Newton's First Law - AP Physics 1

A 2.0 kg book rests on a horizontal table. The book is motionless, and the only interactions are Earth's gravitational pull downward and the table's contact force upward. Which statement about the net force on the book is correct?
Answer: The net force is zero because the upward normal force equals the downward weight. This question tests Newton's first law, which states that an object at rest or moving at constant velocity has zero net force. The book is motionless on the table, meaning its velocity is constant (zero), so the net force must be zero. Two forces act on the book: Earth's gravitational force (weight) pulling downward and the table's normal force pushing upward. Since the book remains at rest, these forces must be equal in magnitude and opposite in direction, resulting in zero net force. Choice A incorrectly assumes that gravity being present means there's a net downward force, reflecting the misconception that motion (or lack of upward motion) requires a net force. When analyzing equilibrium situations, always check if the object's velocity is constant—if so, all forces must balance to give zero net force.
A 0.50 kg cart moves to the right on a level track at constant velocity. A fan on the cart pushes air to the left, providing a constant $1.2\ \text{N}$ thrust to the right. At the same time, a friction force of $1.2\ \text{N}$ acts to the left. Which statement best describes the net force on the cart?
Answer: The net force is zero because the horizontal forces cancel. This question tests your understanding of Newton's first law. The cart moves at constant velocity to the right, which means its velocity is not changing. According to Newton's first law, constant velocity requires zero net force. The fan provides a 1.2 N thrust to the right, while friction provides a 1.2 N force to the left. These horizontal forces are equal in magnitude but opposite in direction, so they cancel out completely, resulting in zero net horizontal force. Choice A incorrectly assumes that having a force source (the fan) means there must be a net force, reflecting the misconception that motion requires net force. When solving force problems, always add forces as vectors—equal and opposite forces always sum to zero.
A crate is pushed across a horizontal floor at constant speed. The applied push is horizontal, kinetic friction acts opposite the motion, and the crate does not accelerate. Which statement about the net force on the crate is correct?
Answer: The net force is zero because the acceleration is zero. This question assesses understanding of Newton's First Law of Motion, which links constant velocity to zero net force. The crate moves at constant speed, indicating no net force acts on it. The horizontal push balances the kinetic friction force, resulting in zero acceleration. All forces sum to zero, allowing steady motion. Choice A is incorrect as it claims net force aligns with motion direction, a misconception that force is required to maintain velocity instead of to change it. For these questions, use the strategy of confirming zero acceleration from constant speed and ensuring net force calculations reflect that balance.
A student pushes a 4.0 kg crate across a level floor at constant velocity. The student's push is horizontal; kinetic friction acts opposite the motion; Earth and the floor provide vertical forces. Which statement is correct?
Answer: The net force is zero because the horizontal forces balance and vertical forces balance. This question assesses understanding of Newton's First Law of Motion, which posits that objects move at constant velocity or stay at rest without a net force. The crate moves across the floor at constant velocity, indicating zero acceleration. Newton's First Law dictates that the net force must be zero for constant velocity. Horizontal forces (student's push and friction) balance, and vertical forces (gravity and normal) balance. Choice A is wrong due to the misconception that motion requires a net force in the direction of motion, but inertia sustains motion without net force. A transferable strategy is to use free-body diagrams to visualize and sum forces to zero in cases of constant velocity or rest.
An elevator moves upward at a constant velocity of $2.0\ \text{m/s}$ . A person of mass $70\ \text{kg}$ stands on a scale inside. The person interacts with Earth (gravity) and with the scale (normal force). Neglect air resistance. Which statement about the forces on the person is correct?
Answer: The normal force equals $mg$ because the net force is zero. This question tests your understanding of Newton's first law. The elevator and person move upward at constant velocity (2.0 m/s), which means their velocity is not changing. According to Newton's first law, constant velocity requires zero net force. The person experiences two vertical forces: the downward gravitational force (weight = mg) and the upward normal force from the scale. Since the net force must be zero, these forces must be equal in magnitude, so the normal force equals mg. Choice A incorrectly suggests the normal force must be greater than mg because of upward motion, reflecting the misconception that motion in a direction requires net force in that direction. Remember: for constant velocity motion, analyze forces exactly as you would for an object at rest.
