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PRACTICE / Relative motion

A faster runner closes the gap

Two runners move in the same direction at 7 m/s and 5 m/s. The slower runner starts 40 m ahead.

When does the faster runner catch up?

Organize the givens & assumptions
  • gap = 40 m
  • vf = 7 m/s
  • vs = 5 m/s

Constant straight-line speeds in the same direction; the stated 40 m is the initial lead.

  1. A8 s8\ \mathrm s
  2. B40/7 s40/7\ \mathrm s
  3. C20 s20\ \mathrm s
  4. D10/3 s10/3\ \mathrm s

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