Gaokao Exam Physics: Mechanics and Kinematics 1 — Questions and Answers
Question 1: A ball is thrown horizontally from a cliff with an initial speed of 10 m/s. What is the magnitude of its vertical velocity component after 2 seconds? (g = 10 m/s², ignore air resistance)
- 10 m/s
- 20 m/s (Correct answer)
- 22 m/s
- 14 m/s
Correct answer: 20 m/s
In projectile motion, the horizontal and vertical components are independent. The vertical component starts at zero and increases due to gravity: v_y = g × t = 10 × 2 = 20 m/s. The horizontal component (10 m/s) does not affect the vertical calculation.
Question 2: A 5 kg box is pushed across a horizontal floor by a 30 N horizontal force. The coefficient of kinetic friction between the box and floor is 0.4. What is the acceleration of the box? (g = 10 m/s²)
- 6 m/s²
- 4 m/s²
- 2 m/s² (Correct answer)
- 1 m/s²
Correct answer: 2 m/s²
Normal force N = mg = 5 × 10 = 50 N. Friction force f = μN = 0.4 × 50 = 20 N. Net force = 30 − 20 = 10 N. By Newton's Second Law, a = F_net / m = 10 / 5 = 2 m/s².
Question 3: A train moving at 72 km/h applies its brakes and decelerates uniformly to rest in 10 seconds. What distance does the train cover while braking?
- 200 m
- 72 m
- 50 m
- 100 m (Correct answer)
Correct answer: 100 m
Convert speed: 72 km/h = 20 m/s. Using s = (u + v)/2 × t = (20 + 0)/2 × 10 = 100 m. Alternatively, find deceleration a = (0 − 20)/10 = −2 m/s², then s = ut + ½at² = 20(10) + ½(−2)(100) = 200 − 100 = 100 m.
Question 4: An object rests on a horizontal table. Which statement correctly describes the weight of the object and the normal force exerted by the table on the object?
- They form an action-reaction pair according to Newton's Third Law
- They are equal in magnitude but act on the same object and are NOT an action-reaction pair (Correct answer)
- The normal force is always greater than the weight
- They act in the same direction and together keep the object stationary
Correct answer: They are equal in magnitude but act on the same object and are NOT an action-reaction pair
Weight and normal force are equal in magnitude and opposite in direction, but both act on the same object — they are a balanced force pair (Newton's First Law), not an action-reaction pair. An action-reaction pair (Newton's Third Law) always acts on two different objects. The true reaction to the object's weight is the gravitational pull the object exerts on the Earth.
Question 5: A 500 g ball traveling at 4 m/s is acted on by a constant net force for 2 seconds, accelerating it to 8 m/s in the same direction. What is the magnitude of the net force?
- 2 N
- 4 N
- 0.5 N
- 1 N (Correct answer)
Correct answer: 1 N
Mass m = 0.5 kg. Change in velocity Δv = 8 − 4 = 4 m/s over t = 2 s, so a = 4/2 = 2 m/s². By Newton's Second Law, F = ma = 0.5 × 2 = 1 N.
Question 6: Object A moves to the right at 3 m/s and object B moves to the left at 5 m/s along the same straight line. What is the velocity of A as observed from B's reference frame?
- 2 m/s to the right
- 2 m/s to the left
- 8 m/s to the left
- 8 m/s to the right (Correct answer)
Correct answer: 8 m/s to the right
Taking rightward as positive, v_A = +3 m/s and v_B = −5 m/s. The velocity of A relative to B is v_A − v_B = 3 − (−5) = 8 m/s, which is positive, meaning 8 m/s to the right. The two objects approach each other at 8 m/s.
Gaokao Exam Physics: Mechanics and Kinematics 1
A ball is thrown horizontally from a cliff with an initial speed of 10 m/s.
What is the magnitude of its vertical velocity component after 2 seconds? (g = 10 m/s², ignore air resistance)