Health Education Systems Inc (HESI) A2 Exam Quiz: Friction And Motion
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Friction And MotionQuestion 1 of 16

A porter applies a steady horizontal force of 200 N to push a gurney. The force of kinetic friction opposing the motion is 45 N. What is the net force that causes the gurney to accelerate?

45 N
155 N
200 N
245 N
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Health Education Systems Inc (HESI) A2 Exam Quiz

Health Education Systems Inc (HESI) A2 Exam Quiz: Friction And Motion

Practice Friction And Motion in Health Education Systems Inc (HESI) A2 Exam with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Friction And Motion, giving you a quick way to practice the rules, question types, and explanations that matter most for Health Education Systems Inc (HESI) A2 Exam.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A porter applies a steady horizontal force of 200 N to push a gurney. The force of kinetic friction opposing the motion is 45 N. What is the net force that causes the gurney to accelerate?

  1. 45 N
  2. 155 N (correct answer)
  3. 200 N
  4. 245 N
Explanation: When you encounter physics problems involving forces and motion, you need to understand that net force determines acceleration. Net force is the vector sum of all forces acting on an object, accounting for their directions. In this problem, you have two horizontal forces acting on the gurney: the applied force (200 N forward) and kinetic friction (45 N backward, opposing motion). To find the net force, subtract the opposing force from the applied force: Fnet=Fapplied−Ffriction=200 N−45 N=155 NF_{net} = F_{applied} - F_{friction} = 200\text{ N} - 45\text{ N} = 155\text{ N} Looking at the wrong answers: Choice A (45 N) gives you only the friction force, ignoring the applied force entirely. Choice C (200 N) represents just the applied force while completely ignoring friction's opposing effect. Choice D (245 N) incorrectly adds the forces together, treating friction as if it helps the motion rather than opposes it. The key insight is that friction always opposes motion, so you subtract it from the applied force when both act along the same line. The net force of 155 N is what actually accelerates the gurney forward according to Newton's second law. For HESI physics questions, remember that net force problems require you to consider force directions carefully. Applied forces and friction typically work in opposite directions, so you'll usually subtract friction from the applied force. Watch for answer choices that represent individual forces rather than the net result—these are common distractors designed to catch students who don't complete the vector addition properly.

Question 2

An object at rest on a horizontal surface has zero frictional force acting on it. However, when a small horizontal force is applied, a static friction force appears. As the applied force increases, the static friction force:

  1. remains at a constant maximum value.
  2. decreases until the object begins to move.
  3. is always zero until the object starts to slide.
  4. increases to match the applied force up to a certain maximum. (correct answer)
Explanation: When you encounter questions about static friction, remember that static friction is a responsive force that adjusts to prevent motion. Unlike kinetic friction, which has a constant value, static friction varies based on the applied force. Static friction works by matching the applied force exactly, preventing the object from accelerating. When you apply a small horizontal force to an object at rest, static friction immediately appears with the same magnitude but in the opposite direction, maintaining equilibrium. As you increase the applied force, static friction increases proportionally to continue balancing it. This continues until the static friction reaches its maximum possible value, determined by fs,max=μsNf_{s,max} = \mu_s N, where μs\mu_s is the coefficient of static friction and NN is the normal force. Beyond this point, the object begins to slide. Answer choice A is incorrect because static friction doesn't remain constant at its maximum value - it only reaches maximum just before sliding begins. Choice B is wrong because static friction increases rather than decreases as applied force grows. Choice C reflects a common misconception that static friction is always zero until motion starts, but static friction actually exists whenever there's an applied force trying to cause motion. Choice D correctly describes how static friction increases to match the applied force up to its maximum limit. For physics questions on the HESI, remember that static friction is always reactive and proportional to the applied force, while kinetic friction remains constant during motion. Focus on understanding the difference between these two types of friction.

Question 3

A medical supply crate is resting on the floor. A second identical crate is placed directly on top of the first one. How does this affect the maximum force of static friction between the bottom crate and the floor?

