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. The net force is downward because the rock is moving downward.
2. The net force is zero because the speed is constant. (correct answer)
3. The buoyant force equals $mg$ because the rock is in water.
4. The buoyant force depends only on the rock’s mass, so it must exceed drag.

Explanation: This question involves equilibrium analysis for objects moving at constant velocity through fluids. The rock experiences weight mg downward, plus buoyant force and drag both upward. Since the rock moves at constant speed, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the sum of upward forces (buoyant force plus drag) must equal the downward weight mg. Choice A incorrectly assumes downward motion requires downward net force, but constant velocity requires zero net force regardless of motion direction.

Question 2

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 3

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 4

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 5

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 6

1. Upward
2. Downward (correct answer)
3. Zero
4. Net force must be zero because only two forces act.

Explanation: This question examines net force direction for objects accelerating downward in fluids. The object experiences buoyant force upward and weight mg downward only. Since the object moves downward while speeding up, its acceleration is downward. By Newton's second law, downward acceleration requires downward net force, meaning the weight exceeds the buoyant force. Choice D incorrectly assumes having only two forces guarantees zero net force, but net force depends on the relative magnitudes of opposing forces.

Question 7

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 8

1. Net force is upward because buoyant force acts upward.
2. Net force is zero. (correct answer)
3. Net force is downward because drag and weight both act downward.
4. Buoyant force must equal $mg$ because the speed is constant.

Explanation: This question tests equilibrium analysis for objects moving at constant velocity in fluids. The object experiences buoyant force upward, weight mg downward, and drag downward. Since the object moves upward at constant speed, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, all forces must balance with upward forces equaling downward forces. Choice A incorrectly assumes upward motion requires upward net force, but constant velocity specifically requires zero net force regardless of motion direction.

Question 9

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 10

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 11

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.

Question 12

1. Net force is downward because the motion is downward.
2. Net force is zero, so $mg = F_B + F_D$. (correct answer)
3. Buoyant force equals $mg$ because speed is constant.
4. Drag must be downward because it acts in the direction of motion.

Explanation: This question tests equilibrium analysis for objects moving at constant velocity downward in fluids. The ball experiences weight mg downward, buoyant force upward, and drag upward. Since the ball moves downward at constant speed, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the downward weight must equal the sum of upward forces, so mg = FB + FD. Choice A incorrectly assumes downward motion requires downward net force, but constant velocity requires zero net force.

Question 13

1. The net force is zero because the cork is in a fluid.
2. The buoyant force is greater than $mg$. (correct answer)
3. The buoyant force equals $mg$ because it will eventually float.
4. The buoyant force depends on the cork’s mass, so it increases as the cork rises.

Explanation: This question involves force analysis for objects accelerating upward in fluids. The cork experiences buoyant force upward and weight mg downward, with negligible drag. Since the cork accelerates upward after release, Newton's second law requires upward net force, meaning the upward buoyant force must exceed the downward weight mg. Therefore, the buoyant force is greater than mg. Choice C incorrectly assumes future floating behavior determines current force relationships, but instantaneous acceleration depends only on the current force imbalance.

Question 14

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 15

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 16

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 17

1. The net force is upward because buoyant force exists.
2. The net force is zero because $F_B = mg$. (correct answer)
3. The buoyant force must be zero since it is not moving.
4. The buoyant force depends only on mass, so $F_B$ must be less than $mg$ for small masses.

Explanation: This question tests equilibrium inference when objects remain stationary without external support in fluids. The block experiences buoyant force upward and weight mg downward only. Since the block remains at rest without support after release, its acceleration is zero, requiring zero net force by Newton's first law. Therefore, the buoyant force must exactly equal mg for perfect equilibrium. Choice A incorrectly assumes buoyant force creates upward net force, but remaining stationary requires force balance.

Question 18

1. Net force is zero because it is already moving upward.
2. Buoyant force is less than $mg$ because it is in water.
3. Buoyant force is greater than $mg$. (correct answer)
4. Buoyant force depends only on mass, so it must equal $mg$.

Explanation: This question examines force relationships when objects accelerate upward in fluids. The object experiences buoyant force upward and weight mg downward, with negligible drag. Since the object rises and speeds up upward, its acceleration is upward. By Newton's second law, upward acceleration requires upward net force, meaning the buoyant force must exceed the weight mg. Choice A incorrectly assumes net force is zero during motion, but acceleration requires non-zero net force in the acceleration direction.

Question 19

1. The buoyant force equals the rock’s weight since it is not moving.
2. The net force is upward because the string pulls upward.
3. The forces balance so $T+F_B=mg$. (correct answer)
4. If the rock is motionless, then no forces act on it.

Explanation: This question tests force equilibrium for a three-force system in a fluid. When the rock is held motionless underwater, its acceleration is zero, so the net force must be zero by Newton's first law. Three forces act vertically: weight (mg) downward, buoyant force (FB) upward, and tension (T) upward. For equilibrium, the upward forces must balance the downward force: T + FB = mg. Since the rock would sink without the string (mg > FB for rocks in water), the tension provides the additional upward force needed for equilibrium. Choice A incorrectly claims FB = mg, which would mean the rock could float without the string. When multiple forces act on a submerged object, sum all forces and set equal to zero for equilibrium.

Question 20

1. The buoyant force increases because the new tank has more fluid.
2. The buoyant force stays equal to the weight, so the net force remains zero. (correct answer)
3. The object must sink because buoyant force depends on container size.
4. The weight decreases because the object is in a different tank.

Explanation: This question tests understanding that buoyant force depends on fluid properties, not container size. When a floating object is at rest in equilibrium, the buoyant force equals its weight regardless of tank size. The buoyant force depends only on the volume of fluid displaced and the fluid's density, both unchanged when moving between tanks of the same fluid. Since the object still floats at rest in the new tank, it remains in equilibrium with FB = mg, so the net force stays zero. Choice A incorrectly suggests buoyant force depends on the total amount of fluid in the container rather than just the displaced volume. Remember that Archimedes' principle depends only on the displaced fluid volume, not the container dimensions.