Spring Forces

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AP Physics 1 › Spring Forces

Questions 1 - 10
1

A cart attached to an ideal spring moves on a frictionless track. Equilibrium is $x=0$ at the spring’s natural length, and +$x$ points right. When the cart is at $x=+0.04,\text{m}$, the spring force on it is $F_s=-2.0,\text{N}$. What is the spring force when the cart is at $x=+0.08,\text{m}$?

$0,\text{N}$

$-1.0,\text{N}$

$-4.0,\text{N}$

$+4.0,\text{N}$

Explanation

This question tests application of Hooke's law to predict spring force at different positions. The spring force is a restoring force that opposes displacement and points toward equilibrium, with its direction given by the negative sign in F_s = -kx. The magnitude is proportional to displacement, so if force is -2.0 N at x = +0.04 m, doubling the displacement to +0.08 m doubles the magnitude while keeping the negative direction. This yields F_s = -4.0 N, as the spring constant k remains consistent. Choice D, +4.0 N, is a distractor that mistakes the sign convention, suggesting a force away from equilibrium. For transferable skills, practice using known force-position pairs to find k, then apply it to new positions for verification.

2

A mass is attached to an ideal spring on a frictionless table. Equilibrium is $x=0$ at the unstretched length, with +$x$ to the right. The mass is pulled to $x=+0.03,\text{m}$ and released. If the displacement were instead $x=+0.06,\text{m}$, how would the spring force magnitude compare?

It would be half as large

It would be zero because the spring is stretched

It would be the same, since direction is unchanged

It would be twice as large

Explanation

This question explores how spring force magnitude varies with displacement in Hooke's law. The restoring force acts opposite to displacement, pulling or pushing toward equilibrium with magnitude |F| = k|x|. Doubling the displacement from +0.03 m to +0.06 m doubles the stretch, thus doubling the force magnitude at release. This proportionality holds as long as the spring remains ideal and within elastic limits. Choice D erroneously claims zero force for stretching, confusing it with equilibrium. A transferable tip is to use proportional reasoning: multiply force by the displacement ratio for quick comparisons in similar scenarios.

3

A block is attached to an ideal spring on a frictionless table. The equilibrium position is $x=0$ at the unstretched spring length, with +$x$ to the right. The block is held at $x=-0.12,\text{m}$, compressing the spring. What is the direction of the spring force on the block?

To the right (positive $x$ direction)

In the direction of the displacement (negative $x$)

To the left (negative $x$ direction)

Zero because the block is momentarily held still

Explanation

This question examines the direction of the spring's restoring force when the spring is compressed. The restoring force always acts to restore the system to equilibrium, pushing outward when the spring is compressed. At x = -0.12 m, the compression means the force on the block is to the right, or positive x-direction, to expand the spring back to x = 0. The force magnitude is proportional to the displacement's absolute value, but direction is key here, following F = -kx which yields a positive force for negative x. Choice A distracts by suggesting the force is to the left, confusing compression with stretching. Remember, to solve similar issues, visualize the spring's state and confirm the force opposes the displacement vector.

4

A block attached to an ideal spring rests on a frictionless surface. Equilibrium is $x=0$ at the natural length; +$x$ is right. At some instant the block is at $x=-0.05,\text{m}$. Which expression gives the spring force on the block?

$F_s=kx$

$F_s=0$

$F_s=-kx$

$F_s=-kv$

Explanation

This question evaluates the correct expression for spring force using Hooke's law. The restoring force is directed opposite to the displacement, toward equilibrium, and its vector form is F_s = -kx, where the negative sign ensures opposition. For x = -0.05 m, this yields a positive force (to the right), proportional to the displacement magnitude. The expression accounts for both magnitude and direction without depending on velocity. Choice C distracts by using velocity v instead of position x, which applies to damping, not ideal springs. When facing formula-based questions, recall Hooke's law and substitute values to verify direction and proportionality.

5

A mass attached to an ideal spring oscillates on a frictionless surface. Equilibrium is $x=0$ at the natural length, and +$x$ is right. If the displacement changes from $x=+0.05,\text{m}$ to $x=+0.15,\text{m}$, how does the magnitude of the spring force change?

It decreases because the spring is stretched more

It depends on the mass’s velocity

It triples

It increases by a factor of $\sqrt{3}$

Explanation

This question probes how spring force magnitude scales with changes in displacement according to Hooke's law. The restoring force opposes displacement and directs toward equilibrium, whether the spring is stretched or compressed. Its magnitude |F| = k|x| is directly proportional to the displacement from equilibrium, independent of velocity or other factors. Changing from x = +0.05 m to x = +0.15 m triples the displacement, thus tripling the force magnitude. Choice D incorrectly ties the force to velocity, which is not a factor in Hooke's law for ideal springs. A general strategy is to compare ratios of displacements to forecast force changes, ensuring you focus on magnitude separately from direction.