A skydiver has reached terminal velocity and continues falling straight down at constant speed. The forces on the skydiver are gravity downward and air resistance upward. The speed remains constant. Which statement about these forces is correct?
Answer: Gravity equals air resistance because the net force is zero. This question tests your understanding of Newton's first law. The skydiver falls at terminal velocity, which means constant speed in a straight line downward. According to Newton's first law, constant velocity requires zero net force. The skydiver experiences two vertical forces: gravity downward and air resistance upward. Since the speed remains constant (no acceleration), these forces must be equal in magnitude and opposite in direction, so gravity equals air resistance. Choice A incorrectly suggests gravity must be greater because of downward motion, reflecting the misconception that motion in a direction requires greater force in that direction. To solve such problems, remember that terminal velocity is just another example of constant velocity motion requiring zero net force.
A crate is pulled across a floor at constant velocity by a horizontal rope. Kinetic friction acts opposite the motion. What is the relationship between the rope's pull and friction?
Answer: They are equal in magnitude because the net horizontal force is zero. This question tests understanding of Newton's first law. The crate moves at constant velocity, which means its acceleration is zero. By Newton's first law, zero acceleration requires zero net horizontal force. Two horizontal forces act on the crate: the rope's pull forward and kinetic friction backward, and these must be equal in magnitude to produce zero net force. Choice A incorrectly assumes that motion requires an unbalanced force, reflecting the misconception that objects need a net force to keep moving. To solve constant velocity problems, always set the net force to zero and balance opposing forces.
A crate is pushed across a floor at constant speed. A student pushes horizontally, kinetic friction acts opposite motion, and the normal force and weight act vertically. Which statement is correct?
Answer: The net force is zero; the push balances friction and vertical forces balance. This question tests Newton's first law for an object moving at constant speed with forces in multiple directions. The crate moves at constant speed, meaning constant velocity and zero acceleration, which requires zero net force. Horizontally, the student's push must equal the kinetic friction force in magnitude. Vertically, the normal force must equal the weight. With forces balanced in both horizontal and vertical directions, the net force is zero. Choice A incorrectly assumes that motion in a direction requires a net force in that direction, demonstrating the misconception that force is needed to maintain motion. To solve multi-force equilibrium problems, analyze forces separately in each direction and ensure they balance in all directions for zero net force.
A book rests on a horizontal table and remains at rest. The book interacts with Earth (gravity) and the table (contact). No other objects touch it, and air effects are negligible. Which claim about the forces on the book is correct?
Answer: The table's normal force on the book equals the book's weight in magnitude. This question assesses understanding of Newton's First Law of Motion, which explains that an object at rest remains at rest unless acted upon by a net external force. The book is at rest on the table, so the net force on it must be zero. This means the downward gravitational force (weight) is exactly balanced by the upward normal force from the table. Since there is no motion, these vertical forces are equal in magnitude and opposite in direction, maintaining equilibrium. Choice B is incorrect because it embodies the misconception that no forces act on stationary objects, ignoring that balanced forces can keep an object at rest without motion. When approaching such problems, identify all forces acting on the object and verify that they sum to zero for cases of rest or constant velocity.
A box sits at rest on a ramp that makes a 20° angle with the horizontal. Static friction acts along the surface, and the ramp exerts a normal force. The box remains motionless. What is true about the net force on the box?
Answer: It is zero because static friction balances the component of gravity down the ramp. This question assesses understanding of Newton's First Law of Motion, which states that an object at rest stays at rest if net force is zero. The box is at rest on the ramp, with zero velocity and thus zero acceleration. Newton's First Law requires the net force to be zero for the box to remain motionless. The component of gravity down the ramp is balanced by static friction up the ramp, and normal force balances the perpendicular component of gravity. Choice B is wrong as it reflects the misconception that motion on an incline requires an unbalanced force down the ramp, but at rest, forces balance completely. A transferable strategy is to resolve forces into components parallel and perpendicular to the surface and set their sums to zero for equilibrium.
