Energy, Kinematics, and the Three Collisions Practice Questions

25 multiple-choice questions on Energy, kinematics, and the three collisions for paramedic and EMT students, from Chapter 30 (Trauma Systems and Mechanism of Injury). Read each question, pick your answer, then tap Show answer to check it and read the explanation.

25 practice questions

1. Kinematics, as applied to trauma, is the study of:

   1. A. How energy and motion cause injury
   2. B. How the body compensates after injury
   3. C. How muscles produce body movement
   4. D. How injuries are graded by severity
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   Answer: A. How energy and motion cause injury

   Understanding how forces were applied lets the paramedic predict injuries before they show. How muscles move the body is kinesiology, not trauma kinematics. The body's compensation after injury is pathophysiology, and grading severity is the job of trauma scores; neither describes how energy causes the injury.

2. Mechanism of injury describes:

   1. A. The anatomic list of injuries that were found
   2. B. The patient's physiological response
   3. C. The severity score assigned at the ED
   4. D. The way in which traumatic injuries occur
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   Answer: D. The way in which traumatic injuries occur

   The mechanism (the forces and how they acted on the body) raises the index of suspicion for injuries not yet apparent. The list of injuries found is the result, not the mechanism.

3. The formula for kinetic energy is:

   1. A. KE = mass x velocity
   2. B. KE = mass x acceleration
   3. C. KE = (mass x velocity squared) / 2
   4. D. KE = (mass squared x velocity) / 2
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   Answer: C. KE = (mass x velocity squared) / 2

   Because velocity is squared, speed matters far more than weight. Mass times acceleration is force, from Newton's second law.

4. A driver says he was going 60 mph, twice the 30 mph speed of another crash you ran today. Compared with that car, his car's kinetic energy at impact:

   1. A. Doubles
   2. B. Increases fourfold
   3. C. Increases eightfold
   4. D. Stays the same
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   Answer: B. Increases fourfold

   KE rises with the square of velocity, so twice the speed gives four times the energy. Doubling would be the effect of doubling mass, not speed.

5. Which change increases the kinetic energy of a moving object the most?

   1. A. Tripling its mass
   2. B. Doubling its mass
   3. C. Tripling its speed
   4. D. Doubling its speed
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   Answer: C. Tripling its speed

   Tripling speed multiplies energy by nine, while tripling mass multiplies it by only three. This is why high-velocity rifle rounds wound more than heavier slow bullets.

6. Newton's first law states that an object in motion will remain in motion until:

   1. A. Acted on by an outside force
   2. B. Its energy is converted to heat
   3. C. It reaches its maximum velocity
   4. D. Its mass is reduced by half
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   Answer: A. Acted on by an outside force

   This law of inertia explains why an unrestrained occupant keeps moving forward after the car stops. Energy conversion is the law of conservation of energy, a separate principle.

7. The law of conservation of energy states that energy:

   1. A. Is destroyed when a vehicle comes to a rest
   2. B. Is created by the force of the impact itself
   3. C. Disappears as a vehicle slows during braking
   4. D. Cannot be created or destroyed, only changed
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   Answer: D. Cannot be created or destroyed, only changed

   In a crash the vehicle's kinetic energy is changed into deformation, heat, sound, and tissue damage. When it is absorbed by the body, injury results.

8. Newton's second law is expressed as:

   1. A. Force = mass x velocity squared
   2. B. Energy = mass x acceleration
   3. C. Force = velocity x time
   4. D. Force = mass x acceleration
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   Answer: D. Force = mass x acceleration

   Rapid deceleration (a large negative acceleration) produces large forces even at modest mass. Mass times velocity squared over two is kinetic energy, not force.

9. A car that brakes to a stop over 100 feet causes fewer injuries than one that hits a wall at the same speed because:

   1. A. Stopping distance spreads the force over time
   2. B. Kinetic energy is lower when the car is braking
   3. C. The occupants can brace before it stops
   4. D. Braking keeps the internal organs from moving
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   Answer: A. Stopping distance spreads the force over time

   The same energy must be dissipated, but a longer stopping distance means lower deceleration and lower forces. Crumple zones and airbags work on this same principle. Bracing does little against crash-level forces, and the organs still decelerate during a hard stop; what changes is how quickly the speed is lost.