6

A mass is attached to an ideal spring on a frictionless track. Equilibrium is $x=0$ and +$x$ is to the right. If the mass is displaced from $x=+0.20,\text{m}$ to $x=-0.20,\text{m}$, how does the spring-force magnitude compare?

It stays the same because $|x|$ is unchanged

It becomes proportional to velocity instead of position

It becomes zero at negative $x$

It doubles because the displacement changes sign

Explanation

This question assesses understanding of spring forces in AP Physics 1, comparing magnitudes at symmetric positions. The restoring force magnitude depends on |x|, not the sign of x, as |F_s| = k|x|. Moving from +0.20 m to -0.20 m keeps |x| the same, so magnitude remains unchanged. The direction flips, but the question asks about magnitude. Choice A is incorrect because doubling does not occur; sign change affects direction, not magnitude. A transferable strategy is to focus on absolute displacement |x| when questions concern force magnitude, separating it from direction.

7

A block attached to an ideal spring has equilibrium at $x=0$ and +$x$ to the right. The block is at $x=+0.25,\text{m}$ (stretched). Which best describes the spring force compared with the force at $x=+0.05,\text{m}$?

Five times larger magnitude, still to the left

Same magnitude, because both are to the left

Five times larger magnitude, to the right

Zero at $x=+0.25,\text{m}$ because the spring is stretched past equilibrium

Explanation

This question assesses understanding of spring forces in AP Physics 1, comparing forces at different positive displacements. The restoring force is proportional to displacement and always points toward equilibrium. At x = +0.05 m, |F_s| = k0.05 m; at +0.25 m, it's k0.25 m, five times larger, and both point left. The direction remains leftward for positive x. Choice D is incorrect because force is not zero; it increases with displacement. A transferable strategy is to calculate the ratio of displacements to determine how force magnitudes scale in similar positions.

8

A block is attached to an ideal spring with equilibrium at $x=0$ and +$x$ to the right. At $x=+0.10,\text{m}$ the spring-force magnitude is $3,\text{N}$. What is the spring-force magnitude at $x=+0.30,\text{m}$?

$6,\text{N}$

$1,\text{N}$

$9,\text{N}$

$3,\text{N}$

Explanation

This question assesses understanding of spring forces in AP Physics 1, particularly calculating force magnitudes. The restoring force is proportional to displacement, with |F_s| = k|x| from Hooke's law. At x = +0.10 m, the magnitude is 3 N, so k = 3 N / 0.10 m = 30 N/m. At x = +0.30 m, |F_s| = 30 N/m * 0.30 m = 9 N. Choice A is incorrect because it ignores the proportionality, suggesting no change in magnitude. A transferable strategy is to find k from one position and apply it to others, ensuring consistent use of Hooke's law.

9

A block is attached to an ideal horizontal spring on a frictionless table. The equilibrium position is defined as $x=0$ where the spring is neither stretched nor compressed, and +$x$ is to the right. The block is held at $x=+0.10,\text{m}$ (spring stretched) and released from rest. At that instant, what is the direction of the spring’s force on the block?

Upward, because the spring pulls along the table surface

To the left (in the $-x$ direction)

To the right (in the +$x$ direction)

Zero, because the block is momentarily at rest

Explanation

This question assesses understanding of spring forces in AP Physics 1, specifically the direction of the restoring force when a spring is displaced from equilibrium. The restoring force of an ideal spring always acts to return the system to its equilibrium position, opposing the displacement. According to Hooke's law, the force is given by F_s = -kx, where the negative sign indicates it is opposite to the displacement x. Thus, when the block is at x = +0.10 m, the force points to the left, in the -x direction. Choice C is incorrect because the force is not zero; even though the block is at rest, the spring is stretched and exerts a force proportional to the displacement. A transferable strategy is to always determine the direction of the spring force by checking if the displacement is positive or negative and recalling that it points toward equilibrium.

10

A block is attached to an ideal spring. The equilibrium position is defined as $x=0$, and +$x$ is to the right. At one moment the block is at $x=-0.12,\text{m}$ (spring compressed). What is the direction of the spring force on the block?

To the right, toward equilibrium

Zero, because force is only present when stretched

To the left, because compression pushes left

In the direction of the block’s velocity

Explanation

This question assesses understanding of spring forces in AP Physics 1, specifically the direction during compression. The restoring force opposes the displacement, pushing back toward equilibrium when compressed. For a negative displacement like x = -0.12 m, F_s = -k(-|x|) = +k|x|, directing the force to the right. This proportionality to displacement ensures the force magnitude matches k|x|. Choice C is incorrect because springs exert force when compressed, not just when stretched. A transferable strategy is to visualize the spring's state—compressed or stretched—and note the force always points toward the equilibrium position.

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