  1. It decreases because the pressure is more concentrated.
  2. It increases because the total normal force on the floor is doubled. (correct answer)
  3. It remains the same because the coefficient of friction is unchanged.
  4. It remains the same because the surface area of contact is unchanged.
Explanation: When you encounter physics problems about friction forces, remember that static friction depends on two key factors: the coefficient of friction and the normal force pressing the surfaces together. The maximum static friction force follows the equation fmax=μs×Nf_{max} = \mu_s \times N, where μs\mu_s is the coefficient of static friction and NN is the normal force. When you place the second crate on top, you're doubling the total weight pressing down on the floor. Since normal force equals the weight of the objects (in this horizontal scenario), the normal force doubles from the weight of one crate to the weight of two crates. This doubled normal force directly doubles the maximum static friction force between the bottom crate and floor. Let's examine why the other options miss the mark. Option A incorrectly suggests that pressure concentration affects friction - but friction depends on normal force, not pressure distribution. Option C acknowledges that the coefficient of friction stays constant (which is true) but incorrectly concludes this means the total friction force is unchanged - it ignores that normal force has doubled. Option D focuses on contact surface area, but static friction is independent of contact area when dealing with solid surfaces. For HESI physics questions, always identify which variables in the friction equation are changing. The coefficient of friction is a material property that stays constant, but normal force changes with the weight of the system. When normal force increases, maximum friction force increases proportionally.

Question 4

Which of the following statements about the force of friction is generally considered NOT true in introductory physics?

  1. Friction can act on objects that are not in motion.
  2. The force of kinetic friction increases significantly as the object's speed increases. (correct answer)
  3. Friction is dependent on the types of materials that are in contact with each other.
  4. The direction of the frictional force is parallel to the surfaces in contact.
Explanation: When you encounter questions about friction forces, focus on the fundamental principles that govern how friction behaves in different situations. Static friction can indeed act on stationary objects to prevent them from moving, making option A correct. When you try to push a heavy box that doesn't budge, static friction is working against your applied force even though there's no motion. The key insight here involves kinetic friction's relationship with speed. In introductory physics, kinetic friction is typically treated as independent of velocity - it remains roughly constant regardless of how fast an object moves across a surface. This makes option B incorrect and therefore our answer. While real-world scenarios can involve speed-dependent friction (like air resistance or complex surface interactions), basic physics models treat kinetic friction as a constant force determined by the coefficient of kinetic friction and the normal force: fk=μkNf_k = \mu_k N. Option C is accurate because different material combinations (like rubber on concrete versus ice on steel) have vastly different coefficients of friction, directly affecting the friction force magnitude. Option D correctly describes friction's direction - it always acts parallel to the contact surfaces, opposing relative motion or potential motion between the surfaces. Remember that HESI science questions often test whether you can distinguish between simplified physics models and real-world complexity. When you see friction questions, think about the basic model first: static friction varies up to a maximum, kinetic friction stays constant, and both depend on materials and normal force, not speed.

Question 5

A wheelchair and its occupant have a combined weight of 900 N. If the coefficient of kinetic friction between the wheelchair's tires and a level hallway floor is 0.04, what is the magnitude of the force of kinetic friction?

  1. 3.6 N
  2. 36 N (correct answer)
  3. 90 N
  4. 22500 N
Explanation: Physics problems involving friction require you to understand the relationship between normal force and frictional force. When you see a question about kinetic friction, remember that the frictional force depends on two factors: the normal force pressing the surfaces together and the coefficient of kinetic friction between the materials. The formula for kinetic friction is: Fk=μk×NF_k = \mu_k \times N, where μk\mu_k is the coefficient of kinetic friction and NN is the normal force. On a level surface, the normal force equals the weight of the object, so N=900 NN = 900 \text{ N}. Substituting into the formula: Fk=0.04×900=36 NF_k = 0.04 \times 900 = 36 \text{ N}. This confirms answer B is correct. Looking at the wrong answers: A) 3.6 N results from incorrectly multiplying 0.04 × 90 instead of 0.04 × 900 – a decimal place error. C) 90 N comes from dividing the weight by 10 rather than multiplying by the coefficient, suggesting confusion about the friction formula. D) 22,500 N results from dividing 900 by 0.04 instead of multiplying, which would give an impossibly large friction force that exceeds the object's weight. For HESI physics problems, always write down the relevant formula first, then carefully identify each variable from the problem. Pay special attention to decimal calculations with coefficients of friction, as they're typically small numbers (less than 1) that can easily lead to calculation errors if you're not methodical.

Question 6

A maintenance worker applies grease to the hinges of a noisy door in a patient's room. What is the primary function of the grease in this context?

