GED Science Quiz: Apply Motion And Force

What this quiz covers

This quiz focuses on Apply Motion And Force, giving you a quick way to practice the rules, question types, and explanations that matter most for GED Science.

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.

Question 1

1. The First Law, because the skater's inertia keeps them moving.
2. The Second Law, because the force causes the skater to accelerate.
3. The Third Law, because the wall pushes back on the skater. (correct answer)
4. The Law of Universal Gravitation, because it holds the skater on the ice.

Explanation: When you encounter questions about forces and motion, focus on identifying the specific cause-and-effect relationship being described. This scenario involves action-reaction pairs, which is the hallmark of Newton's Third Law. The correct answer is C because Newton's Third Law states that for every action, there is an equal and opposite reaction. When the skater pushes against the wall (action force), the wall simultaneously pushes back on the skater with equal force in the opposite direction (reaction force). This reaction force from the wall is what propels the skater backward across the ice. Let's examine why the other options miss the mark. Option A incorrectly identifies the cause of motion - while the First Law explains why the skater continues gliding after pushing off (inertia), it doesn't explain what initially caused the backward motion. The skater was at rest before pushing the wall. Option B focuses on acceleration, but the Second Law (F=ma) describes how forces affect motion rather than explaining the source of the force that moves the skater. Option D mentions gravitational force, which does keep the skater on the ice surface, but gravity acts downward and doesn't explain the horizontal backward motion. For GED Science questions about Newton's Laws, look for these key indicators: First Law involves objects maintaining their state of motion, Second Law connects force with acceleration and mass, and Third Law always involves paired forces between two objects. When you see "pushes off" or "recoil," think Third Law - there's always a reaction force doing the work.

Question 2

1. The table pushing up on the book with an equal and opposite force.
2. The air pressure pushing down on the top surface of the book.
3. The friction between the book and the table that prevents it from sliding.
4. The book pulling up on the entire Earth with an equal and opposite force. (correct answer)

Explanation: Newton's Third Law states that for every action, there is an equal and opposite reaction. The key insight is identifying which forces are actually paired together as action-reaction forces. When Earth's gravity pulls the book downward, the reaction force must be the book pulling back on Earth with equal magnitude but in the opposite direction. This is exactly what choice D describes. The gravitational force between two objects is always mutual - Earth pulls on the book, and simultaneously, the book pulls on Earth with the same strength upward. Now let's examine why the other options miss the mark. Choice A describes the normal force from the table pushing up on the book. While this force does balance the gravitational force (keeping the book at rest), it's not the reaction force to gravity. The normal force is a contact force between the book and table, completely separate from the gravitational interaction between book and Earth. Choice B identifies air pressure, which is unrelated to gravitational forces and doesn't form an action-reaction pair with gravity. Choice C mentions friction, but friction only exists when there's sliding or attempted sliding - it's not the reaction to gravitational pull. The common mistake here is confusing "balanced forces" with "action-reaction pairs." Forces can balance without being reaction pairs. The table's upward push balances gravity, but it's not gravity's reaction force. Remember: action-reaction pairs always involve the same type of force acting between the same two objects, just in opposite directions. Look for forces acting on the same objects, not just forces that balance each other.

Question 3

1. The force of the gases pushing on the rocket is equal in magnitude and opposite in direction to the force of the rocket pushing on the gases. (correct answer)
2. The force of the rocket pushing on the gases is greater than the force of the gases pushing on the rocket.
3. The hot gases push against the surrounding air, which in turn pushes the rocket forward with an equal force.
4. The gravitational force on the rocket is the reaction force to the thrust produced by the engine.

