Based on the scenario below, choose the best answer.A car is driving up a slope at a constant velocity. This means:

Question

BlackTom AI · Accepted Answer

Newton's first law (equilibrium) implies that an object moving at constant velocity has zero net force acting on it.

1. Relevant knowledge points and concepts
- Newton's first and second laws: constant velocity ⇒ acceleration a = 0 ⇒ ΣF = 0.
- Forces on an inclined plane: weight mg (acts vertically), normal force N (perpendicular to surface), frictional force f (parallel to surface, opposing motion), and the engine/drive force Fdrive (if the car is powered).
- Components of weight: parallel to slope = mg sinθ (down the slope); perpendicular to slope = mg cosθ.

2. Step-by-step reasoning
- Because the car moves up the slope at constant speed, acceleration along and perpendicular to the surface is zero.
- Take axes: x along the slope (positive up the slope), y perpendicular (out of the surface).
- Along x: ΣFx = Fdrive - f - mg sinθ = 0. This says the driving force balances friction and the downhill component of weight.
- Perpendicular: ΣFy = N - mg cosθ = 0 ⇒ N = mg cosθ.

3. Intermediate steps (equations)
- ΣFx = 0 ⇒ Fdrive = f + mg sinθ.
- ΣFy = 0 ⇒ N = mg cosθ.

4. Why the correct answer is correct
- The statement "the net force is zero" (option b) follows directly from ΣF = ma with a = 0. None of the other options state the general condition required by constant velocity.

5. Why other options are incorrect
- a. "Frictional force equals the normal force." Friction is not generally equal to the normal force; static/kinetic friction magnitude is related by f ≤ μN or f = μkN only for limiting/kinetic cases, not universally equal.
- c. "The weight force equals the frictional force." Weight (mg) is generally much larger and acts vertically; its components appear in balance equations, but mg ≠ f in general.
- d. "The weight force equals the normal force." On an incline N = mg cosθ, which is less than mg for θ ≠ 0; equality holds only for a horizontal surface.

6. Conclusion
Therefore the best answer is b. The net force is zero. This matches the provided correct answer.

Based on the scenario below, choose the best answer.A car is driving up a slope at a constant velocity. This means:单项选择题

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