  1. To increase the normal force within the hinge.
  2. To decrease the coefficient of friction between the hinge surfaces. (correct answer)
  3. To increase the surface area of contact within the hinge.
  4. To change the type of friction from static to kinetic.
Explanation: This question tests your understanding of friction and how lubricants work in mechanical systems. When you encounter problems about reducing unwanted motion or noise, think about the forces and surface interactions involved. Grease functions as a lubricant by creating a thin film between the metal surfaces of the hinge. This film prevents direct metal-to-metal contact, which significantly reduces the coefficient of friction - the measure of how much two surfaces resist sliding against each other. With less friction, the hinge moves more smoothly and quietly, eliminating the squeaking noise. The grease essentially makes the surfaces "slipperier" relative to each other. Let's examine why the other options miss the mark. Choice A is incorrect because grease doesn't increase the normal force (the perpendicular force pressing the surfaces together) - if anything, it might slightly separate the surfaces. Choice C is wrong because grease actually reduces effective surface contact by creating a barrier layer between the metal surfaces. Choice D misunderstands friction types - the hinge already involves kinetic friction when moving, and grease doesn't change the type of friction but rather reduces the magnitude of frictional force. For HESI physics questions, remember that lubrication problems almost always focus on reducing friction coefficients. When you see scenarios involving squeaky hinges, sticky mechanisms, or any application of oils or greases, the primary goal is typically to reduce friction between surfaces, not to change forces or contact areas.

Question 7

Two identical crash carts, A and B, are on the same tiled floor. Cart A is empty, while Cart B is loaded with heavy equipment, making it four times heavier than Cart A. How does the kinetic friction experienced by Cart B compare to that of Cart A when both are pushed?

  1. The friction on Cart B is four times greater than on Cart A. (correct answer)
  2. The friction on Cart B is one-fourth that of Cart A.
  3. The friction is the same for both carts because the floor surface is the same.
  4. The friction is the same for both carts because their wheels are identical.
Explanation: When you encounter physics problems involving friction in healthcare settings, remember that kinetic friction depends on two key factors: the coefficient of friction between surfaces and the normal force (which equals the object's weight on a horizontal surface). The kinetic friction force is calculated as Fk=μk×NF_k = μ_k × N, where μkμ_k is the coefficient of kinetic friction and NN is the normal force. Since both carts are on the same tiled floor with identical wheels, the coefficient of friction remains constant. However, the normal force changes dramatically with weight. Cart B, being four times heavier than Cart A, exerts four times the normal force on the floor. Therefore, Cart B experiences four times the kinetic friction of Cart A. Looking at the wrong answers: Option B suggests Cart B has one-fourth the friction, which reverses the relationship—this would only be true if friction decreased with weight, which contradicts physics. Option C incorrectly assumes that identical floor surfaces alone determine friction, ignoring the crucial role of weight in the normal force calculation. Option D makes the same error by focusing only on identical wheels while overlooking how the different loads affect the force pressing the wheels against the floor. The correct answer is A—Cart B experiences four times greater friction than Cart A. For HESI physics questions, always identify what stays constant versus what changes. Here, the surface and wheels are constant, but the weight (and thus normal force) varies, directly affecting friction proportionally.

Question 8

The maximum force of static friction between a heavy equipment box and the floor is 300 N. A hospital employee pushes horizontally on the box with a steady force of 250 N. What is the magnitude of the friction force acting on the box?

  1. 50 N
  2. 250 N (correct answer)
  3. 300 N
  4. 550 N
Explanation: When you encounter friction problems, you need to distinguish between static friction (when an object isn't moving) and kinetic friction (when it's sliding). The key principle is that static friction adjusts to match the applied force, up to its maximum limit. Here, the box remains stationary because the applied force (250 N) is less than the maximum static friction (300 N). When an object isn't moving, static friction equals exactly the applied horizontal force to maintain equilibrium. Think of static friction as a responsive force that "fights back" with just enough strength to prevent motion. Since the employee applies 250 N horizontally and the box doesn't move, the friction force must be exactly 250 N in the opposite direction. This balance keeps the net force at zero, satisfying Newton's first law. Looking at the wrong answers: A) 50 N incorrectly suggests friction is the difference between applied force and maximum friction, but friction doesn't work this way. C) 300 N assumes friction always acts at its maximum value, but static friction only reaches maximum when the object is on the verge of sliding. D) 550 N incorrectly adds the forces together, which has no physical meaning in this context. Study tip for HESI: Remember that static friction is a "matching" force—it equals the applied force as long as that force doesn't exceed the maximum. Only when you push harder than the maximum does the object start sliding, and then kinetic friction takes over. Always check whether the object is moving or stationary first.

