Kinetic and Static Friction - AP Physics 1

A crate is pushed to the right across a rough floor and is observed to be sliding right at constant speed. The friction is kinetic; other forces are the push, weight, and normal force. Which direction is the friction force on the crate?
Answer: To the left This question assesses understanding of kinetic friction in AP Physics 1, emphasizing its direction relative to motion. Static friction adjusts to prevent impending motion, matching applied forces up to μ_s N. Kinetic friction, however, provides a constant opposing force of μ_k N once sliding begins, always acting opposite to the velocity. In this scenario, with the crate sliding right at constant speed, kinetic friction must point left to oppose the motion and balance the push. A common distractor is choice D, claiming zero friction due to constant speed, but friction is necessary to counteract the push for zero net force. A transferable strategy is to identify the direction of velocity and remember friction opposes it for kinetic cases.
A block on a rough horizontal surface is pushed to the right with $4\,\text{N}$ and remains at rest. The friction is static; other forces are weight and normal force. Which statement about the friction magnitude is necessarily true?
Answer: It equals $4\,\text{N}$ This question assesses understanding of static friction in AP Physics 1, distinguishing it from its maximum value. Static friction equals the applied force when it's less than μ_s N, keeping the object stationary. Kinetic friction would apply if the push exceeded the maximum, but here the block rests. Thus, friction must be exactly 4 N to balance the push. A common distractor is choice A, assuming it equals μ_s N, but that's only at the onset of motion, not necessarily here. A transferable strategy is to use equilibrium conditions to set friction equal to other parallel forces when motion doesn't occur.
A block is on a rough horizontal table and is pulled to the right. It is sliding right, so friction is kinetic; other forces are weight, normal, and the pull. Which expression gives the magnitude of the friction force?
Answer: $f=\mu_k N$ This problem tests understanding of kinetic friction magnitude. When an object slides across a surface, kinetic friction has a constant magnitude given by f = μₖN, where μₖ is the coefficient of kinetic friction and N is the normal force. This value is independent of the sliding speed or the magnitude of the applied force. On a horizontal surface, the normal force equals the weight, making the friction calculation straightforward. Choice C (f equals the pull) would only be true at constant velocity, but kinetic friction equals μₖN regardless of acceleration. For sliding objects, always use f = μₖN to find kinetic friction magnitude.
A cart is sliding to the right on a rough level track after being given a push, and no one is touching it now. Kinetic friction acts; other forces are weight and normal. What is the direction of the friction force on the cart?
Answer: To the left, opposite the sliding direction This question tests understanding of kinetic friction on a coasting object. When an object slides across a surface with no applied force, kinetic friction is the only horizontal force acting. Since the cart slides to the right, kinetic friction points to the left, opposing this motion and causing deceleration. Kinetic friction always opposes sliding motion with magnitude μₖN. Choice A incorrectly suggests friction maintains motion rather than opposing it. When an object slides freely, kinetic friction points opposite to velocity and causes it to slow down.
A $2.0\,\text{kg}$ block rests on a rough horizontal table. A student pulls horizontally with a $6.0\,\text{N}$ force, and the block remains at rest. The friction is static; other forces are the block's weight and the table's normal force. What is the magnitude of the friction force on the block?
Answer: $6.0\,\text{N}$ This question assesses understanding of static friction in AP Physics 1, focusing on its role in preventing motion. Static friction adjusts its magnitude to exactly match the applied force, up to a maximum value given by μ_s times the normal force, to keep the object at rest. In contrast, kinetic friction acts with a constant magnitude of μ_k times the normal force when the object is sliding. Here, since the block remains at rest under a 6.0 N pull, the static friction force must equal 6.0 N to balance the applied force. A common distractor is choice C, suggesting it's greater than 6.0 N, but that would imply a net force preventing equilibrium. A transferable strategy is to always draw a free-body diagram and set the net force to zero for objects in equilibrium.
A block slides up a rough incline after being launched upward, slowing as it moves. The friction is kinetic; other forces are weight and the normal force. What is the direction of the kinetic friction force on the block?
Answer: Down the incline This problem tests kinetic friction direction on an incline. Kinetic friction always opposes the direction of sliding motion between surfaces. Since the block slides up the incline, kinetic friction points down the incline, opposing this upward motion. Both gravity's parallel component and kinetic friction point down the incline, causing the upward-moving block to decelerate. Choice A (up the incline) would incorrectly help rather than oppose the motion. The strategy is: kinetic friction opposes velocity direction along the surface, not the direction of acceleration.
A block rests on a rough horizontal surface. A horizontal force of $8\,\text{N}$ is applied to the right, but the block does not move. The friction force is static; other forces are weight and normal. Which statement about the static friction magnitude is correct?
Answer: It must equal $8\,\text{N}$ This problem tests understanding of static friction matching applied forces. When an object remains at rest under an applied force, static friction adjusts to exactly balance that force, maintaining equilibrium. Since the 8 N force doesn't cause motion, static friction must equal 8 N in the opposite direction. Static friction can range from zero up to its maximum value μₛN, taking whatever value is needed for equilibrium. Choice A incorrectly assumes static friction always equals its maximum value, even when less is sufficient. For stationary objects, static friction equals the applied force (up to the maximum μₛN).
