Health Education Systems Inc (HESI) A2 Exam Quiz: Forces Mass And Acceleration
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Forces Mass And AccelerationQuestion 1 of 19

An object experiences a net force F, which causes it to have an acceleration a. If the net force is tripled (3F) and the mass of the object is quadrupled (4m), what will be the new acceleration?

3/4 a
4/3 a
7 a
12 a
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Health Education Systems Inc (HESI) A2 Exam Quiz

Health Education Systems Inc (HESI) A2 Exam Quiz: Forces Mass And Acceleration

Practice Forces Mass And Acceleration 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 Forces Mass And Acceleration, 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.

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Question 1

An object experiences a net force F, which causes it to have an acceleration a. If the net force is tripled (3F) and the mass of the object is quadrupled (4m), what will be the new acceleration?

  1. 3/4 a (correct answer)
  2. 4/3 a
  3. 7 a
  4. 12 a
Explanation: When you encounter physics problems involving force and acceleration, immediately think of Newton's second law: F=maF = ma, which can be rearranged to a=Fma = \frac{F}{m}. This fundamental relationship shows that acceleration is directly proportional to force and inversely proportional to mass. Starting with the original situation where force F produces acceleration a in mass m, we have a=Fma = \frac{F}{m}. Now let's find the new acceleration when force becomes 3F and mass becomes 4m: anew=3F4m=34⋅Fm=34aa_{new} = \frac{3F}{4m} = \frac{3}{4} \cdot \frac{F}{m} = \frac{3}{4}a So the new acceleration is 34a\frac{3}{4}a, making A correct. Let's examine why the other answers are wrong. Answer B (4/3 a) incorrectly inverts the fraction—this would result from mistakenly thinking acceleration increases when mass increases. Answer C (7a) suggests simply adding the multipliers (3 + 4 = 7), which completely ignores how force and mass relate in Newton's second law. Answer D (12a) comes from multiplying the factors (3 × 4 = 12), again missing the inverse relationship between mass and acceleration. Remember this key pattern for HESI physics questions: when both force and mass change, set up the ratio Fnewmnew\frac{F_{new}}{m_{new}} and compare it to Foriginalmoriginal\frac{F_{original}}{m_{original}}. The most common trap is forgetting that mass appears in the denominator—doubling mass cuts acceleration in half, while doubling force doubles acceleration.

Question 2

A gurney with a mass of 40 kg is pushed with a constant velocity of 1.5 m/s across a level floor. What is the net force on the gurney?

  1. 0 N (correct answer)
  2. 26.7 N
  3. 40 N
  4. 60 N
Explanation: When you encounter physics problems involving motion, always start by identifying what type of motion is occurring. This question describes constant velocity motion, which is the key to understanding the forces at play. According to Newton's First Law of Motion, an object moving at constant velocity has zero acceleration. Since force equals mass times acceleration (F=maF = ma), when acceleration is zero, the net force must also be zero. The gurney moves at a steady 1.5 m/s, meaning its velocity isn't changing—no speeding up, slowing down, or changing direction. This makes A) 0 N correct. Even though someone is pushing the gurney and friction opposes the motion, these forces are balanced, resulting in zero net force. B) 26.7 N likely comes from incorrectly dividing mass by velocity (40 kg ÷ 1.5 m/s), but this calculation has no physical meaning in force problems. C) 40 N represents a common mistake of confusing mass (40 kg) with force—remember that mass and force are completely different quantities with different units. D) 60 N probably results from multiplying mass by velocity (40 kg × 1.5 m/s), but this gives momentum (kg⋅m/s), not force (Newtons). Remember this pattern: constant velocity always means zero net force, regardless of the actual velocity value or mass involved. On physics questions, if you see "constant velocity," immediately think "balanced forces" and "zero acceleration."

Question 3

A nurse pushes a 50 kg patient in a wheelchair, causing an acceleration of 0.5 m/s². If the mass of the patient and wheelchair were doubled, what force would be required to produce the same acceleration?