Question 9

A physical therapist assists a patient in sliding their foot towards their body to bend their knee. The patient's heel is dragging along the surface of the therapy table. In which direction does the force of kinetic friction act?

  1. Perpendicular to the surface of the table, pushing upward.
  2. Away from the patient's body, opposing the direction of motion. (correct answer)
  3. Towards the patient's body, in the same direction as the motion.
  4. The frictional force is zero because the therapist is providing the force.
Explanation: When you encounter physics questions about friction on the HESI, focus on the fundamental principle that kinetic friction always opposes the direction of motion. This is a key concept that applies regardless of what other forces are present. In this scenario, the patient's foot is sliding toward their body along the table surface. Since kinetic friction occurs when two surfaces move relative to each other, and it always acts to resist that motion, the frictional force must point in the opposite direction of the foot's movement. Therefore, the friction acts away from the patient's body, opposing the motion. Let's examine why each incorrect answer fails: Answer A describes a normal force, not friction. The normal force does act perpendicular to surfaces, but friction acts parallel to the contact surface, not perpendicular to it. Answer C suggests friction acts in the same direction as motion, which violates the fundamental nature of friction—it would actually help the motion rather than resist it, which is impossible. Answer D incorrectly assumes that because the therapist provides force, friction disappears. However, friction depends only on the relative motion between surfaces and the normal force pressing them together, not on what causes the motion. For HESI physics questions, remember this simple rule: kinetic friction always opposes motion. It doesn't matter who or what causes the movement—if two surfaces are sliding against each other, friction will act opposite to the direction of that sliding motion.

Question 10

Which of the following factors would have the LEAST effect on the magnitude of the kinetic friction acting on a hospital bed being slid across a level floor?

  1. The weight of the patient lying in the hospital bed.
  2. The surface area of the bed's casters in contact with the floor. (correct answer)
  3. The materials from which the floor and the casters are made.
  4. The presence of water or another liquid spilled on the floor.
Explanation: When you encounter physics problems about friction in healthcare settings, focus on the fundamental equation for kinetic friction: Fk=μk×NF_k = \mu_k \times N, where μk\mu_k is the coefficient of kinetic friction and NN is the normal force (typically equal to weight on a level surface). The correct answer is B because kinetic friction is independent of contact area. This might seem counterintuitive, but increasing surface area spreads the same normal force over a larger area, reducing pressure per unit area proportionally. The total friction force remains unchanged. This is a well-established principle in physics that applies regardless of whether you're moving a hospital bed or sliding a book across a table. Let's examine why the other factors do matter: A is incorrect because patient weight directly increases the normal force N, proportionally increasing friction force. C is wrong because different material combinations have different coefficients of friction μk\mu_k - for example, rubber on concrete versus metal on tile creates vastly different friction values. D is incorrect because liquids typically act as lubricants, significantly reducing the coefficient of friction and making the bed much easier to slide. For HESI physics questions, remember that friction depends on "how hard surfaces press together" (normal force) and "how much the materials resist sliding" (coefficient of friction), but never on "how much area touches." This counterintuitive concept appears frequently on standardized exams because it challenges common misconceptions about friction.

Question 11

A healthcare worker pushes a linen cart down a long, level corridor at a constant velocity. Which statement must be true about the horizontal forces acting on the cart?

  1. The force applied by the worker is greater than the total frictional force.
  2. The net force on the cart is positive in the direction of motion.
  3. The force of kinetic friction is zero because the velocity is constant.
  4. The force applied by the worker is equal in magnitude to the total frictional force. (correct answer)
Explanation: When you encounter physics problems involving constant velocity motion, remember that constant velocity means zero acceleration, which requires zero net force according to Newton's first law. Since the cart moves at constant velocity down the corridor, all horizontal forces must be perfectly balanced. The worker applies a forward force to push the cart, while friction acts backward to resist the motion. For the net force to be zero (required for constant velocity), these opposing forces must be equal in magnitude. This makes answer choice D correct. Let's examine why the other options are wrong. Choice A suggests the applied force exceeds friction, which would create a net forward force and cause the cart to accelerate, contradicting the constant velocity condition. Choice B claims the net force is positive in the direction of motion, but any net force would again cause acceleration, not constant velocity. Choice C incorrectly states that kinetic friction is zero during constant velocity motion. In reality, kinetic friction still exists whenever surfaces slide past each other - it's the net force that's zero, not the individual forces. The key insight is distinguishing between individual forces and net force. Individual forces (like friction) can still act on an object moving at constant velocity, but they must sum to zero. For HESI physics questions, always check whether motion is described as constant velocity, accelerating, or at rest. Constant velocity is your cue that forces are balanced, making this a statics problem rather than a dynamics problem.