A 2.0 kg book rests on a horizontal table and remains at rest for 10 s. The book interacts with Earth (gravity) and the table (contact). Which statement about the net force on the book is correct?
Answer: The net force is zero because the upward normal force equals the book's weight. This question assesses understanding of Newton's First Law of Motion, which states that an object at rest remains at rest unless acted upon by a net force. Since the book is at rest on the table, its velocity is zero and constant, implying that the acceleration is zero. According to Newton's First Law, zero acceleration means the net force on the book must be zero. The downward gravitational force (weight) is balanced by the upward normal force from the table, resulting in no net force. Choice A is incorrect because it reflects the misconception that motion (or in this case, the lack of falling) requires an unbalanced force in the direction of gravity, but actually, constant velocity—including zero velocity—requires zero net force. A transferable strategy is to identify if an object's velocity is constant (including zero); if yes, draw a free-body diagram to confirm that all forces balance to zero net force.
A car moves east at constant velocity $20\,\text{m/s}$ on a level road. Air drag acts west, and the road exerts a static friction force east on the tires. What must be true about these horizontal forces?
Answer: They are equal in magnitude and opposite in direction. This question tests understanding of Newton's first law. The car moves at constant velocity, which means its acceleration is zero. By Newton's second law, zero acceleration requires zero net force, so all forces acting on the car must balance. In the horizontal direction, two forces act: air drag westward and static friction from the road eastward, and these must be equal in magnitude and opposite in direction. Choice A incorrectly suggests unbalanced forces, which would cause the car to accelerate rather than maintain constant velocity—this reflects the misconception that motion requires a net force. When analyzing constant velocity situations, always verify that opposing forces balance to produce zero net force.
A drone flies horizontally at constant velocity. The rotors provide an upward force equal to the drone's weight, and a forward thrust balances air drag. Which statement about the net force is correct?
Answer: The net force is zero because forces are balanced in both horizontal and vertical directions. This question tests Newton's first law for motion with forces in two perpendicular directions. The drone flies horizontally at constant velocity, meaning zero acceleration and zero net force. In the vertical direction, the upward rotor force equals the downward weight. In the horizontal direction, the forward thrust equals the backward air drag. With forces balanced in both directions, the net force is zero. Choice A incorrectly assumes that forward motion requires a forward net force, demonstrating the misconception that motion requires force in the direction of movement. When analyzing constant velocity motion with multiple forces, check that forces balance separately in each perpendicular direction to ensure zero net force overall.
A boat moves east across a lake at constant velocity. The engine provides a steady thrust east, while water drag acts west. Vertical forces from buoyancy and weight also act. The boat's speed and direction remain unchanged. Which statement best describes the net force on the boat?
Answer: The net force is zero because all forces balance. This question tests Newton's First Law for multi-directional forces at constant velocity. The boat moves east at constant velocity, meaning both its speed and direction remain unchanged. Newton's First Law states that constant velocity requires zero net force—all forces in all directions must balance. Horizontally, the eastward engine thrust must equal the westward water drag, while vertically, the upward buoyancy must equal the downward weight. The misconception that motion direction determines net force direction ignores that constant velocity always means balanced forces. When analyzing objects with forces in multiple directions, check that forces balance separately in each direction (horizontal and vertical) for true equilibrium.
A car travels east on a straight, level road at a constant speed of $20\ \text{m/s}$ . The engine provides a forward traction force on the car, while air resistance and rolling resistance act opposite the motion. The speed remains constant for several seconds. What can be concluded about the net force on the car?
Answer: The net force is zero because the forward and resistive forces balance. This question tests your understanding of Newton's first law. The car travels at constant speed (20 m/s) in a straight line, which means its velocity is not changing. According to Newton's first law, when an object maintains constant velocity, the net force on it must be zero. The forward traction force from the engine must exactly balance the combined resistive forces (air resistance and rolling resistance) acting opposite to the motion. Since these forces balance, the net force is zero. Choice A incorrectly assumes that motion eastward requires a net force eastward, which is a common misconception that motion requires force. To analyze such problems, focus on whether velocity is changing—constant velocity always means zero net force.