Explanation: When you encounter rocket propulsion questions, you're dealing with Newton's Third Law of Motion: for every action, there's an equal and opposite reaction. The key insight is that rockets don't need air to push against—they create their own reaction force. In rocket propulsion, the rocket engine pushes hot gases downward and backward with tremendous force. According to Newton's Third Law, those gases simultaneously push back on the rocket with exactly the same magnitude of force, but in the opposite direction (upward). This reaction force is what propels the rocket forward. The forces form what physicists call an "action-reaction pair"—they're always equal in magnitude and opposite in direction. Choice A correctly describes this relationship. The rocket pushes on the gases with force X downward, and the gases push on the rocket with force X upward. Choice B violates Newton's Third Law by suggesting unequal forces. Action-reaction pairs are always equal in magnitude—this is a fundamental principle. Choice C reflects a common misconception that rockets need air to push against. This is wrong because rockets work perfectly in the vacuum of space where there's no air. The rocket creates its own reaction mass through expelled gases. Choice D confuses two completely different forces. Gravity pulls the rocket downward, while engine thrust pushes it upward. These aren't action-reaction pairs—they're separate forces acting on the same object. Remember: Newton's Third Law pairs always involve two different objects acting on each other with equal and opposite forces. Look for this reciprocal relationship in physics problems.

Question 4

1. They will move backward with a velocity that depends on the mass of the ball and person. (correct answer)
2. They will remain stationary because their mass is greater than the ball's.
3. They will move forward in the same direction as the ball, but more slowly.
4. They will move backward only if the skateboard has frictionless wheels.

Explanation: When you see a physics problem involving objects moving in opposite directions after an interaction, think about Newton's Third Law and conservation of momentum. These principles govern how forces and motion work when objects push or pull on each other. When the person throws the ball forward, they exert a force on the ball. By Newton's Third Law, the ball exerts an equal and opposite force back on the person. This backward force will push the person and skateboard in the opposite direction from the ball's motion. The velocity of the person depends on both their mass and the ball's mass - specifically, the momentum transferred. Since momentum is conserved, the person's momentum backward equals the ball's momentum forward. A heavier ball or lighter person results in greater backward velocity for the person. Choice A correctly identifies that the person moves backward with velocity depending on the masses involved. Choice B is wrong because mass alone doesn't determine whether motion occurs - even a light ball can cause backward motion if thrown hard enough. The key is momentum transfer, not just mass comparison. Choice C incorrectly suggests the person moves forward, which violates Newton's Third Law - forces always act in opposite directions. Choice D is wrong because this backward motion happens regardless of friction. While friction might slow the skateboard down after the throw, it doesn't prevent the initial backward motion from occurring. Remember: whenever objects interact by pushing or pulling, they always exert equal and opposite forces on each other, causing motion in opposite directions.

Question 5

1. 0 kg·m/s (correct answer)
2. 1,200 kg·m/s
3. Approximately 11,760 kg·m/s, due to gravity.
4. It cannot be determined without knowing the car's potential energy.

Explanation: When you encounter momentum problems, remember that momentum measures how much motion an object has. Momentum is calculated using the formula $p = mv$, where p is momentum, m is mass, and v is velocity. In this problem, you have a 1,200 kg car that is "at rest" in a parking lot. The key phrase here is "at rest," which means the car's velocity is zero. When you substitute into the momentum formula: $p = (1,200 \text{ kg})(0 \text{ m/s}) = 0 \text{ kg·m/s}$. Any number multiplied by zero equals zero, so the momentum is zero. Let's examine why the other answers are incorrect. Answer B (1,200 kg·m/s) incorrectly assumes the car has some velocity, perhaps by mistaking mass for momentum or forgetting that velocity is zero. Answer C (approximately 11,760 kg·m/s due to gravity) represents a fundamental misunderstanding—while gravity acts on the car, it doesn't contribute to momentum when the car isn't moving. The number 11,760 appears to come from multiplying mass by gravitational acceleration (9.8 m/s²), but that would give you weight or force, not momentum. Answer D (cannot be determined without potential energy) confuses momentum with energy concepts—potential energy is completely irrelevant to calculating momentum. For momentum problems on the GED, always identify the object's velocity first. If something is at rest, stopped, or stationary, its momentum is automatically zero regardless of its mass. Don't let large masses fool you into thinking there must be momentum without motion.

Question 6

1. The acceleration is doubled from its original value. (correct answer)
2. The acceleration remains unchanged but the velocity increases.
3. The acceleration is reduced to one-half of its original value.
4. The acceleration is quadrupled from its original value.