10. In the three-collision concept of a motor vehicle crash, the first collision is:

    1. A. The occupant striking the interior
    2. B. The vehicle striking an object
    3. C. The organs striking the body wall
    4. D. Loose objects striking the occupant
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    Answer: B. The vehicle striking an object

    The vehicle collision is followed by the body collision and then the organ collision. Damage to the vehicle shows how much energy was involved.

11. In the three-collision concept, the second collision is:

    1. A. The vehicle striking a second vehicle head-on
    2. B. Internal organs striking each other
    3. C. The occupant striking the vehicle interior
    4. D. The vehicle striking the ground
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    Answer: C. The occupant striking the vehicle interior

    The body collision produces visible injuries such as steering wheel bruises and starred windshields. A second vehicle impact is still a vehicle collision.

12. The third collision in the three-collision concept involves:

    1. A. The occupant striking the steering column
    2. B. Internal organs striking structures in the body
    3. C. Rescuers striking the vehicle during extrication
    4. D. The vehicle striking a fixed object
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    Answer: B. Internal organs striking structures in the body

    Organs continue moving after the body stops and strike the skull, ribs, or spine, or tear at their attachments. These injuries are often hidden and are the most life-threatening.

13. Some texts describe a fourth collision in a crash, which is:

    1. A. Unsecured objects striking the occupant
    2. B. A second vehicle striking the car
    3. C. The occupant striking the airbag
    4. D. Organs rebounding back to their position
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    Answer: A. Unsecured objects striking the occupant

    Loose cargo, groceries, and unrestrained passengers become projectiles that hit occupants. Organs moving inside the body are part of the third collision. The occupant striking the airbag is part of the second collision, and a second vehicle hitting the car starts a new vehicle collision rather than being the fourth collision.

14. A driver's chest struck a steering column, leaving a bruise. The injury most worrisome for the third (organ) collision is:

    1. A. Fracture of the steering column
    2. B. Abrasion of the anterior chest wall skin
    3. C. Bruising of the pectoral muscles
    4. D. Myocardial contusion or aortic injury
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    Answer: D. Myocardial contusion or aortic injury

    The heart and aorta continue forward after the chest wall stops, causing contusion or a tear. The bruise itself is the second collision, and the column fracture is the first.

15. A 30-year-old driver struck a bridge abutment at 55 mph. She has chest pain radiating to her back and unequal arm pressures. Deceleration injury to the aorta most commonly occurs:

    1. A. At the aortic valve in the left ventricle
    2. B. At the bifurcation into the iliac arteries
    3. C. Just distal to the left subclavian artery
    4. D. Where the renal arteries leave the aorta
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    Answer: C. Just distal to the left subclavian artery

    The ligamentum arteriosum anchors the aorta there, so the mobile arch shears against the fixed descending aorta. Most patients die at the scene, but survivors may have only subtle signs.

16. Rapid deceleration can tear the liver because it is anchored by the:

    1. A. Ligamentum arteriosum
    2. B. Ligament of Treitz
    3. C. Ligamentum teres
    4. D. Inguinal ligament
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    Answer: C. Ligamentum teres

    The liver keeps moving and is cut by its own ligament attachment, sometimes called a cleaving injury. The ligamentum arteriosum is the aortic anchor.

17. The two main types of force that produce blunt trauma are:

    1. A. Compression and shear
    2. B. Cavitation and fragmentation
    3. C. Radiation and convection
    4. D. Ionization and conduction
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    Answer: A. Compression and shear

    Compression crushes tissue between surfaces, and shear tears tissue where mobile and fixed parts meet. Cavitation and fragmentation describe penetrating projectiles.

18. A 45-year-old unrestrained driver hit a tree at 40 mph. He has no visible injuries and says he feels fine. Your best approach is to:

    1. A. Release him because he has no complaints
    2. B. Assume serious injury and assess fully
    3. C. Focus the exam on the areas he says hurt
    4. D. Wait for injuries to appear before treating
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    Answer: B. Assume serious injury and assess fully

    The mechanism carries enough energy to cause hidden injury, and adrenaline and distracting factors can mask pain. Focusing only on complaints risks missing internal hemorrhage.

