Train A moves at 20 m/s. A passenger in A sees a train B of length 100 m pass him completely in 10 s. Which speeds of B are possible?
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Question

Train A moves at 20 m/s. A passenger in A sees a train B of length 100 m pass him completely in 10 s. Which speeds of B are possible?

G1 Physics · Accepted Answer

The magnitude of relative speed must be 100/10=10 m/s. For same-direction motion, |v_B−20|=10 gives v_B=30 or 10 m/s. If B were opposite at 40 m/s, relative speed would be 60 m/s.