Explanation: When you encounter questions about force and motion, you're working with Newton's Second Law of Motion, which states that force equals mass times acceleration: $F = ma$. This fundamental relationship tells us how force, mass, and acceleration are connected. To solve this problem, you need to analyze what happens when force changes while mass stays constant. From Newton's Second Law, we can rearrange the equation to solve for acceleration: $a = \frac{F}{m}$. This shows that acceleration is directly proportional to force when mass remains unchanged. If you double the net force (make it 2F) while keeping mass the same, the new acceleration becomes: $a_{new} = \frac{2F}{m} = 2 \times \frac{F}{m} = 2a_{original}$. Therefore, doubling the force doubles the acceleration, making choice A correct. Let's examine why the other options are wrong. Choice B incorrectly suggests acceleration stays the same while velocity increases—this misses that changing force must change acceleration according to Newton's Second Law. Choice C states acceleration is cut in half, which would only happen if you doubled the mass instead of the force. Choice D claims acceleration quadruples, but this confuses the direct proportional relationship—quadrupling would require the force to increase four times, not double. For GED Science success, remember that Newton's Second Law problems often test whether you understand direct versus inverse relationships. Force and acceleration have a direct relationship (one doubles, the other doubles), while mass and acceleration have an inverse relationship (one doubles, the other halves).

Question 7

1. A feather floating in the air.
2. A large, stationary cargo ship. (correct answer)
3. A student sitting at a desk.
4. A baseball traveling at 90 miles per hour.

Explanation: Inertia questions test your understanding that inertia depends solely on an object's mass, not its motion or position. Inertia is an object's resistance to changes in its state of motion—the more massive an object, the greater its inertia. A large, stationary cargo ship (B) has the greatest inertia because it has by far the most mass among these options. Cargo ships typically weigh thousands of tons, making them extremely resistant to changes in motion. This is why massive ships need powerful tugboats to start moving and require long distances to stop. Let's examine why the other choices have less inertia: A feather (A) has very little mass, so it has minimal inertia despite appearing to "resist" motion due to air resistance—that's a force effect, not inertia. A student sitting at a desk (C) has moderate mass compared to the other objects, giving them moderate inertia, but nowhere near that of a massive ship. A baseball traveling at 90 mph (D) is the trickiest distractor because its high speed might seem important, but inertia depends only on mass, not velocity. The baseball's small mass means it has relatively little inertia regardless of how fast it's moving. Remember this key principle: when comparing inertia, ignore motion and focus entirely on mass. The most massive object always has the greatest inertia. On the GED, watch for questions that try to confuse you by mentioning speed or describing objects in motion—inertia is purely about mass.

Question 8

1. The object must be stationary and not moving at all.
2. The object must be slowing down and coming to a stop.
3. The object must be accelerating at a constant rate.
4. The object's velocity must be constant (which includes being at rest). (correct answer)

Explanation: When you encounter questions about net force and motion, you're dealing with Newton's First Law of Motion, also known as the law of inertia. This fundamental principle states that when the net force on an object is zero, the object will maintain its current state of motion. The correct answer is D because zero net force means zero acceleration (from Newton's Second Law: $F = ma$). When acceleration is zero, velocity cannot change - it must remain constant. This constant velocity could be any value, including zero (which means the object is at rest). So whether an object is moving at 5 m/s or sitting completely still, both situations are possible when net force equals zero. Let's examine why the other options are incorrect. Choice A is too restrictive - it assumes the only possibility is being stationary, but an object can maintain any constant velocity when net force is zero. Choice B describes deceleration, which requires a net force opposite to the direction of motion, contradicting our zero net force condition. Choice C describes constant acceleration, which is impossible when net force is zero since acceleration equals net force divided by mass. Remember this key relationship: zero net force always equals zero acceleration, which always equals constant velocity. On the GED Science exam, questions about forces and motion often test whether you understand that "constant velocity" includes both moving at steady speed and being at rest - both are valid states when forces are balanced.

Question 9

1. The force of gravity pulling the crate down is greater than the applied horizontal force.
2. The crate's inertia is too large for the applied force to overcome its resting state.
3. The force of static friction between the crate and the floor is equal and opposite to the applied force. (correct answer)
4. The person is not applying enough force to counteract the crate's total weight.

