Asked by Christian
A picture of 3 firefighters fighting a fire in a building.
Why are more firefighters needed to help hold the fire hose when the increase of water flow increases the force of the water?
When the force of the water increases, the reaction force pushing the hose back also increases.
When the force of the water increases, the reaction force pushing the hose back decreases.
When the force of the water increases, the reaction force pushing the hose back stays the same.
All Answers 1
Answered by
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When the force of the water increases, the reaction force pushing the hose back also increases.
Why: by Newton’s third law and conservation of momentum, the water leaving the nozzle carries momentum; to accelerate that water the nozzle (and hose) feels an equal and opposite thrust. For a steady jet the thrust F ≈ ṁ v, where ṁ is mass flow rate and v is exit velocity (with ṁ = ρ A v, so F ≈ ρ A v^2 for a fixed nozzle area). As flow/velocity increases the backward force grows, so more firefighters are needed to oppose that larger reaction.
Why: by Newton’s third law and conservation of momentum, the water leaving the nozzle carries momentum; to accelerate that water the nozzle (and hose) feels an equal and opposite thrust. For a steady jet the thrust F ≈ ṁ v, where ṁ is mass flow rate and v is exit velocity (with ṁ = ρ A v, so F ≈ ρ A v^2 for a fixed nozzle area). As flow/velocity increases the backward force grows, so more firefighters are needed to oppose that larger reaction.
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