Explain why a ball thrown horizontally from the top of a cliff and a ball dropped vertically from the same point at the same instant will reach the ground at the same time, despite the thrown ball travelling a greater total distance.

IB DP Physics Standard Level (2023 syllabus) — A.1 Kinematics · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The horizontal and vertical motions of the thrown ball are independent of each other.
Both balls experience the same downward acceleration due to gravity (g) in the vertical direction.
Both balls start with zero vertical velocity, so they undergo identical vertical motion.
Because the vertical motion is identical for both balls, they take the same time to fall the same vertical height, regardless of the horizontal distance covered.

Examiner tips

  • Use the word ‘independent’ to show separation of motions. Mention that the initial vertical velocity is zero for both. Show that time of fall depends only on vertical motion, not horizontal distance.

Common mistakes

  • Confusing horizontal motion with vertical time of fall. Forgetting to state that both start with zero vertical velocity. Saying the balls fall at the same speed instead of same time.

Mark scheme (4 marks)

  1. The horizontal and vertical motions of the thrown ball are independent of each other.
  2. Both balls experience the same downward acceleration due to gravity (g) in the vertical direction.
  3. Both balls start with zero vertical velocity, so they undergo identical vertical motion.
  4. Because the vertical motion is identical for both balls, they take the same time to fall the same vertical height, regardless of the horizontal distance covered.

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