AP Physics 1 Quiz: Fluids And Newtons Laws

What this quiz covers

This quiz focuses on Fluids And Newtons Laws, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Physics 1.

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. Net force is upward because it is moving upward.
2. Net force is zero, so buoyant force equals $mg$. (correct answer)
3. Buoyant force must be greater than $mg$ to keep it moving.
4. Buoyant force depends only on mass, so it increases after the push.

Explanation: This question involves equilibrium analysis for neutrally buoyant objects after external impulses. The sphere experiences buoyant force upward and weight mg downward, with negligible drag. Since the sphere moves upward at constant speed after the push, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the buoyant force must equal mg for force balance. Choice A incorrectly assumes upward motion requires upward net force, but constant velocity motion requires zero net force.

Question 2

1. The buoyant force equals $mg$, so the net force is zero. (correct answer)
2. The buoyant force is greater than $mg$ because the cube is rising.
3. The net force is upward because motion upward requires upward net force.
4. The buoyant force depends only on the cube's mass, so it must be constant.

Explanation: This question examines force analysis when an object moves at constant velocity in a fluid. The cube experiences buoyant force upward and weight mg downward, with negligible drag initially. Since the cube rises at constant speed, its acceleration is zero, requiring zero net force by Newton's first law. This means the upward buoyant force must exactly equal the downward weight mg. Choice B incorrectly assumes upward motion requires upward net force, but constant velocity motion requires zero net force regardless of direction.

Question 3

1. Buoyant force equals $mg$. (correct answer)
2. Buoyant force is half of $mg$ because half the volume is submerged.
3. Net force is upward because some volume is above water.
4. Buoyant force depends only on mass, so submergence fraction is irrelevant.

Explanation: This question examines equilibrium analysis for partially submerged floating objects. The object floats at rest with half its volume submerged, experiencing buoyant force upward and weight mg downward. Since the object is in equilibrium (zero acceleration), Newton's first law requires zero net force, meaning the buoyant force must exactly equal mg regardless of submersion fraction. Choice B incorrectly assumes buoyant force scales linearly with submerged fraction, but equilibrium demands complete force balance.

Question 4

1. The buoyant force is less than $mg$ because the block is only partially submerged.
2. The net force is upward because the block is floating.
3. The buoyant force equals $mg$. (correct answer)
4. There are no forces on the block because it is at rest.

Explanation: This question tests equilibrium analysis for floating objects in fluids. The wooden block floats at rest, experiencing buoyant force upward and weight mg downward with no other vertical forces. Since the block is in equilibrium (zero acceleration), Newton's first law requires the net force to be zero, meaning upward and downward forces must balance exactly. Therefore, the buoyant force must equal mg. Choice A incorrectly assumes partial submersion reduces buoyant force below mg, but equilibrium demands force balance regardless of submersion fraction.

Question 5

1. Net force is downward because velocity is downward.
2. Net force is zero, so $mg = F_B + F_D$. (correct answer)
3. Buoyant force equals $mg$ because acceleration is zero.
4. Drag must be downward because the block is moving downward.

Explanation: This question tests equilibrium analysis when objects have zero acceleration while moving. The block experiences weight mg downward, buoyant force upward, and drag upward. Since the block has zero acceleration at that instant, Newton's first law requires zero net force, meaning upward forces must equal downward forces. Therefore, mg = FB + FD, with weight balancing the sum of buoyant force and drag. Choice C incorrectly assumes buoyant force alone equals weight, ignoring the additional upward drag force.

Question 6

1. The net force is zero because the ball is already moving upward.
2. The net force is upward because the buoyant force is greater than $mg$. (correct answer)
3. The net force is downward because the ball is less massive than the displaced water.
4. The buoyant force equals the ball's mass times $g$ because it is a floating object.

Explanation: This question tests understanding of net force analysis in fluids when objects undergo acceleration. The ball experiences buoyant force upward and weight mg downward, and since it's moving upward and speeding up, the acceleration is upward. By Newton's second law, upward acceleration requires upward net force, which means the buoyant force must exceed the weight. Choice A incorrectly assumes that upward motion alone determines net force, but motion and net force are independent—net force determines acceleration, not velocity.

Question 7

1. Buoyant force is greater than $mg$ at that moment. (correct answer)
2. Buoyant force equals $mg$ because the block is a floater.
3. Net force is zero because it was floating before being pushed.
4. Buoyant force depends only on mass, so it cannot change when pushed down.

