Hose recoil from momentum change

Steps:

• Convert mass flow rate to SI units: 90 kg/min ÷ 60 s/min = 1.5 kg/s.
• Water exits with horizontal velocity 20 m/s, starting from rest inside hose.
• Recoil force F = (dm/dt) × Δv = 1.5 kg/s × 20 m/s = 30 N (backward on hose).
• Person applies equal forward force to hold hose stationary.

Why A is correct:

• Force equals rate of momentum change of water (F = ṁv), per Newton's second law applied to fluid thrust.

Why the others are wrong:

• B: Incorrect; overestimates by factor of 9, possibly misapplying units or velocity.
• C: Incorrect; assumes 90 kg/s without converting minutes to seconds (90 × 20 = 1800 N).
• D: Incorrect; extreme overestimate, perhaps multiplying by extra factors like 60 s/min twice.

Final answer: A