Hose recoil force from momentum change

Steps:

• Convert mass flow rate to SI units: 90 kg/min = 1.5 kg/s.
• Water emerges at 20 m/s horizontally; assume initial velocity inside hose is 0 m/s.
• Rate of momentum change = mass flow rate × velocity change = 1.5 kg/s × 20 m/s.
• Force required = 30 N (by Newton's second law, F = dp/dt).

Why A is correct:

• Matches the thrust force formula F = (dm/dt) × v, where dm/dt is mass flow rate and v is exit velocity.

Why the others are wrong:

• B: Uses unconverted 90 kg/min × 20 m/s = 1800 kg m/s², ignoring unit conversion.
• C: Equals 90 kg/min × 20 m/s without dividing by 60 s/min.
• D: Likely from erroneous calculation like 90 kg/s × 20 m/s × something extra.

Final answer: A