Explanation: This question tests force analysis when floating objects are displaced and released. The block experiences buoyant force upward and weight mg downward, with negligible drag. Since the block accelerates upward after being pushed down and released, Newton's second law requires upward net force. When pushed below its equilibrium position, the buoyant force increases while weight remains constant, making buoyant force greater than mg. Choice B incorrectly assumes buoyant force equals weight regardless of displacement, but buoyant force varies with submerged volume.

Question 8

1. Buoyant force equals $mg$ because the block is not moving.
2. Buoyant force is greater than $mg$. (correct answer)
3. Tension must be upward to keep the block from sinking.
4. There is no buoyant force because the block is tied down.

Explanation: This question tests equilibrium analysis for submerged objects held by external forces. The block experiences buoyant force upward, weight mg downward, and string tension downward. Since the block is at rest, Newton's first law requires zero net force, so upward forces must equal downward forces. This means the buoyant force alone must exceed mg to balance both weight and downward tension. Choice A incorrectly assumes equilibrium means buoyant force equals weight, ignoring the additional downward tension force that must also be balanced.

Question 9

1. The buoyant force is greater than $mg$ because oil is dense.
2. The net force is zero because the sphere starts from rest.
3. The weight $mg$ is greater than the buoyant force. (correct answer)
4. The buoyant force depends only on the sphere's mass.

Explanation: This question involves analyzing net force when an object begins accelerating downward in a fluid. The sphere experiences weight mg downward and buoyant force upward, with negligible drag at release. Since the sphere accelerates downward from rest, the net force must be downward by Newton's second law. This requires the downward weight to exceed the upward buoyant force. Choice A incorrectly suggests the buoyant force is larger because oil is dense, but density affects buoyant force magnitude through displaced volume, not by making it automatically larger than weight.

Question 10

1. Downward, because the velocity is downward
2. Zero, because only two forces act
3. Upward, because it is slowing while moving downward (correct answer)
4. Upward only if buoyant force depends on mass

Explanation: This question tests understanding the relationship between velocity, acceleration, and net force in fluids. The buoy moves downward but is slowing, meaning its acceleration is opposite to its velocity—therefore upward. Two forces act: buoyant force FB upward and weight mg downward. Since acceleration is upward, net force must be upward, requiring FB > mg. Choice A incorrectly assumes net force direction matches velocity direction, not recognizing that slowing down means acceleration opposes velocity. The key principle is that net force determines acceleration direction, not velocity direction—when slowing, acceleration opposes motion.

Question 11

1. The net force is upward because the fluid pushes up.
2. The net force is downward. (correct answer)
3. The net force is zero because it is moving.
4. Buoyant force must equal $mg$ if it is in a fluid.

Explanation: This question tests net force analysis when objects accelerate downward in fluids. The object experiences weight mg downward and buoyant force upward, with negligible drag. Since the object has increasing downward speed, its acceleration is downward. By Newton's second law, downward acceleration requires downward net force, meaning the weight must exceed the buoyant force. Choice A incorrectly assumes fluids create upward net force, but acceleration direction determines net force direction regardless of the fluid medium.

Question 12

1. Upward
2. Downward (correct answer)
3. Zero
4. Zero because motion upward implies balanced forces.

Explanation: This question tests net force direction when objects decelerate while moving upward in fluids. The object experiences weight mg downward and buoyant force upward only. Since the object moves upward but slows down, its acceleration is downward (opposite to velocity direction). By Newton's second law, downward acceleration requires downward net force, meaning weight exceeds buoyant force. Choice D incorrectly assumes upward motion implies balanced forces, but deceleration requires net force opposing motion direction.

Question 13

1. $F_B = mg$, so the net force is zero. (correct answer)
2. $F_B > mg$ because it is underwater.
3. $mg > F_B$ because it is a solid cube.
4. There are no forces because it does not move.

Explanation: This question tests understanding of equilibrium when objects remain stationary upon release in fluids. The cube experiences buoyant force upward and weight mg downward only. Since the cube remains at rest after release (no acceleration), Newton's first law requires zero net force, meaning upward and downward forces must be equal in magnitude. Therefore, FB = mg, creating equilibrium. Choice B incorrectly assumes objects automatically move when forces exceed weight, but equilibrium occurs when forces balance exactly.