  1. 12.5 N
  2. 25 N
  3. 50 N (correct answer)
  4. 100 N
Explanation: Physics problems involving force and motion are testing your understanding of Newton's second law: F=maF = ma, where force equals mass times acceleration. When you see questions asking about changing conditions, focus on how each variable affects the others. Let's start by finding the original force. With a 50 kg mass and 0.5 m/s² acceleration: F=ma=50 kg×0.5 m/s2=25 NF = ma = 50 \text{ kg} \times 0.5 \text{ m/s}^2 = 25 \text{ N}. Now, if the mass doubles to 100 kg but we want the same 0.5 m/s² acceleration: F=100 kg×0.5 m/s2=50 NF = 100 \text{ kg} \times 0.5 \text{ m/s}^2 = 50 \text{ N}. Looking at the wrong answers: A) 12.5 N represents halving the original force instead of doubling it—this misconception comes from incorrectly thinking that doubling mass requires less force. B) 25 N is the original force needed for the lighter mass, but this ignores the fact that the mass has doubled. D) 100 N comes from incorrectly thinking you need to double both the mass effect and something else, perhaps confusing this with momentum or kinetic energy formulas. The correct answer is C) 50 N because doubling the mass while maintaining the same acceleration requires exactly double the force. Remember this pattern: in F=maF = ma, force and mass are directly proportional when acceleration stays constant. If mass doubles, force must double. If mass triples, force triples. This direct relationship is key for HESI physics problems involving Newton's laws.

Question 4

A force of 40 N is applied to a 10 kg object, causing it to accelerate at 3 m/s². What is the magnitude of the frictional force acting on the object?

  1. 10 N (correct answer)
  2. 30 N
  3. 40 N
  4. 70 N
Explanation: When you encounter physics problems involving forces and acceleration, you need to consider that multiple forces can act on an object simultaneously. Newton's Second Law tells us that the net force equals mass times acceleration, not that any single applied force equals mama. Here, you have an applied force of 40 N, but the object only accelerates at 3 m/s². Using Newton's Second Law, the net force is: Fnet=ma=10 kg×3 m/s2=30 NF_{net} = ma = 10 \text{ kg} × 3 \text{ m/s}^2 = 30 \text{ N} Since the net force (30 N) is less than the applied force (40 N), there must be a force opposing the motion. The friction force works against the applied force: Fnet=Fapplied−FfrictionF_{net} = F_{applied} - F_{friction}, so 30=40−Ffriction30 = 40 - F_{friction}, giving us Ffriction=10 NF_{friction} = 10 \text{ N}. Answer A (10 N) correctly represents this opposing frictional force. Answer B (30 N) represents the net force, not the friction force specifically. Answer C (40 N) would mean friction equals the applied force, resulting in zero acceleration, which contradicts the given 3 m/s² acceleration. Answer D (70 N) incorrectly adds the applied force and net force, showing a misunderstanding of how forces combine. Remember for the HESI: when an object accelerates less than the applied force alone would predict, look for opposing forces like friction. Always distinguish between individual forces, net force, and the relationship Fnet=maF_{net} = ma. The net force determines acceleration, but individual forces can be much different.

Question 5

Two identical boxes, A and B, are on a frictionless surface. A constant force F is applied to box A, which then pushes box B. How does the magnitude of the force that box A exerts on box B compare to the magnitude of the force F?

