The boat moves in still water at a speed of 5m / s. The speed of the river is 8m / s. What is the speed of the boat relative to the shore?

| January 12, 2021 | education

Vt = 8 m / s.
Vк / в = 5 m / s.
Vk -?
According to the law of relativity of motion, the speed of a body relative to a stationary coordinate system Vк (relative to the coast) is the vector sum: the speed of the moving coordinate system relative to a stationary Vт (current velocity relative to the coast) and the body speed relative to the moving coordinate system Vк / в (boat speed relative ).
The boat is moving with the flow.
The speed of the boat relative to the coast Vk will be the sum of the current velocity Vt and the speed of the boat relative to the water Vk / w: Vk = Vt + Vk / w.
Vk = 8 m / s + 5 m / s = 13 m / s.
The boat is moving against the current.
The speed of the boat relative to the coast in this case, Vk will be the difference between the speed of the current Vt and the speed of the boat relative to the water Vk / w: Vk = Vt – Vk / w.
Vk = 8 m / s – 5 m / s = 3 m / s.
The boat’s own speed is not enough to overcome the speed of the current, so it will move with the current at a speed Vк = 3 m / s.
Answer: the speed of the boat relative to the coast is Vк = 13 m / s when it moves with the current and Vк = 3 m / s when it moves against the current.



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