Question 14

1. Downward, because the balloon starts from rest.
2. Upward, because acceleration is upward. (correct answer)
3. Zero, because buoyant force always equals weight in a fluid.
4. Cannot be determined without the balloon's mass.

Explanation: This question tests understanding of force direction when objects accelerate from rest in fluids. The balloon experiences buoyant force upward and weight mg downward, with negligible drag at release. Since the balloon accelerates upward from rest, Newton's second law requires the net force to be upward, in the same direction as the acceleration. This means the buoyant force exceeds the weight. Choice A incorrectly assumes starting from rest determines force direction, but initial motion state doesn't affect the relationship between net force and acceleration direction.

Question 15

1. $F_B > mg$
2. $F_B = mg$
3. $mg > F_B$ (correct answer)
4. $F_B$ depends only on the object's mass, so no comparison is possible.

Explanation: This question tests force comparison when objects accelerate downward in fluids. The object experiences weight mg downward and buoyant force upward, with negligible drag. Since the object accelerates downward, Newton's second law requires downward net force, meaning the downward weight must exceed the upward buoyant force. Therefore, mg > FB. Choice D incorrectly claims buoyant force depends only on mass, but buoyant force actually depends on displaced fluid volume and fluid density, not the object's mass directly.

Question 16

1. Upward
2. Downward (correct answer)
3. Zero
4. Cannot be determined because buoyant force depends on mass.

Explanation: This question tests net force direction analysis when objects accelerate in fluids. The cube experiences weight mg downward and buoyant force upward, with negligible drag. Since the cube falls through water and speeds up downward, its acceleration is downward. By Newton's second law, downward acceleration requires downward net force, meaning weight exceeds buoyant force. Choice C incorrectly assumes net force could be zero despite changing speed, but acceleration requires non-zero net force in the direction of acceleration.

Question 17

1. $F_B = mg$
2. $F_B > mg$
3. $F_B = mg + F_D$ (correct answer)
4. $mg = F_B + F_D$ because it is moving upward.

Explanation: This question examines force relationships for objects moving at constant velocity upward in fluids. The ball experiences buoyant force upward, weight mg downward, and drag downward. Since the ball moves upward at constant speed, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the upward buoyant force must equal the sum of downward forces: FB = mg + FD. Choice A incorrectly sets buoyant force equal to weight only, ignoring the additional downward drag force that must also be balanced.

Question 18

1. The buoyant force equals $mg$.
2. The buoyant force equals $mg - T$.
3. The buoyant force equals $mg + T$. (correct answer)
4. There is no buoyant force because the object is tied.

Explanation: This question involves equilibrium analysis for light objects held submerged by external forces. The object experiences buoyant force upward, weight mg downward, and tension downward while at rest. Since the object is in equilibrium, Newton's first law requires zero net force, so the upward buoyant force must equal the sum of downward forces. Therefore, the buoyant force equals mg + T, with buoyant force balancing both weight and downward tension. Choice A incorrectly assumes buoyant force equals weight only, ignoring the additional downward tension.

Question 19

1. The net force is upward because two upward forces act.
2. The net force is zero, so $F_B + T = mg$. (correct answer)
3. Buoyant force equals $mg$ because the rock is underwater.
4. Tension must act downward to keep the rock from rising.

Explanation: This question tests equilibrium analysis for objects held stationary by external forces in fluids. The rock experiences buoyant force upward, tension upward, and weight mg downward while at rest. Since the rock is in equilibrium, Newton's first law requires zero net force, so upward forces must equal downward forces. Therefore, FB + T = mg, with both upward forces combining to balance the weight. Choice A incorrectly assumes multiple upward forces automatically create upward net force, but equilibrium requires force balance regardless of force directions.

Question 20

1. Buoyant force is greater than $mg$ because it is submerged.
2. Buoyant force equals $mg$. (correct answer)
3. Weight must be greater than buoyant force, but motion is prevented by the fluid.
4. There are no forces because it is not moving.

Explanation: This question tests equilibrium analysis for submerged objects at rest without external constraints. The block experiences buoyant force upward and weight mg downward, with negligible drag. Since the block remains at rest without strings or other supports, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the buoyant force must exactly equal mg for equilibrium. Choice A incorrectly assumes submerged objects always have buoyant force exceeding weight, but equilibrium requires force balance.