19. A falling brick and a nail fired from a nail gun strike a worker's thigh with the same kinetic energy. The nail penetrates deeply while the brick only bruises. The main reason is the difference in:

    1. A. Total energy delivered to the body
    2. B. Density of the skin that was struck
    3. C. Direction of the force at impact
    4. D. Surface area of energy transfer
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    Answer: D. Surface area of energy transfer

    The same energy concentrated on a tiny point (a nail, a knife tip, a bullet) exceeds the strength of skin and penetrates, while the same energy spread across a broad surface (a brick, an airbag) is more likely to bruise or fracture. Total energy is wrong because the stem fixes it as equal. Skin density is wrong because the same thigh skin was struck by both objects. Direction can change which structures are hit but does not explain penetration versus bruising at equal energy.

20. Cavitation can occur with blunt trauma as well as penetrating trauma. In blunt trauma the cavity is typically:

    1. A. Temporary, as tissue is pushed away briefly
    2. B. Permanent, with an open tract to the skin surface
    3. C. Caused by gas formed during the impact
    4. D. Larger than any cavity a bullet makes
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    Answer: A. Temporary, as tissue is pushed away briefly

    A blow pushes tissue away from the point of impact and it rebounds, leaving damage that may not be visible. A permanent tract open to the skin is typical of a projectile.

21. The density of tissue affects injury from energy transfer because:

    1. A. Less dense tissue bleeds more freely
    2. B. Air-filled organs resist all injury
    3. C. Density matters in burns but not trauma
    4. D. Denser tissue absorbs more energy
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    Answer: D. Denser tissue absorbs more energy

    Solid organs such as the liver absorb more energy and fracture, while elastic, air-filled lung tissue tends to stretch. Air-filled organs are still injured, particularly by blast pressure.

22. At a crash scene, which vehicle finding should most raise your index of suspicion for serious occupant injury?

    1. A. A cracked side mirror
    2. B. A bent steering wheel
    3. C. A deployed driver airbag
    4. D. A broken taillight
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    Answer: B. A bent steering wheel

    A deformed steering wheel means the driver's chest or abdomen struck it with great force, so look for chest and upper abdominal injury. A deployed airbag means the car decelerated quickly but often protected the driver.

23. Why should paramedics evaluate the mechanism of injury rather than rely only on the initial physical exam?

    1. A. The mechanism alone sets the triage level
    2. B. Physical findings are unreliable in adults
    3. C. Many serious injuries are hidden at first
    4. D. The mechanism predicts each exact injury
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    Answer: C. Many serious injuries are hidden at first

    Internal bleeding and organ injury may not produce signs until compensation fails. The mechanism complements the exam and vital signs but never replaces them. It raises suspicion without predicting exact injuries, and the 2021 field triage guideline weighs it alongside injury patterns, mental status, and vital signs rather than letting it set the triage level by itself.

24. A 7-year-old is struck in the abdomen by a heavy metal playground swing. Which factor most determines how much energy is transferred to his abdominal organs?

    1. A. The size of the bruise on his skin
    2. B. The swing's mass and speed at impact
    3. C. How tense his abdominal muscles were
    4. D. The thickness of his clothing
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    Answer: B. The swing's mass and speed at impact

    Kinetic energy equals half the mass times velocity squared, so the swing's mass and especially its speed determine the energy delivered; how abruptly it stops determines the force. Bruise size is wrong because serious solid-organ injury can occur with little or no bruising, especially in children with compliant abdominal walls. Muscle tension and clothing may change slightly how the force is spread, but they do not set how much energy the swing carries.

25. A driver wearing a seat belt in a modern car with crumple zones hits a wall at the same speed as a driver in an older car without them. The belted driver in the newer car is less injured mainly because:

    1. A. The newer car carries less kinetic energy at impact
    2. B. The seat belt stops him sooner than the car
    3. C. The car absorbs energy and lengthens stopping time
    4. D. The airbag lowers the car's impact speed
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    Answer: C. The car absorbs energy and lengthens stopping time

    Crumple zones and belts extend the time over which the occupant decelerates, lowering the force. Kinetic energy at impact is the same at equal mass and speed. The belt ties him to the car so he slows with it rather than ahead of it, and the airbag cushions him after impact without changing the speed at which the car hits the wall.