Explanation: When you encounter a question about forces and motion, think about Newton's laws and how forces interact to create equilibrium or movement. In this scenario, the crate remains stationary because all forces acting on it are balanced. Since the person applies a horizontal force but the crate doesn't move, there must be an equal and opposite force preventing motion. This opposing force is static friction between the crate and floor, which automatically adjusts to match the applied force up to its maximum limit. The correct answer is C because static friction provides exactly the right amount of resistance to keep the crate in equilibrium. Let's examine why the other options are incorrect. Option A confuses vertical and horizontal forces – gravity acts downward while the applied force acts horizontally, so they don't directly oppose each other. Option B misapplies the concept of inertia. While inertia explains why objects at rest tend to stay at rest, it's not a force that actively resists motion; rather, it's friction that provides the actual opposing force. Option D incorrectly relates horizontal force to vertical weight. The person doesn't need to overcome the crate's weight to move it horizontally – they need to overcome friction. For GED Science questions involving forces, remember that when an object remains stationary despite an applied force, look for the specific force that's providing resistance. Static friction is often the answer when dealing with objects on surfaces, as it's the force that prevents sliding motion.

Question 10

1. The gravitational force between the satellite and the Earth. (correct answer)
2. The satellite's engine thrust, which pushes it forward continuously.
3. The outward centrifugal force balancing the satellite's weight.
4. The force from the initial launch that keeps it moving in a circle.

Explanation: When you encounter questions about orbital mechanics, focus on identifying the fundamental forces at work. Satellites stay in orbit because they're constantly falling toward Earth while moving fast enough sideways that they keep missing it. The gravitational force between the satellite and Earth provides the centripetal force needed to keep the satellite moving in a circular path. This attractive force continuously pulls the satellite toward Earth's center, causing its straight-line motion to curve into an orbit. Without gravity, the satellite would fly off in a straight line into space. Let's examine why the other options are incorrect: Option B suggests engine thrust keeps the satellite moving. Once in orbit, satellites don't need continuous thrust - they maintain their speed due to Newton's first law of motion (objects in motion stay in motion). Engines are only used for occasional adjustments. Option C mentions centrifugal force balancing weight. Centrifugal force isn't a real force - it's what you feel when you're in a rotating reference frame. The actual physics involves centripetal force (provided by gravity) pulling inward. Option D claims the initial launch force maintains the orbit. The launch only gets the satellite to the right speed and altitude. Once there, the launch force is gone, and gravity alone maintains the orbital motion. Remember this key principle: in orbital mechanics, gravity isn't the enemy of staying in space - it's what makes orbits possible. When you see questions about satellites, planets, or moons, always consider gravity as the force providing the centripetal acceleration needed for circular motion.

Question 11

1. unbalanced, because the bowstring is applying a force.
2. unbalanced, because of the arrow's potential energy.
3. balanced, because the arrow is not moving. (correct answer)
4. balanced, because gravity is not acting on the arrow.

Explanation: When you encounter physics problems about forces and motion, the key is understanding Newton's First Law: an object at rest or moving at constant velocity has balanced forces acting on it. In this scenario, the arrow is stationary (not moving) while the archer holds it steady. This means all forces acting on the arrow must be balanced - they cancel each other out. The bowstring pulls the arrow backward, but this force is exactly balanced by the archer's hand pushing forward to keep the arrow in position. Gravity pulls the arrow downward, but the archer's grip provides an upward force to counteract this. Since the net force is zero, the forces are balanced. Looking at the wrong answers: Choice A incorrectly assumes that because the bowstring applies a force, the forces must be unbalanced. However, a force can exist without creating unbalanced conditions if other forces counteract it. Choice B confuses energy with force - potential energy refers to stored energy in the system, not whether forces are balanced or unbalanced. These are completely different physics concepts. Choice D makes a false claim that gravity isn't acting on the arrow. Gravity always acts on objects with mass; it's just being balanced by other forces in this situation. Remember this pattern: if an object isn't accelerating (speeding up, slowing down, or changing direction), then the forces acting on it are balanced, regardless of how many individual forces might be present. Focus on the object's motion, not just the presence of forces.

Question 12

1. Newton's Third Law, which states that forces occur in equal and opposite pairs.
2. The law of conservation of energy, which relates to energy transformation.
3. Newton's First Law of Motion, which describes an object's inertia. (correct answer)
4. The principle of universal gravitation, which describes the attraction between masses.