  1. The force on box B is equal to F.
  2. The force on box B is greater than F.
  3. The force on box B is less than F. (correct answer)
  4. The relationship cannot be determined without knowing the mass.
Explanation: When you encounter problems involving forces and multiple objects, you need to analyze the system carefully using Newton's laws, particularly considering how forces are transmitted through connected objects. In this scenario, both boxes accelerate together as a single system under the applied force F. Since the boxes are identical (same mass m), the total mass being accelerated is 2m. Using Newton's second law, the acceleration of the entire system is a=F2ma = \frac{F}{2m}. Now, to find the force that box A exerts on box B, consider box B in isolation. Box B has mass m and accelerates at a=F2ma = \frac{F}{2m}. The only horizontal force acting on box B is the push from box A. Using Newton's second law for box B alone: FA on B=m×F2m=F2F_{A \text{ on } B} = m \times \frac{F}{2m} = \frac{F}{2}. Therefore, the force on box B is less than F, making C correct. Choice A incorrectly assumes the entire applied force is transmitted to box B, ignoring that box A also needs force to accelerate itself. Choice B defies physics—no mechanism exists to amplify the applied force. Choice D represents a common misconception; while mass affects acceleration, the force relationship can be determined through Newton's laws regardless of the specific mass values, as long as the masses are equal. Remember: when objects are connected and accelerating together, the force between them is always less than the total applied force because each object requires force for its own acceleration. Always consider the system as a whole first, then analyze individual components.

Question 6

If the net force on an object is doubled while its mass is held constant, what happens to its acceleration?

  1. It is quartered.
  2. It is halved.
  3. It is doubled. (correct answer)
  4. It is quadrupled.
Explanation: When you encounter physics problems involving force, mass, and acceleration, you're dealing with Newton's Second Law of Motion, which states that F=maF = ma, where F is net force, m is mass, and a is acceleration. To solve this problem, you need to understand the direct relationship between force and acceleration when mass remains constant. If we rearrange Newton's equation to solve for acceleration, we get a=Fma = \frac{F}{m}. This shows that acceleration is directly proportional to force when mass is constant. If the original force is F and produces acceleration a, then when the force is doubled (2F), the new acceleration becomes anew=2Fm=2×Fm=2aa_{new} = \frac{2F}{m} = 2 \times \frac{F}{m} = 2a. Therefore, doubling the force doubles the acceleration, making C correct. Looking at the wrong answers: A suggests the acceleration is quartered (divided by 4), which would only happen if you somehow confused force with mass and applied an inverse square relationship that doesn't exist here. B indicates the acceleration is halved, which would occur if you mistakenly thought force and acceleration were inversely related. D claims the acceleration is quadrupled, which might result from incorrectly applying a square relationship (22=42^2 = 4) rather than recognizing the direct proportionality. Remember this key principle: in F=maF = ma, force and acceleration have a direct, one-to-one relationship when mass is constant. Whatever factor changes the force will change the acceleration by exactly the same factor. This direct proportionality is fundamental to many physics problems on standardized exams.

Question 7

A car with mass M and a truck with mass 4M are traveling at the same velocity. They both apply their brakes and experience the same constant braking force. Which statement is true about the time it takes them to stop?

  1. The car will take four times as long to stop as the truck.
  2. The truck will take four times as long to stop as the car. (correct answer)
  3. They will both take the same amount of time to stop.
  4. The truck will take twice as long to stop as the car.
Explanation: When you encounter physics problems involving force and motion, focus on Newton's second law and the relationship between force, mass, and acceleration. Both vehicles experience the same braking force but have different masses. Using Newton's second law (F=maF = ma), we can find their accelerations. The car has acceleration acar=FMa_{car} = \frac{F}{M}, while the truck has acceleration atruck=F4M=14⋅FMa_{truck} = \frac{F}{4M} = \frac{1}{4} \cdot \frac{F}{M}. The truck's acceleration is one-fourth that of the car's. Since both start at the same velocity and must reach zero velocity, we can use v=v0+atv = v_0 + at (where final velocity = 0). Solving for time: t=−v0at = \frac{-v_0}{a}. The car stops in time tcar=v0F/Mt_{car} = \frac{v_0}{F/M}, while the truck stops in time ttruck=v0F/4M=4⋅v0F/Mt_{truck} = \frac{v_0}{F/4M} = 4 \cdot \frac{v_0}{F/M}. Therefore, the truck takes four times longer to stop. Answer A incorrectly reverses the relationship—it suggests the lighter object takes longer to stop. Answer C assumes mass doesn't matter, ignoring that the same force produces different accelerations on different masses. Answer D gives the wrong ratio, perhaps confusing the mass ratio (4:1) with the time ratio. Remember this key principle: when the same force acts on objects of different masses, the more massive object experiences less acceleration and takes longer to change its motion. The time ratio equals the mass ratio when forces are equal.

