Relative motion: the general idea

Frame-switching: add the frame's velocity to everything — positions, velocities, accelerations.

Relative motion: the general idea0 min · Free lecture

In this lesson

  • Frame-switching: add the frame's velocity to everything — positions, velocities, accelerations.

Watch a ball from a moving train: its path changes even though the physics doesn't.

v_AB = v_A − v_B in general. Accelerations: a_AB = a_A − a_B (only equal when the frame accelerates uniformly — pseudo forces come later).

Choose the frame that makes the most quantities zero: usually the faster-moving body.

v⃗A/B=v⃗A−v⃗B\vec{v}_{A/B} = \vec{v}_A - \vec{v}_B

Relative velocity (vector)

Worked example

The acceleration of a body starting from rest is a = 2 m/s² for 4 s, then zero for 2 s. Find its velocity at t = 6 s.

  1. Area under the a–t graph = change in velocity.
  2. Area = 2 m/s² × 4 s = 8 m/s (the last 2 s add nothing).

Answer: 8

Transcript (0 min)
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Relative motion: the general idea — FemtoLearn.
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Frame-switching: add the frame's velocity to everything — positions, velocities, accelerations.
Printable notes (free)

Practice

Free · 4 questions with full solutions
  1. Q1 · Numerical · difficulty 2/5

    The acceleration of a body starting from rest is a = 2 m/s² for 4 s, then zero for 2 s. Find its velocity at t = 6 s.

  2. Q2 · MCQ · difficulty 2/5

    A particle moves with constant velocity. Which a–t graph describes it?

    • A horizontal line at a = 0
    • A horizontal line at a = 2 m/s²
    • A straight line through the origin
    • A parabola
  3. Q3 · Numerical · difficulty 2/5

    A body moves with v = 10 m/s for 3 s, then v = 5 m/s for 2 s in the same direction. Find the total distance.

  4. Q4 · MCQ · difficulty 2/5

    The slope of a velocity–time graph gives:

    • Displacement
    • Acceleration
    • Speed
    • Distance