A student pushes a box across a floor, then stops pushing. Immediately after the push ends, the box continues moving but slows down. During this slowing interval, the box interacts with Earth (gravity), the floor (normal and kinetic friction), and air (drag). Which statement about the net force is correct during the slowing interval?
Answer: The net force is backward because resistive forces are unbalanced. This question tests Newton's First Law by examining what happens when forces become unbalanced. During the slowing interval, the box's velocity is decreasing, which means it is accelerating (in the negative direction). Newton's First Law tells us that changing velocity requires a non-zero net force. After the push ends, only resistive forces remain: kinetic friction and air drag both act backward, with no forward force to balance them. The misconception that "forward inertia" provides a force misunderstands that inertia is a property describing resistance to acceleration, not a force itself. To identify net force direction, look at how velocity changes—slowing down means net force opposes the motion direction.
A 0.50 kg drone hovers at a constant altitude with zero vertical velocity. The drone's rotors push air downward; Earth pulls downward. Which is true about the net force on the drone?
Answer: Net force is zero because the upward thrust equals the weight. This question assesses understanding of Newton's First Law of Motion, emphasizing that objects maintain constant velocity (including zero) without a net force. The drone is hovering at constant altitude with zero vertical velocity, so its vertical acceleration is zero. Per Newton's First Law, this constant velocity means the net force must be zero. The upward thrust from the rotors balances the downward gravitational force (weight), resulting in no net force. Choice A is incorrect as it suggests the misconception that motion or hovering requires an unbalanced force upward, but actually, balanced forces maintain constant velocity. A transferable strategy is to recognize states of rest or constant motion and verify that opposing forces are equal in magnitude for zero net force.
A hockey puck slides on nearly frictionless ice at constant velocity $5.0\ \text{m/s}$ north. After it leaves the player's stick, it continues without changing speed or direction; air resistance is negligible. Which statement about the net force on the puck is correct while it slides freely?
Answer: The net force is zero because the puck's velocity is constant. This question assesses understanding of Newton's First Law of Motion, which states that an object in uniform motion remains in that state without a net external force. The puck is sliding at a constant velocity of 5.0 m/s north, indicating zero net force acting on it. With negligible friction and air resistance, no unbalanced forces alter its motion, so it continues straight at constant speed. This demonstrates inertia, where the puck resists changes to its velocity. Choice A is wrong as it suggests a net force is needed to maintain motion, a common misconception that force is required for constant velocity rather than for acceleration. For similar scenarios, remember to equate constant velocity with zero net force and draw free-body diagrams to confirm balanced forces.
A boat travels north at constant velocity on calm water. The engine provides thrust north, and water drag acts south. What must be true about the net force on the boat?
Answer: The net force is zero because the thrust and drag are equal in magnitude. This question tests Newton's first law for horizontal motion at constant velocity. The boat travels north at constant velocity, meaning its acceleration is zero and the net force must be zero. Two horizontal forces act: engine thrust north and water drag south. For the boat to maintain constant velocity, these forces must be equal in magnitude and opposite in direction, resulting in zero net force. Choice A incorrectly assumes that northward motion requires a north net force because thrust causes the motion, illustrating the misconception that motion requires force. When analyzing constant velocity situations, focus on the current state of motion (constant velocity) rather than what initially caused the motion—constant velocity always means zero net force.
A 0.20 kg ball falls straight downward at constant velocity through air. Earth exerts gravity downward; air exerts a resistive force upward. Which statement about the forces is correct?
Answer: Gravity equals air resistance because the net force is zero. This question assesses understanding of Newton's First Law of Motion, explaining terminal velocity as constant velocity with zero net force. The ball falls downward at constant velocity, meaning its acceleration is zero. According to Newton's First Law, zero acceleration implies the net force is zero. The downward gravitational force equals the upward air resistance force. Choice A is incorrect because it embodies the misconception that motion requires the driving force (gravity) to be greater, but at constant velocity, forces balance. A transferable strategy is to recognize terminal velocity as a balance point and compare magnitudes of opposing forces like drag and weight.

Start this quiz