Question 8

A box is pushed across a floor with a constant applied force. If the box accelerates, which statement must be true regarding the forces acting on it?

  1. The applied force is equal to the force of friction.
  2. The applied force is greater than the force of friction. (correct answer)
  3. The applied force is less than the force of friction.
  4. The applied force is equal to the weight of the box.
Explanation: When you encounter physics problems involving forces and motion, remember Newton's second law: net force determines acceleration. If an object accelerates, the net force acting on it cannot be zero. For a box being pushed across a floor, two main horizontal forces act on it: the applied force (pushing forward) and friction (resisting backward). The net force equals applied force minus friction force. Since the box accelerates forward, this net force must be positive, meaning the applied force exceeds the friction force. Option B correctly states that the applied force is greater than the force of friction. This is the only way to produce the forward acceleration described in the problem. Option A suggests the applied force equals the friction force. If these forces were equal, they would cancel out, creating zero net force and zero acceleration. The box would move at constant velocity, not accelerate. Option C claims the applied force is less than friction. This would create a net force opposing motion, causing the box to decelerate (negative acceleration), which contradicts the given information. Option D states the applied force equals the box's weight. Weight acts vertically downward while the applied force acts horizontally. These forces are perpendicular and unrelated to each other in this scenario. Study tip: For HESI physics questions involving motion, always identify all forces acting on an object and determine the net force. Remember that acceleration requires an unbalanced force in the direction of acceleration.

Question 9

Two forces act on a 5 kg object. Force F1 is 15 N to the right, and force F2 is 25 N to the left. What is the magnitude and direction of the object's acceleration?

  1. 2 m/s² to the left (correct answer)
  2. 2 m/s² to the right
  3. 8 m/s² to the left
  4. 8 m/s² to the right
Explanation: When you encounter force and acceleration problems, you're applying Newton's second law: F=maF = ma. The key is recognizing that forces in opposite directions subtract from each other, and the net force determines both the magnitude and direction of acceleration. First, find the net force by considering direction. Taking rightward as positive: Force F1 = +15 N (right) and Force F2 = -25 N (left). The net force is Fnet=15N−25N=−10NF_{net} = 15 N - 25 N = -10 N. The negative sign indicates the net force points left. Next, apply Newton's second law: a=Fnetm=−10N5kg=−2m/s2a = \frac{F_{net}}{m} = \frac{-10 N}{5 kg} = -2 m/s². The magnitude is 2 m/s², and the negative sign confirms the direction is to the left. Looking at the wrong answers: Answer B (2 m/s² to the right) has the correct magnitude but wrong direction—this happens if you ignore which force is stronger. Answer C (8 m/s² to the left) results from incorrectly adding the forces (15 + 25 = 40 N) instead of finding their difference, then dividing by mass incorrectly. Answer D (8 m/s² to the right) combines both errors: adding forces and getting the direction wrong. The correct answer is A: 2 m/s² to the left. Remember this pattern: always establish a positive direction first, then subtract opposing forces to find the net force. The stronger force determines the direction of acceleration. On physics problems, magnitude and direction errors are common traps—work methodically through both components.

Question 10

Which of the following scenarios is the best example of Newton's First Law of Motion (inertia)?

