AP Physics Physics: Classical Mechanics & Kinematics 3 — Questions and Answers
Question 1: A ball rolls off a horizontal table of height 1.25 m with a horizontal speed of 3 m/s. How long does it take to hit the ground? (g = 10 m/s²)
- 0.25 s
- 0.5 s (Correct answer)
- 1.0 s
- 1.5 s
Correct answer: 0.5 s
Using h = ½gt²: 1.25 = ½(10)t², so t² = 0.25, t = 0.5 s.
Question 2: Newton's third law states that for every action there is an equal and opposite reaction. When a rocket expels gas downward, the reaction force:
- Acts on the gas in the downward direction
- Acts on the rocket in the upward direction (Correct answer)
- Cancels out, resulting in no net force
- Acts on the rocket in the downward direction
Correct answer: Acts on the rocket in the upward direction
The expelled gas pushes down (action), and by Newton's third law the rocket is pushed upward (reaction).
Question 3: An object moves with velocity described by v(t) = 4t − 2 m/s. What is the displacement from t = 1 s to t = 3 s?
- 8 m
- 12 m (Correct answer)
- 14 m
- 16 m
Correct answer: 12 m
Displacement = ∫₁³ (4t−2)dt = [2t²−2t]₁³ = (18−6)−(2−2) = 12 − 0 = 12 m.
Question 4: A 3 kg object moving at 6 m/s collides with a stationary 3 kg object and they stick together. What is their final velocity?
- 6 m/s
- 4 m/s
- 3 m/s (Correct answer)
- 2 m/s
Correct answer: 3 m/s
By conservation of momentum: (3)(6) = (3+3)v, so v = 18/6 = 3 m/s.
Question 5: What is the centripetal acceleration of an object moving in a circle of radius 4 m at a speed of 8 m/s?
- 2 m/s²
- 8 m/s²
- 16 m/s² (Correct answer)
- 32 m/s²
Correct answer: 16 m/s²
Centripetal acceleration a_c = v²/r = 64/4 = 16 m/s².
Question 6: A force of 50 N is applied at 60° to the horizontal to push a crate. What is the horizontal component of the force? (cos60° = 0.5)
- 25 N (Correct answer)
- 43 N
- 50 N
- 87 N
Correct answer: 25 N
Horizontal component = F·cosθ = 50 × 0.5 = 25 N.
Question 7: Which kinematic equation would you use to find displacement when initial velocity, final velocity, and time are known but acceleration is not needed?
- v = v₀ + at
- d = v₀t + ½at²
- v² = v₀² + 2ad
- d = ½(v₀ + v)t (Correct answer)
Correct answer: d = ½(v₀ + v)t
The equation d = ½(v₀ + v)t uses only initial velocity, final velocity, and time.
AP Physics Physics: Classical Mechanics & Kinematics 3
A ball rolls off a horizontal table of height 1.25 m with a horizontal speed of 3 m/s.
How long does it take to hit the ground? (g = 10 m/s²)