Gaokao Exam Physics: Mechanics and Kinematics 2 — Questions and Answers

Question 1: A car accelerates uniformly from rest to 20 m/s in 5 seconds. What is its acceleration?

4 m/s² (Correct answer)
100 m/s²
2.5 m/s²
10 m/s²

Correct answer: 4 m/s²

Acceleration a = Δv/Δt = (20-0)/5 = 4 m/s².

Uniform acceleration means the velocity changes at a constant rate. Using the formula a = (v - v₀)/t, where v = 20 m/s (final velocity), v₀ = 0 (starts from rest), and t = 5 s: a = (20 - 0)/5 = 4 m/s². This is a fundamental kinematics calculation covered in Chinese high school physics (必修1). Note that acceleration is a vector — its direction is the same as the direction of velocity change (here, same as motion direction).

Question 2: According to Newton's Third Law, when a horse pulls a cart forward, the cart pulls the horse backward with equal force. Why does the horse-cart system still accelerate forward?

Newton's Third Law action-reaction pairs act on different objects; the net force on the horse from the ground exceeds the backward pull from the cart (Correct answer)
The horse exerts more force than the cart exerts back
Newton's Third Law doesn't apply during acceleration
The cart's weight cancels the backward force on the horse

Correct answer: Newton's Third Law action-reaction pairs act on different objects; the net force on the horse from the ground exceeds the backward pull from the cart

Action-reaction forces always act on different objects and cannot cancel each other. The horse accelerates because the ground pushes it forward (friction) more than the cart pulls it back. The NET force on the horse-cart system is analyzed separately.

Newton's Third Law: the horse pulls the cart forward with force F, and the cart pulls the horse backward with equal force F (action-reaction pair on different objects). These cannot cancel because they act on different objects. Analyzing the horse alone: forces include the cart's backward pull (F) and the ground's forward friction (F_ground). If F_ground > F, the horse accelerates forward. The key insight is that action-reaction pairs never act on the same object, so they never directly cancel in a single free-body diagram. This is a classic Gaokao conceptual question.

Question 3: A 2 kg object slides down a frictionless incline of 30°. What is its acceleration along the incline? (g = 10 m/s²)

5 m/s² (Correct answer)
10 m/s²
8.66 m/s²
2 m/s²

Correct answer: 5 m/s²

The component of gravity along the incline: a = g·sin30° = 10 × 0.5 = 5 m/s². Mass cancels out.

On a frictionless incline of angle θ = 30°, the forces on the object are: normal force (perpendicular to slope, doesn't affect motion along slope) and gravity (mg, vertically downward). The component of gravity along the incline is mg·sin30°. Applying Newton's Second Law along the incline: ma = mg·sin30° → a = g·sin30° = 10 × 0.5 = 5 m/s². Notably, acceleration is independent of mass — all objects slide down a frictionless incline at the same acceleration regardless of mass.

Question 4: The unit of momentum in the SI system is:

kg·m/s (Correct answer)
kg·m/s²
N·m
J/s

Correct answer: kg·m/s

Momentum p = mv. Units: kg × m/s = kg·m/s. This is also equivalent to N·s (Newton-second), since N = kg·m/s².

Momentum (动量) is defined as p = mv, where m is mass (kg) and v is velocity (m/s). The SI unit of momentum is therefore kg·m/s (kilogram-metre per second). This can also be expressed as N·s, since 1 N = 1 kg·m/s², so 1 N·s = 1 kg·m/s. The impulse-momentum theorem (冲量—动量定理) states Ft = Δp, connecting force×time (impulse, in N·s) to momentum change (in kg·m/s). These concepts form a major section of Chinese high school physics mechanics.

Question 5: In projectile motion (抛体运动), what happens to the horizontal velocity component throughout the flight (ignoring air resistance)?

It remains constant (Correct answer)
It increases due to gravity
It decreases due to gravity
It becomes zero at the highest point

Correct answer: It remains constant

In projectile motion without air resistance, the horizontal direction has no force acting on it, so horizontal velocity remains constant throughout the flight (Newton's First Law).

In ideal projectile motion (no air resistance), the motion is resolved into independent horizontal and vertical components. Horizontally: no force acts (gravity is vertical), so by Newton's First Law, horizontal velocity vₓ = v₀cosθ remains constant throughout the flight. Vertically: gravity (g downward) causes constant downward acceleration, so vertical velocity changes continuously. This independence of horizontal and vertical motions is the fundamental principle of projectile motion (平抛运动 for horizontal throw, 斜抛运动 for angled throw) in Chinese physics 必修2.

Question 6: What does the area under a velocity-time (v-t) graph represent?

Displacement (Correct answer)
Acceleration
Force
Distance traveled only for constant velocity

Correct answer: Displacement

The area under a v-t graph represents displacement (位移). This follows from the definition: displacement = average velocity × time, and graphically this is the area under the v-t curve.

The velocity-time (v-t) graph is fundamental to kinematics in Chinese high school physics. The slope of the v-t graph represents acceleration (a = dv/dt). The area under the v-t graph represents displacement (s = ∫v dt). This is true for all types of motion — uniform, uniformly accelerated, or non-uniform. For a uniformly accelerating object from v₀ to v in time t, the area under the v-t graph is a trapezoid: s = (v₀ + v)t/2. Understanding v-t and s-t graphs is essential for Gaokao mechanics problems.

Gaokao Exam Physics: Mechanics and Kinematics 2

A car accelerates uniformly from rest to 20 m/s in 5 seconds.
What is its acceleration?