Explanation: When you encounter questions about objects in motion and what happens when forces change, you're dealing with Newton's Laws of Motion. The key is identifying which specific law explains the observed behavior. In this scenario, the passenger continues moving forward at the car's original speed even after the car brakes. This perfectly demonstrates Newton's First Law of Motion, also known as the law of inertia. This law 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. The passenger was moving with the car, and when the car suddenly stops, the passenger's body has no force acting on it to make it stop, so it continues moving forward. Looking at the wrong answers: Choice A, Newton's Third Law, deals with action-reaction force pairs (like when you push a wall, it pushes back with equal force), which isn't what's happening here. Choice B, conservation of energy, involves energy changing from one form to another, but this question is about motion continuing due to inertia, not energy transformation. Choice D, universal gravitation, explains the attractive force between masses like Earth and objects, but gravity isn't the primary factor causing the forward motion. For GED Science questions about motion, remember that inertia problems typically involve objects continuing their motion when external conditions change suddenly. Look for scenarios where something keeps moving in the same direction when forces are removed or changed - that's usually Newton's First Law.

Question 13

1. The truck exerts a greater force on the car than the car exerts on the truck.
2. The car and the truck exert equal and opposite forces on each other. (correct answer)
3. The car exerts a greater force on the truck than the truck exerts on the car.
4. The net force on the system is zero, so no forces are exerted.

Explanation: When you encounter collision problems, you're dealing with Newton's Third Law of Motion, which states that for every action, there's an equal and opposite reaction. This fundamental principle applies to all interactions between objects, regardless of their size or mass. During the head-on collision between the car and truck, both vehicles exert forces on each other simultaneously. According to Newton's Third Law, these forces must be equal in magnitude but opposite in direction. The truck pushes on the car with exactly the same force that the car pushes back on the truck. This might seem counterintuitive since the truck is much larger, but force pairs are always equal. Answer B correctly states that the car and truck exert equal and opposite forces on each other, which is precisely what Newton's Third Law requires. Answer A is wrong because it assumes the larger object exerts more force, confusing force with the effects of force. While the truck may cause more damage due to its greater mass, the actual forces during collision are equal. Answer C makes the opposite error, incorrectly suggesting the smaller car somehow exerts more force. Answer D misunderstands the concept entirely. While the net force on the entire system might be zero, this doesn't mean no forces exist between the objects—it means the forces cancel out when considering the whole system. Remember this key distinction: Newton's Third Law governs the forces between objects (always equal and opposite), while Newton's Second Law (F=ma) explains why those equal forces produce different effects on objects with different masses.

Question 14

1. Both vehicles will stop in the same amount of time.
2. The truck will stop more quickly because it has more momentum.
3. The truck will stop more quickly because of its larger tires.
4. The sports car will stop more quickly because it has less inertia. (correct answer)

Explanation: When you encounter physics problems involving forces and motion, focus on Newton's second law and the concept of inertia. This question tests your understanding of how mass affects an object's resistance to changes in motion. Since both vehicles experience the same braking force and start at the same speed, we can use Newton's second law: $F = ma$. Rearranging gives us $a = F/m$. With identical forces but different masses, the lighter sports car will experience greater deceleration than the heavier truck. Greater deceleration means the sports car reaches zero velocity faster. This happens because the sports car has less inertia—its smaller mass makes it easier to change its motion. Looking at the wrong answers: Choice A incorrectly suggests mass doesn't matter when the same force is applied. In reality, mass is crucial in determining acceleration. Choice B makes a fundamental error about momentum. While the truck does have more momentum initially, this actually works against it—more momentum means it's harder to stop, not easier. The truck's greater momentum requires more time to overcome with the same braking force. Choice C focuses on tire size, which isn't relevant to this physics problem. The question specifies identical braking forces, so factors like tire contact aren't being considered. Remember this key relationship: for identical forces, lighter objects accelerate more than heavier ones. On GED science questions, when you see scenarios comparing objects of different masses under the same force, immediately think about how mass inversely affects acceleration.

Question 15

1. Box B will accelerate at a greater rate than Box A.
2. Box A will accelerate at a greater rate than Box B. (correct answer)
3. Both boxes will accelerate at the same rate because the force is the same.
4. Neither box will accelerate unless the surface has friction.

