Correct answer: A

The correct answer is An object's tendency to maintain its state of motion unless a net force acts.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: Newton's first law is the law of inertia: zero net force means constant velocity, including rest.
3. Check units and direction: The correct answer is An object's tendency to maintain its state of motion unless a net force acts.

Correct answer: B

The correct answer is $\vec F_{net}=m\vec a$.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: The net force on an object equals mass times acceleration, with force and acceleration pointing in the same net direction.
3. Check units and direction: The correct answer is $\vec F_{net}=m\vec a$.

Correct answer: C

The correct answer is Forces between interacting objects occur in equal-magnitude, opposite-direction pairs.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: The paired forces act on different interacting objects, so they do not cancel on one free-body diagram.
3. Check units and direction: The correct answer is Forces between interacting objects occur in equal-magnitude, opposite-direction pairs.

Correct answer: D

The correct answer is A force that opposes sliding motion between surfaces.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: Kinetic friction acts opposite the relative sliding motion and depends on the surfaces and normal force.
3. Check units and direction: The correct answer is A force that opposes sliding motion between surfaces.

Correct answer: A

The correct answer is $3\,\text{m/s}^2$.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: Using $a=F_{net}/m$, $a=12/4=3\,\text{m/s}^2$.
3. Check units and direction: The correct answer is $3\,\text{m/s}^2$.

Correct answer: B

The correct answer is $W=Fd$.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: For parallel force and displacement, work is force multiplied by displacement; more generally $W=Fd\cos\theta$.
3. Check units and direction: The correct answer is $W=Fd$.

Correct answer: C

The correct answer is $K=\frac{1}{2}mv^2$.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: Translational kinetic energy is $K=\frac{1}{2}mv^2$, so doubling speed quadruples kinetic energy.
3. Check units and direction: The correct answer is $K=\frac{1}{2}mv^2$.

Correct answer: D

The correct answer is $U_g=mgh$.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: Near Earth's surface, gravitational potential energy relative to a chosen reference is $U_g=mgh$.
3. Check units and direction: The correct answer is $U_g=mgh$.

Correct answer: A

The correct answer is The sum of kinetic and potential energy remains constant.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: If friction and other nonconservative work are absent, $K_i+U_i=K_f+U_f$.
3. Check units and direction: The correct answer is The sum of kinetic and potential energy remains constant.

Correct answer: B

The correct answer is The rate at which work is done or energy is transferred.

1. Identify the physics model: Determine whether the problem concerns motion, force, energy, momentum, waves, electricity, magnetism, or experimental data.
2. Apply the relationship: Average power is $P=\frac{W}{\Delta t}$, and power describes how quickly energy is transferred.
3. Check units and direction: The correct answer is The rate at which work is done or energy is transferred.