A book is at rest on a rough horizontal desk. A student applies a $3\,\text{N}$ horizontal force to the right, and the book still does not move. The friction is static; other forces are weight and normal force. What is the direction of the friction force on the book?
Answer: To the left This problem tests static friction direction when preventing motion. Static friction acts to prevent sliding by opposing the tendency to move. The 3 N force pushes rightward, creating a tendency for the book to slide right, so static friction must act leftward to prevent this motion. Since the book remains at rest, the leftward static friction exactly balances the rightward applied force at 3 N. Choice D (zero) incorrectly assumes friction is absent when objects are at rest. The strategy is: static friction opposes the direction the object would move if friction were absent.
A book rests on a rough horizontal table. A student gently pulls it to the right with a $3.0\,\text{N}$ force, and the book does not move. The friction is static; other forces are weight and normal force. Which statement about the static friction is correct?
Answer: It is $3.0\,\text{N}$ to the left This question tests the concept of kinetic and static friction in AP Physics 1, dealing with equilibrium under a sub-maximal applied force. Static friction matches the applied force in magnitude but opposes it in direction, up to the limit of μ_s N, to maintain no motion. Since the book doesn't move with a 3.0 N pull to the right, static friction must be 3.0 N to the left for net force zero. Kinetic friction would only apply if motion started, with magnitude μ_k N. Choice D is a distractor, suggesting friction exceeds 3.0 N, but static friction equals the applied force when it's less than the maximum. Always use Newton's laws to balance forces in equilibrium and verify if friction is below or at its maximum for stationary objects.
A $1.5\,\text{kg}$ block is held at rest against a vertical rough wall by a horizontal push of $20\,\text{N}$ into the wall. The friction is static; other forces are weight and the wall's normal force. What is the direction of the friction force on the block?
Answer: Upward This question assesses understanding of static friction in AP Physics 1, including its direction in non-horizontal setups. Static friction adjusts to counteract tendencies to move, up to μ_s N, preventing sliding. Kinetic friction would oppose actual motion if sliding occurred, but here the block is at rest. The friction force points upward to balance the downward weight, as the horizontal push creates the normal force enabling friction. A common distractor is choice B, thinking it's downward with gravity, but friction opposes the potential slide down. A transferable strategy is to determine the direction of impending motion and set friction to oppose it.
A sled slides leftward on rough level ground after being released, slowing down as it moves. The friction force is kinetic; other forces are weight and the normal force. What is the direction of the kinetic friction force on the sled?
Answer: Rightward This problem tests kinetic friction direction for a decelerating object. Kinetic friction always opposes the direction of sliding motion. Since the sled slides leftward across the ground, kinetic friction acts rightward, opposing this leftward motion. This rightward friction force causes the leftward-moving sled to decelerate. The friction acts horizontally because it opposes sliding along the horizontal surface. Choice A (leftward) would incorrectly accelerate the sled rather than slow it. The strategy is: kinetic friction points opposite to velocity, regardless of whether the object speeds up or slows down.
A block is sliding to the right on a rough horizontal surface. The friction is kinetic; other forces are weight and normal force. Which statement correctly compares kinetic and static friction coefficients for the same surfaces?
Answer: $\mu_k$ is typically less than $\mu_s$ This question tests the concept of kinetic and static friction in AP Physics 1, comparing their coefficients. Static friction can range from zero to μ_s N, providing a higher threshold to start motion than to maintain it. Kinetic friction is μ_k N, and for most surfaces, μ_k is less than μ_s, meaning it's easier to keep an object moving than to start it moving. This is because surface irregularities interlock more when stationary. Choice A is a distractor, reversing the typical relationship by claiming μ_k > μ_s, which is rarely true. To remember this, think of real-world examples like pushing a heavy box: more force to start than to continue sliding.
A box is on a rough horizontal surface. A horizontal force $F$ pulls it to the right. The box is at rest but is about to start moving to the right. The friction force is static; other forces are weight and normal. What is the magnitude of the friction force at this instant?
Answer: $f_s=\mu_s N$ This problem tests understanding of maximum static friction. When an object is on the verge of sliding, static friction reaches its maximum value of μₛN. At this critical point, static friction no longer adjusts to match the applied force but instead equals its maximum possible value. Before this point, static friction would equal F, but at the threshold of motion, it specifically equals μₛN. Choice A (f = F) would be correct if the box weren't about to move, but misses that we're at the maximum static friction limit. When an object is about to slide, static friction equals μₛN regardless of the applied force magnitude.
A crate is pushed across a rough horizontal floor and is sliding to the right at constant speed. The friction force is kinetic; other forces are the push, weight, and normal force. What is the direction of the friction force on the crate?