Explanation: When you encounter physics problems involving force and motion, think about Newton's Second Law: Force equals mass times acceleration, or $F = ma$. This fundamental relationship tells us how force, mass, and acceleration are connected. Since the same constant force is applied to both boxes, we can rearrange the equation to solve for acceleration: $a = \frac{F}{m}$. This shows that acceleration is inversely proportional to mass when force remains constant. Box A (10 kg) will experience acceleration of $a_A = \frac{F}{10}$, while Box B (20 kg) will experience acceleration of $a_B = \frac{F}{20}$. Since Box A has half the mass of Box B, it will accelerate twice as fast. Therefore, Box A will accelerate at a greater rate than Box B, making answer B correct. Answer A is wrong because it suggests the heavier object accelerates faster, which contradicts Newton's Second Law. Answer C represents a common misconception—while the applied force is identical, the resulting acceleration depends on both force AND mass. Equal forces don't produce equal accelerations when masses differ. Answer D is incorrect because objects can definitely accelerate on frictionless surfaces; friction isn't required for acceleration to occur when a force is applied. Remember this key relationship: when the same force acts on objects of different masses, the lighter object always accelerates more. On the GED, physics problems often test whether you can apply Newton's Second Law correctly, so practice rearranging $F = ma$ to solve for the unknown variable.

Question 16

1. the force of gravity on the skydiver becomes zero.
2. the skydiver's mass is exactly balanced by her volume.
3. the upward force of air resistance equals the downward force of gravity. (correct answer)
4. the skydiver has fallen for a specific amount of time, usually one minute.

Explanation: When objects fall through air, they experience two competing forces that determine their motion. Understanding how these forces interact is key to solving terminal velocity problems. As a skydiver falls, gravity pulls her downward with a constant force equal to her weight. However, air resistance pushes upward against her motion, and this resistance force increases as her speed increases. Initially, gravity is stronger than air resistance, so she accelerates downward. But as she falls faster, air resistance grows until it eventually equals the gravitational force. At this point, the net force becomes zero, acceleration stops, and she continues falling at a constant speed—terminal velocity. The correct answer is C because terminal velocity occurs precisely when the upward air resistance force equals the downward gravitational force, creating equilibrium. Option A is wrong because gravity never becomes zero—it remains constant throughout the fall. Option B incorrectly relates mass and volume, which have nothing to do with force balance during terminal velocity. The relationship between mass and volume determines density, not terminal velocity. Option D is incorrect because terminal velocity depends on force balance, not a predetermined time period. Different objects with different shapes, masses, and surface areas reach terminal velocity at different times. Remember this key principle: terminal velocity always results from balanced forces, not from gravity disappearing or time limits. Look for force equilibrium in any terminal velocity question on the GED.

Question 17

1. A helicopter hovering motionless in the air.
2. A train traveling in a straight line at a constant speed of 80 mph.
3. A book resting motionless on a tabletop.
4. A satellite orbiting Earth at a constant speed of 17,000 mph. (correct answer)

Explanation: When you encounter questions about acceleration, remember that acceleration isn't just about speeding up—it's any change in velocity, which includes changes in speed OR direction. The satellite in option D is the correct answer because even though it maintains a constant speed of 17,000 mph, it's constantly changing direction as it follows its circular orbital path around Earth. Since velocity is a vector quantity (having both magnitude and direction), any change in direction constitutes acceleration. This type is called centripetal acceleration, where the object accelerates toward the center of the circular path. Let's examine why the other options don't show acceleration. Option A describes a motionless helicopter—with zero velocity and no change in motion, there's no acceleration. Option B shows a train moving in a straight line at constant speed, meaning both the magnitude and direction of velocity remain unchanged, so acceleration is zero. Option C presents a stationary book, which like the helicopter, has no motion and therefore no acceleration. The key trap here is assuming acceleration only occurs when objects speed up or slow down. Many students incorrectly eliminate option D because they focus on the "constant speed" and miss that the direction is continuously changing. Remember this pattern: any object moving in a curved path—whether it's a car turning a corner, a planet orbiting the sun, or a ball swinging on a string—is accelerating due to its changing direction, even if its speed stays the same.