  1. A rocket accelerates upwards by expelling gas downwards.
  2. It is more difficult to push a heavy cart than a light one.
  3. A patient lurches forward when a wheelchair suddenly stops. (correct answer)
  4. A ball thrown in the air follows a curved path back to the ground.
Explanation: Newton's First Law of Motion states that an object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by an external force. This principle of inertia is fundamental to understanding how objects resist changes to their motion. When you examine option C, you see a perfect demonstration of inertia in action. The patient's body is moving forward with the wheelchair at a constant velocity. When the wheelchair suddenly stops due to an external force (brakes or obstruction), the patient's body continues moving forward at the same velocity because no force has been applied directly to the patient to stop their motion. This forward lurching is inertia - the tendency to maintain the existing state of motion. Option A describes Newton's Third Law (action-reaction pairs), where the rocket pushes gas down and experiences an equal and opposite upward force. Option B illustrates Newton's Second Law (F=ma), showing how greater mass requires more force to achieve the same acceleration. Option D demonstrates projectile motion under gravitational influence, which involves Newton's Second Law as gravity continuously acts on the ball to curve its path. The key distinction is that inertia specifically refers to an object's resistance to changes in motion, not the forces causing acceleration or the action-reaction relationships between objects. Study tip: When identifying Newton's First Law on the HESI, look for scenarios involving sudden starts, stops, or direction changes where one object continues its original motion while another object changes. Think "resistance to change in motion."

Question 11

A block is sliding to the right on a surface with friction and is slowing down. Which direction is the net force on the block?

  1. To the right, in the direction of motion.
  2. To the left, opposite the direction of motion. (correct answer)
  3. Downward, due to gravity.
  4. There is no net force because the block is moving.
Explanation: When you encounter physics problems involving motion and forces, remember that Newton's First Law tells us an object will maintain constant velocity unless acted upon by a net force. Since this block is slowing down, its velocity is changing, which means there must be a net force acting on it. The correct answer is B because the net force must be opposite to the direction of motion. The block moves right but slows down, so the net force points left. This net force comes from friction, which always opposes motion. According to Newton's Second Law (F=maF = ma), when acceleration is opposite to velocity direction (deceleration), the net force must also be opposite to the velocity direction. Answer A is incorrect because if the net force were in the direction of motion, the block would speed up, not slow down. A rightward force would cause rightward acceleration, making the block go faster. Answer C misunderstands what "net force" means. While gravity does pull downward, it's balanced by the normal force from the surface. The question asks for the net force, which is the sum of all forces. The unbalanced horizontal forces (friction opposing motion) create the net force. Answer D reflects a common misconception that moving objects don't experience net forces. However, objects moving at constant velocity have zero net force, while objects changing speed (like this decelerating block) must have a net force. Remember: whenever you see an object changing speed or direction, there's always a net force involved. The net force direction matches the acceleration direction, not necessarily the motion direction.

Question 12

An object of mass 'm' is dropped from a great height. As it falls, it experiences air resistance, which increases with speed. The object reaches terminal velocity when:

  1. the force of gravity on the object becomes zero.
  2. the acceleration of the object is at its maximum.
  3. the upward force of air resistance equals the downward force of gravity. (correct answer)
  4. the object's mass begins to decrease due to air friction.
Explanation: When you encounter physics problems involving falling objects and air resistance, focus on the forces acting on the object and how they change during the fall. This is fundamentally about force equilibrium and Newton's laws. As an object falls, two main forces act on it: gravity (pulling downward) and air resistance (pushing upward). Initially, gravity is stronger, so the object accelerates downward. However, as the object's speed increases, air resistance grows stronger. Terminal velocity occurs when these opposing forces balance perfectly - the upward air resistance force equals the downward gravitational force. At this point, the net force is zero, so acceleration becomes zero, and the object falls at constant velocity. Looking at the wrong answers: Choice A incorrectly suggests gravity disappears, but gravitational force remains constant (mgmg) throughout the fall. Choice B misunderstands the physics - acceleration is actually at its maximum at the very beginning of the fall when air resistance is minimal, not at terminal velocity where acceleration is zero. Choice D introduces a nonsensical concept, as air friction doesn't reduce an object's mass. Choice C correctly identifies that terminal velocity occurs when the upward air resistance force equals the downward gravitational force, creating equilibrium. Study tip: For HESI physics questions about motion, always identify all forces acting on an object and remember that equilibrium (balanced forces) means zero acceleration, not zero motion. When forces balance, velocity becomes constant.