Question 12

A patient is being moved on a low-friction transfer board. If the staff doubles the constant speed at which they slide the patient, what is the approximate effect on the force of kinetic friction?

  1. The kinetic friction is approximately halved.
  2. The kinetic friction is approximately doubled.
  3. The kinetic friction remains essentially unchanged. (correct answer)
  4. The kinetic friction increases by a factor of four.
Explanation: When you encounter physics questions on healthcare exams, focus on the fundamental relationships between variables. This question tests your understanding of kinetic friction, which is crucial for safe patient transfers. Kinetic friction depends on only two factors: the coefficient of kinetic friction (a property of the surfaces in contact) and the normal force (the weight pressing the surfaces together). The formula is Fk=μk×NF_k = μ_k × N, where μkμ_k is the coefficient and NN is the normal force. Notice that velocity doesn't appear in this equation at all. When staff doubles the transfer speed, neither the coefficient of kinetic friction nor the normal force changes. The transfer board's surface properties remain the same, and the patient's weight pressing down stays constant. Therefore, the kinetic friction force remains essentially unchanged, making C correct. Option A suggests friction decreases with higher speed, which confuses kinetic friction with other phenomena like air resistance at very high speeds. Option B assumes friction increases proportionally with speed, which would make patient transfers increasingly difficult as speed increases—this isn't what happens with kinetic friction. Option D implies a quadratic relationship with speed, which again has no basis in the kinetic friction formula. Remember this key distinction: kinetic friction is speed-independent once sliding begins. This principle helps healthcare workers understand that the force needed to maintain patient movement on transfer devices stays constant regardless of how quickly they perform the transfer, making procedures more predictable and safer.

Question 13

If the gravitational pull on the moon is about 1/6th that of Earth, how would the force of kinetic friction on a rover sliding on the moon's surface compare to the same rover sliding on Earth?

  1. It would be 6 times greater on the moon.
  2. It would be approximately the same on the moon.
  3. It would be about 1/6th as much on the moon. (correct answer)
  4. It would be zero on the moon due to the vacuum.
Explanation: Physics questions involving gravity and friction test your understanding of how forces interact in different gravitational environments. The key principle here is that kinetic friction depends directly on the normal force, which is affected by gravitational pull. Kinetic friction follows the formula Fk=μk×NF_k = \mu_k \times N, where μk\mu_k is the coefficient of kinetic friction and NN is the normal force. On a flat surface, the normal force equals the object's weight, which is mgmg (mass times gravitational acceleration). Since the moon's gravity is 1/6th of Earth's, the rover weighs 1/6th as much on the moon. This directly reduces the normal force by the same factor, making kinetic friction 1/6th as strong. Choice A incorrectly reverses the relationship—it suggests friction increases when gravity decreases, which contradicts the physics. Choice B assumes friction remains constant regardless of gravitational changes, ignoring how normal force affects friction. This might seem logical if you think friction depends only on surface properties, but weight is crucial. Choice D falls into a common misconception about space environments. While the moon has no atmosphere, it does have a solid surface, so friction still exists between contacting surfaces—the vacuum doesn't eliminate friction. Remember that friction problems often test whether you understand the connection between weight and normal force. When gravitational acceleration changes, weight changes proportionally, and so does friction. This principle applies to any scenario involving different gravitational environments, from other planets to elevators accelerating up or down.

Question 14

Which sequence correctly describes the frictional force as a person pushes a stationary, heavy box until it slides across a room at a constant speed?