Answer: To the left This problem tests understanding of kinetic friction direction. Kinetic friction always opposes the direction of sliding motion between surfaces. Since the crate slides to the right across the floor, kinetic friction acts to the left, opposing this rightward motion. The constant speed indicates balanced horizontal forces: the rightward push equals the leftward kinetic friction. Choice B (upward) confuses friction with the normal force, which acts perpendicular to surfaces. The strategy is: kinetic friction always points opposite to the velocity of the sliding object.
A $4.0\,\text{kg}$ block on a rough level surface is pulled horizontally to the right with $12\,\text{N}$ and accelerates to the right. The friction is kinetic; other forces are weight and normal force. Which statement about the kinetic friction magnitude is supported?
Answer: It must be less than $12\,\text{N}$ This question tests the concept of kinetic and static friction in AP Physics 1, analyzing accelerated motion with kinetic friction. Static friction adjusts up to μ_s N but switches to kinetic once motion begins, with constant magnitude μ_k N opposing the motion. Since the block accelerates to the right under a 12 N pull, the net force is rightward, meaning the applied force exceeds kinetic friction. Thus, kinetic friction must be less than 12 N. Choice B is a distractor, suggesting friction > 12 N, which would cause leftward acceleration, not rightward. For acceleration problems, apply Newton's second law: net force equals mass times acceleration, and isolate friction accordingly.
A $2.0\,\text{kg}$ block rests on a horizontal table. A student pulls it to the right with a $3.0\,\text{N}$ horizontal force, and the block remains at rest. The friction force is static; other forces are weight and the table's normal force. What is the magnitude of the friction force on the block?
Answer: $3.0\,\text{N}$ This problem tests understanding of static friction and equilibrium. When the block remains at rest despite the applied force, the net force must be zero, meaning all forces balance. Static friction adjusts its magnitude to exactly match the applied force (up to its maximum value of μₛN), preventing motion. Since the 3.0 N pull doesn't cause motion, static friction must provide an equal 3.0 N force in the opposite direction. Choice A (0 N) incorrectly assumes no friction acts on a stationary object. For objects in equilibrium, identify all forces and recognize that static friction self-adjusts to maintain zero net force.
A $2.0\,\text{kg}$ block rests on a horizontal table. A student pulls it with a $6\,\text{N}$ horizontal force, and the block remains at rest. The friction force is static, and the only other forces are weight and the normal force. What is the magnitude of the friction force on the block?
Answer: $6\,\text{N}$ This problem tests understanding of static friction and equilibrium. When the block remains at rest despite the applied force, the net force must be zero, meaning all forces balance. The student pulls with 6 N to the right, so static friction must exert 6 N to the left to maintain equilibrium. Static friction adjusts its magnitude (up to a maximum of μₛN) to exactly balance applied forces and prevent motion. Choice A (0 N) incorrectly assumes no friction acts when the object is at rest. The key strategy is: for objects at rest, static friction equals the applied force magnitude (not the maximum possible friction).
A $2.0\,\text{kg}$ block rests on a horizontal table. A student pulls horizontally with $3.0\,\text{N}$ , and the block remains at rest. Static friction acts between block and table; other forces are weight and the normal force. What is the magnitude of the static friction force on the block?
Answer: $3.0\,\text{N}$ This question tests understanding of static friction. When an object remains at rest despite an applied force, static friction adjusts to exactly balance that force. Since the block is in equilibrium with a 3.0 N pull to one direction, static friction must provide 3.0 N in the opposite direction to maintain zero net force. Static friction can vary from 0 up to μₛN, adjusting as needed to prevent motion. Choice A (0 N) incorrectly assumes no friction acts when at rest, while choices C and D give general expressions rather than the specific value. When an object is stationary under an applied force, static friction equals the applied force magnitude.
A sled is pulled to the right across snow and is sliding at constant speed. Kinetic friction acts; other forces are weight, normal, and the pull. Which statement about the kinetic friction magnitude is correct?
Answer: It must equal the pull's horizontal component This question tests understanding of kinetic friction at constant speed. When an object moves at constant velocity, the net force must be zero by Newton's first law. For horizontal motion, the horizontal component of the pull must exactly balance kinetic friction. Kinetic friction has magnitude μₖN regardless of speed, but this must equal the pull's horizontal component for equilibrium. Choice B incorrectly uses static friction coefficient, while C violates equilibrium conditions. At constant speed, kinetic friction magnitude equals the horizontal applied force.
A box is pushed on a rough floor with an $8\,\text{N}$ horizontal force to the right and does not move. The friction is static; other forces are weight and normal force. Which quantity must equal $8\,\text{N}$ ?
Answer: The static friction force magnitude This problem tests static friction in equilibrium conditions. When the box remains at rest under an 8 N rightward push, static friction must provide an equal 8 N force leftward to maintain zero net force. Static friction adjusts its magnitude to match applied forces, up to its maximum value μₛN. The actual static friction (8 N) may be less than the maximum possible static friction. Choice C (μₛN) represents the maximum possible static friction, not necessarily the actual value. The strategy is: for stationary objects, static friction magnitude equals the applied force magnitude, not the maximum possible value.

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