Question 18

1. In the direction of the car's motion (tangent to the circle).
2. Toward the center of the circle. (correct answer)
3. Away from the center of the circle.
4. There is no net force because the speed is constant.

Explanation: When you see questions about objects moving in circular paths, you're dealing with circular motion and centripetal force. The key insight is that even when speed is constant, the velocity is constantly changing because direction is changing. For any object moving in a circle, there must be a net force pointing toward the center of the circle. This centripetal force is what keeps the object moving in its circular path rather than flying off in a straight line. In the case of the toy car, this inward force could come from friction between the wheels and the surface, tension in a string, or the banking of a track. Choice A is incorrect because a force in the direction of motion (tangent to the circle) would change the car's speed, not its direction. Since the speed is constant, the net force cannot be tangential. Choice C is wrong because a force pointing away from the center would cause the object to spiral outward, not maintain a circular path. Choice D represents a common misconception. Many students think that constant speed means no acceleration and therefore no net force. However, acceleration includes changes in direction, not just changes in speed. Since the car is constantly changing direction as it moves in a circle, it is accelerating and must have a net force acting on it. Remember: In circular motion problems, always look for the force that points toward the center of the circle. Constant speed doesn't mean no force—it means the force is perpendicular to the motion.

Question 19

1. The force of gravity pulling the person back toward the Earth's surface.
2. The resistance of the air pushing against the person's body as they rise.
3. The force of the person's muscles contracting to propel them upward.
4. The Earth exerting an equal and opposite upward force on the person. (correct answer)

Explanation: Newton's Third Law states that for every action, there is an equal and opposite reaction. When analyzing force pairs, you need to identify two objects exerting forces directly on each other. When you jump, your legs push down on the Earth with a certain force. According to Newton's Third Law, the Earth must simultaneously push back up on you with an equal force in the opposite direction. This upward force from the Earth is what actually propels you into the air. The action-reaction pair consists of: (1) your legs pushing down on Earth, and (2) Earth pushing up on your legs. Answer D correctly identifies this reaction force - the Earth exerting an equal and opposite upward force on the person. Answer A describes gravity, which is a separate force that acts on you throughout the jump. Gravity isn't the reaction to your legs pushing on Earth; it's an independent attractive force between you and Earth that exists whether you're jumping or not. Answer B refers to air resistance, which only occurs as you move through the air and isn't directly related to your legs pushing off the ground. Air resistance is a friction-like force, not part of the action-reaction pair from your jump. Answer C describes the internal force of your muscles, but Newton's Third Law deals with forces between different objects, not internal forces within your body. Your muscle contraction is what enables the push, but isn't the reaction force. Remember: Newton's Third Law always involves two different objects pushing or pulling on each other with equal and opposite forces simultaneously.

Question 20

1. The car has a constant velocity and is therefore not accelerating.
2. The car has zero acceleration because its speed is not changing.
3. The car's speed is changing, but its velocity remains constant.
4. The car's velocity is changing, and therefore it is accelerating. (correct answer)

Explanation: When you encounter physics problems about objects changing direction, the key distinction to understand is between speed and velocity. Speed is how fast something moves, while velocity includes both speed AND direction. The car in this problem maintains a constant speed of 50 mph, but since it's rounding a curve, its direction is continuously changing. Because velocity depends on both speed and direction, the car's velocity is changing even though its speed stays the same. According to physics, any change in velocity means the object is accelerating - this includes speeding up, slowing down, OR changing direction. Looking at the wrong answers: Choice A incorrectly assumes that constant speed means constant velocity, ignoring the direction change. Choice B makes the common mistake of thinking acceleration only occurs when speed changes - but acceleration actually occurs whenever velocity changes in any way. Choice C has it backwards, claiming speed changes while velocity stays constant, which contradicts the given information about constant speed. Choice D correctly identifies that the car's velocity is changing (due to the direction change) and therefore the car is accelerating. This type of acceleration is called centripetal acceleration, which always points toward the center of the curved path. Remember this key principle for GED science questions: acceleration doesn't just mean "going faster." Any change in velocity - whether in magnitude, direction, or both - means acceleration is occurring. Watch for problems involving circular motion, curved paths, or direction changes where this concept applies.