Relative Velocity using a Boat crossing a River
A boat points straight across the river, but due to the rivers speed the
boat travels both across and down stream. The small diagram below shows
the velocity of the boat relative to the shore (red arrow) as the vector
sum of the rivers velocity relative to the shore (horizontal arrow) and
the boats velocity relative to the river (vertical arrow).
In order for a boat to travel straight across the river without moving
down stream, the boat must point at some angle up stream. By doing this,
the boat has a component of its velocity pointing upstream. If that
component is the same magnitude as the rivers downstream velocity, the two
velocties cancel and the boat will travel straight across the river. The
small diagram below shows
the velocity of the boat relative to the shore (red arrow) as the vector
sum of the rivers velocity relative to the shore (horizontal arrow) and
the boats velocity relative to the river (angled arrow).
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