Question 13

An astronaut has a mass of 80 kg on Earth. What are the astronaut's mass and weight on the Moon, where the acceleration due to gravity is approximately 1/6th that of Earth (g ≈ 1.6 m/s²)?

  1. Mass = 13.3 kg, Weight = 128 N
  2. Mass = 80 kg, Weight = 128 N (correct answer)
  3. Mass = 80 kg, Weight = 784 N
  4. Mass = 13.3 kg, Weight = 21.3 N
Explanation: When you encounter physics problems involving different gravitational environments, remember that mass and weight are fundamentally different quantities. Mass measures the amount of matter in an object and remains constant regardless of location, while weight is the gravitational force acting on that mass and varies with gravitational field strength. The astronaut's mass stays 80 kg whether on Earth or the Moon because the amount of matter in their body doesn't change. To find weight, use the formula: Weight = mass × gravitational acceleration. On the Moon, where g=1.6 m/s2g = 1.6 \text{ m/s}^2, the calculation is: Weight = 80 kg×1.6 m/s2=128 N80 \text{ kg} \times 1.6 \text{ m/s}^2 = 128 \text{ N}. Choice A incorrectly divides the mass by 6, mistakenly thinking mass changes with gravity. This reflects a common misconception that mass and weight are the same thing. Choice C uses Earth's gravitational acceleration (9.8 m/s29.8 \text{ m/s}^2) instead of the Moon's, giving 80×9.8=784 N80 \times 9.8 = 784 \text{ N}. Choice D compounds two errors: it wrongly calculates mass as 13.3 kg, then multiplies by Moon's gravity to get 21.3 N. The correct answer is B: Mass = 80 kg, Weight = 128 N. For HESI success, always distinguish between mass (intrinsic property, measured in kg) and weight (gravitational force, measured in Newtons). When gravitational acceleration changes, only weight changes—never mass. This distinction appears frequently in physics and astronomy questions on standardized exams.

Question 14

A 10 kg object is initially at rest. A net force of 20 N acts on it for 4 seconds. If the force is then removed, which statement best describes the object's motion?

  1. The object will immediately stop moving.
  2. The object will continue to move at a constant velocity. (correct answer)
  3. The object will slow down and eventually stop due to inertia.
  4. The object will continue to accelerate at a constant rate.
Explanation: When you encounter physics problems involving forces and motion, think about Newton's laws of motion, particularly the first and second laws that govern how objects behave when forces are applied and removed. Let's work through this step-by-step. Initially, the 10 kg object is at rest. When the 20 N force acts for 4 seconds, we can find the final velocity using Newton's second law. First, calculate acceleration: a=F/m=20 N/10 kg=2 m/s2a = F/m = 20\text{ N}/10\text{ kg} = 2\text{ m/s}^2. After 4 seconds, the velocity becomes: v=at=2×4=8 m/sv = at = 2 \times 4 = 8\text{ m/s}. The key insight is what happens when the force is removed. According to Newton's first law (law of inertia), an object in motion stays in motion at constant velocity unless acted upon by an external force. Since the problem states the force is removed and doesn't mention friction or other forces, the object continues moving at 8 m/s indefinitely. Choice A is wrong because removing the force doesn't cause immediate stopping - that would require an opposing force. Choice C misunderstands inertia; inertia actually keeps objects moving, it doesn't slow them down. Slowing would require friction or air resistance. Choice D is incorrect because acceleration requires a net force, and we're told the force is removed. Remember this pattern: when forces are removed in idealized physics problems, objects continue at whatever velocity they had reached. The absence of force means zero acceleration, not zero velocity. Watch for problems that test whether you confuse force with motion itself.

Question 15

A small car and a large truck collide head-on. During the collision, which of the following statements is true?