  1. Kinetic friction acts, then is overcome by static friction which remains constant.
  2. Static friction increases from zero, is overcome, and is then replaced by a smaller, constant kinetic friction. (correct answer)
  3. Friction is zero, then kinetic friction appears and increases as the box speeds up.
  4. A constant static friction must be overcome, after which friction becomes zero.
Explanation: This question tests your understanding of static versus kinetic friction and how frictional forces change during the transition from rest to motion. When you first push the stationary box, static friction opposes your applied force. Static friction isn't constant—it increases proportionally with your pushing force, up to a maximum value. The box remains stationary as long as your push doesn't exceed this maximum static friction. Once your force surpasses the maximum static friction threshold, the box begins to slide. At this point, kinetic friction takes over, and importantly, kinetic friction is always smaller than maximum static friction for the same surfaces. Since the box moves at constant speed, the kinetic friction exactly balances your continued pushing force. Option B correctly captures this sequence: static friction increases from zero as you push harder, gets overcome when you exceed its maximum, then gets replaced by smaller, constant kinetic friction. Option A reverses the friction types—kinetic friction cannot act on a stationary object, and static friction doesn't "overcome" kinetic friction. Option C incorrectly states friction is initially zero. Even the slightest push on a stationary object creates static friction immediately. Option D suggests static friction is constant from the start, which contradicts how static friction actually works—it varies with applied force up to its maximum. Remember this key relationship: static friction can vary but kinetic friction is constant, and kinetic friction is always less than maximum static friction. This explains why objects are harder to start moving than to keep moving.

Question 15

A nurse finds it significantly harder to start pushing a heavy supply cart from a standstill than to keep it moving at a constant speed. This phenomenon is best explained by the fact that:

  1. the cart's inertia is greatest when it is at rest and must be overcome.
  2. the force of kinetic friction is greater than the maximum force of static friction.
  3. the coefficient of static friction is greater than the coefficient of kinetic friction. (correct answer)
  4. the cart's mass decreases slightly once it is in motion, reducing the required force.
Explanation: When you encounter physics concepts in healthcare contexts, remember that the principles governing everyday objects also apply to medical equipment and patient movement. This scenario illustrates the difference between static and kinetic friction. Static friction acts on objects at rest, while kinetic friction acts on moving objects. The coefficient of static friction (μs\mu_s) represents the maximum friction force before an object starts sliding, while the coefficient of kinetic friction (μk\mu_k) represents the friction force during motion. For virtually all material pairs, μs>μk\mu_s > \mu_k, meaning it takes more force to initiate movement than to maintain it. Answer C correctly identifies this fundamental relationship. The higher coefficient of static friction means you must overcome greater resistance to start the cart moving than to keep it rolling. Answer A misuses the concept of inertia. While inertia (resistance to change in motion) exists at rest, it's not "greatest" at rest—inertia depends only on mass, which remains constant. The difficulty starting movement comes from friction, not inertia. Answer B reverses the relationship. Kinetic friction is actually less than maximum static friction, which is exactly why movement becomes easier once started. Answer D suggests mass changes during motion, which violates conservation of mass. The cart's mass remains constant regardless of its motion state. For the HESI exam, remember that friction problems often test whether you understand the static-versus-kinetic distinction. This concept applies to patient transfers, equipment movement, and understanding why starting any motion (like moving a patient) requires more initial force than maintaining the movement.

Question 16

An ambulance skids to a halt on a dry road with its wheels locked. The primary force causing the ambulance to slow down is:

  1. static friction between the road and the non-rotating tires.
  2. kinetic friction between the road and the sliding tires. (correct answer)
  3. the normal force exerted by the road on the tires.
  4. the inertia of the ambulance resisting the change in motion.
Explanation: When you encounter physics questions about moving objects and forces, focus on identifying what type of motion is occurring and which forces are acting on the object. In this scenario, the ambulance's wheels are locked, meaning they're not rotating as the vehicle slides forward. This creates a specific type of friction interaction with the road surface. Since the tires are sliding against the road rather than rolling, kinetic friction is the primary force opposing the ambulance's motion and causing it to decelerate. Option B is correct because kinetic friction acts between two surfaces sliding past each other - exactly what happens when locked wheels skid across pavement. This friction force opposes the direction of motion and converts the ambulance's kinetic energy into heat. Option A is incorrect because static friction only occurs when surfaces are not sliding relative to each other. Once the wheels lock and begin sliding, we're dealing with kinetic, not static friction. Option C misidentifies the force type. The normal force acts perpendicular to the road surface and supports the ambulance's weight, but it doesn't directly cause the vehicle to slow down. Normal force does affect the magnitude of friction (Ffriction=μ×NF_{friction} = \mu \times N), but it's not the primary stopping force. Option D confuses a property with a force. Inertia describes an object's tendency to maintain its current motion, but it's not a force that acts on the object. Forces like friction overcome inertia to change motion. Remember: when objects slide against surfaces, kinetic friction is always the key force to consider for motion changes.