking and struck vehicles
The mass ratio between the striking and struck vehicles directly determines how momentum is partitioned, making it the dominant factor in delta-V for the struck vehicle.
Question 3: In crash dynamics, 'restitution phase' refers to:
- The period during which vehicles deform prior to maximum engagement
- The period during which vehicles spring apart after reaching maximum crush (Correct answer)
- The post-crash deceleration to final rest
- The time during which airbags deploy
Correct answer: The period during which vehicles spring apart after reaching maximum crush
The restitution phase follows maximum engagement, during which stored elastic energy releases and the vehicles begin to separate.
Question 4: A vehicle traveling at 30 mph skids to a stop on dry asphalt with a drag factor of 0.75. Approximately how far does it skid before stopping?
- 40 ft
- 60 ft
- 80 ft (Correct answer)
- 100 ft
Correct answer: 80 ft
Using the skid formula d = S²/(30f): d = 30²/(30 × 0.75) = 900/22.5 = 40 ft.
Question 5: Which of the following best describes 'impulse' in the context of crash dynamics?
- The peak force during impact
- The product of force and the time over which it acts (Correct answer)
- The energy stored in vehicle deformation
- The change in velocity of one vehicle
Correct answer: The product of force and the time over which it acts
Impulse equals force multiplied by the duration of its application (J = F×Δt), and by the impulse-momentum theorem, it equals the change in momentum.
Question 6: During a rollover event, which energy component must be accounted for in addition to translational kinetic energy?
- Chemical potential energy
- Rotational kinetic energy (Correct answer)
- Gravitational potential energy of the engine block only
- Surface tension energy
Correct answer: Rotational kinetic energy
A rolling vehicle possesses rotational kinetic energy (½Iω²) in addition to translational kinetic energy (½mv²), and both must be considered in a full energy analysis.
Question 7: In pedestrian impact reconstruction, the 'wrap projection' distance is used to estimate the vehicle's speed because it reflects:
- How far the pedestrian walked before impact
- The horizontal distance the pedestrian traveled through the air after being struck by the hood (Correct answer)
- The distance the vehicle traveled after braking
- The rollout distance of the vehicle's tires
Correct answer: The horizontal distance the pedestrian traveled through the air after being struck by the hood
Wrap projection distance is the horizontal distance a pedestrian travels as a projectile after being struck and launched over the vehicle hood, correlated to impact speed through validated formulas.
