Where, Not When
Maya swims straight across a 60 m river at 1.2 m/s. She reaches the far bank in 50 s, exactly as in a still lake, and steps out 50 m downstream.
Why did the current take her sideways without slowing her crossing?
Page 1
Science · Honors Physics
Chapter 1: Kinematics with Calculus Readiness
Every velocity is measured with respect to something.
Maya swims straight across a 60 m river at 1.2 m/s. She reaches the far bank in 50 s, exactly as in a still lake, and steps out 50 m downstream.
Why did the current take her sideways without slowing her crossing?
Make your guess to turn the page.
Still to come: 10 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students merge two questions: how long a crossing takes and where it ends. They expect a current to slow a straight-across crossing, because it must be fought. The book sets that against Maya’s equal times and a lab where 25.0 s never changes. Ask: does the current have any part across the river? The idea to reach is that it changes where you land, not when.
About the Michelson–Morley figures — The 1887 paper gives an effective light path of about 11 m by repeated reflection and an expected shift of about 0.4 fringe. The observed shift was less than a twentieth of that. Earth’s orbital speed, about 30 km/s, was the expected ether wind. The one-in-a-trillion figure is uv/c² for u = v = 300 m/s.