  1. The force exerted by the truck on the car is greater than the force exerted by the car on the truck.
  2. The force exerted by the car on the truck is greater than the force exerted by the truck on the car.
  3. The forces exerted on each other are equal in magnitude. (correct answer)
  4. The truck experiences a greater acceleration than the car.
Explanation: When you encounter collision problems, you're dealing with Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. This fundamental principle applies regardless of the sizes or masses of the objects involved. During any collision, both objects exert forces on each other that are exactly equal in magnitude but opposite in direction. This is a universal law of physics - it doesn't matter if it's a small car hitting a massive truck, two cars of equal size, or even a bug hitting a windshield. The forces are always equal and opposite. Choice A incorrectly suggests the truck exerts a greater force on the car because of its larger size. This reflects a common misconception that bigger objects automatically exert bigger forces during collisions. Choice B makes the opposite error, implying the smaller car somehow exerts more force. Both of these violate Newton's Third Law. Choice D confuses force with acceleration. While the forces are equal, the accelerations are definitely not - the lighter car will experience much greater acceleration (and deceleration) than the heavy truck because acceleration equals force divided by mass (a=F/ma = F/m). Since the car has less mass but experiences the same force, it accelerates more dramatically. The correct answer is C - the forces are always equal in magnitude during any collision, regardless of the objects' sizes. Remember this key distinction for physics questions: equal forces doesn't mean equal effects. The smaller object will always experience greater acceleration and more dramatic consequences, even though the forces are identical.

Question 16

A 2 kg book rests on a table. The force of gravity pulls the book down with a force of approximately 20 N. What prevents the book from accelerating downwards?

  1. The book's inertia, which resists any change in motion.
  2. The reaction force of the book pulling up on the Earth.
  3. The normal force from the table pushing up on the book with 20 N. (correct answer)
  4. The force of air pressure pushing up on the bottom of the book.
Explanation: When you encounter physics problems about objects at rest, think about Newton's First Law and the concept of equilibrium. An object at rest stays at rest when all forces acting on it are balanced. The book remains stationary because the normal force from the table pushes upward with exactly 20 N, perfectly balancing the 20 N gravitational force pulling downward. This upward normal force is the table's response to the book's weight pressing down on it. When forces are balanced (net force = 0), there's no acceleration, so the book stays put. Let's examine why the other options miss the mark: Option A incorrectly suggests inertia prevents acceleration. While inertia describes an object's tendency to maintain its current state of motion, it doesn't actively prevent forces from causing acceleration. Inertia is a property, not a force that counteracts gravity. Option B mentions Newton's Third Law correctly—the book does pull up on Earth—but this reaction force acts on the Earth, not the book. It doesn't explain what prevents the book itself from falling. Option D overestimates air pressure's role. While air pressure exists, it's negligible compared to the book's weight and acts roughly equally on all surfaces of the book anyway. The key insight is recognizing that stationary objects require balanced forces. When you see equilibrium problems on the HESI, always look for the force that directly opposes the obvious one—in this case, the normal force opposing gravity.

Question 17

Increasing the mass of an object that is being pushed across a rough surface will increase the force of friction. This is because friction is directly proportional to:

  1. the acceleration of the object.
  2. the surface area in contact.
  3. the normal force. (correct answer)
  4. the applied force.
Explanation: When you encounter friction problems, remember that friction fundamentally depends on two factors: the nature of the surfaces in contact and how hard they're pressed together. The force of friction is calculated using the equation Ff=μNF_f = \mu N, where μ\mu is the coefficient of friction and NN is the normal force. The normal force represents how hard one surface presses against another. When you increase an object's mass, you increase its weight, which increases the normal force pressing it against the surface. Since friction is directly proportional to this normal force, more mass means more friction. Looking at the incorrect options: Choice A is wrong because acceleration doesn't determine friction force—friction actually opposes motion and can affect acceleration, but acceleration itself isn't a factor in the friction equation. Choice B is incorrect because friction depends on the normal force, not the contact area. Surprisingly, a wider tire doesn't necessarily create more friction than a narrower one if the normal force stays the same. Choice D is wrong because the applied force (the push you give the object) doesn't determine friction—friction exists based on the normal force regardless of how hard you push horizontally. The key insight is that friction is about how hard surfaces press together (normal force), not how fast they move, how much area touches, or how hard you push sideways. For HESI physics questions, always remember that friction problems come down to identifying what affects the normal force—and mass is the primary factor since weight equals mass times gravity.

Question 18

When a person is standing in an elevator that is accelerating upwards, their apparent weight is greater than their actual weight. This is because:

  1. the person's mass has increased due to the acceleration.
  2. the force of gravity on the person has increased.
  3. the floor must push up with a force greater than gravity to cause acceleration. (correct answer)
  4. the air pressure inside the elevator increases with upward motion.
Explanation: This question tests your understanding of Newton's laws of motion and the difference between weight and apparent weight. When analyzing elevator problems, focus on the forces acting on the person and how acceleration affects the normal force from the floor. When the elevator accelerates upward, the floor must exert an upward force on you that's greater than your weight to produce that acceleration. According to Newton's second law (F = ma), the net upward force equals your mass times the acceleration. Since you need a net upward force, the normal force from the floor must exceed the downward gravitational force. This extra force creates the sensation of feeling heavier – your apparent weight increases because the floor pushes up on you with greater force. Let's examine why the other options are incorrect. Choice A suggests your mass increases, but mass is an intrinsic property that doesn't change with acceleration – only the forces acting on you change. Choice B claims gravity increases, but gravitational force remains constant at Earth's surface regardless of the elevator's motion. Choice D mentions air pressure, but pressure changes in elevators are minimal and don't significantly affect your perceived weight. The key insight is that apparent weight depends on the normal force you feel, not your actual weight. When you accelerate upward, you feel heavier because the floor pushes harder against you. Study tip: Remember that in acceleration problems, always identify all forces and apply Newton's second law. The "feeling" of weight change comes from changes in contact forces, not from changes in gravity or mass.

Question 19

An object has a weight of 98 N on Earth (where g ≈ 9.8 m/s²). If a net force of 20 N is applied to this object on a frictionless surface, what is its acceleration?

  1. 0.20 m/s²
  2. 2.0 m/s² (correct answer)
  3. 4.9 m/s²
  4. 10.0 m/s²
Explanation: When you encounter physics problems involving forces and motion, you need to connect weight, mass, and Newton's second law. Weight is the gravitational force on an object, while mass is the amount of matter it contains. First, find the object's mass using the relationship W=mgW = mg. Since the weight is 98 N and g=9.8 m/s2g = 9.8 \text{ m/s}^2: m=Wg=98 N9.8 m/s2=10 kgm = \frac{W}{g} = \frac{98 \text{ N}}{9.8 \text{ m/s}^2} = 10 \text{ kg} Now apply Newton's second law, F=maF = ma, to find acceleration: a=Fm=20 N10 kg=2.0 m/s2a = \frac{F}{m} = \frac{20 \text{ N}}{10 \text{ kg}} = 2.0 \text{ m/s}^2 Looking at the wrong answers: Choice A (0.20 m/s²) likely results from incorrectly dividing the applied force by the weight instead of the mass, giving 2098≈0.20\frac{20}{98} ≈ 0.20. Choice C (4.9 m/s²) comes from mistakenly using half the gravitational acceleration value or making calculation errors with the given numbers. Choice D (10.0 m/s²) represents the object's mass value but incorrectly applied as acceleration, showing confusion between different physical quantities. The correct answer is B (2.0 m/s²). Study tip: Always distinguish between weight (force in Newtons) and mass (kilograms) in physics problems. Weight depends on gravity's strength, but mass stays constant. When applying forces, you need mass for Newton's second law, so convert weight to mass first using m=W/gm = W/g.