Question

Consider the following nozzle attached to a tripod. The nozzle mass is 10kg when filled with water and has a 5cm diameter. The tripod is rated for 1800N of holding force. A firefighter was standing 60cm behind the nozzle and was hit by the nozzle when the tripod suddenly failed. You have been hired as an accident reconstructionist and you determine that the flow rate increased to the point where it did break at 1800N. You must state in your report the water velocity and flow rate at this failure point. Also, what was the speed of the nozzle when it struck the firefighter?

Answer 1

The tripod is rated for a holding force of 1800 N i.e., it can hold the thrust force of the nozzle up to 1800 N The tripod fail if the thrust reaches this value thrust of nozzle is equal to mass flow rate times the exist velocity T = mV_e Mass flow rate m = rho A_e v_e rho = density of fluid A_e = -exist area T = rho A_e v^2_e v_e = exit velocity At point of failure T = rho A_e V^2_e = 1000 times pi/4 times 0.05^2 times v^2_e = 1800 N V_e = squareroot 916.7 = 30.3 m/s Flow rate at failure point Q = A_e v_e = pi/4 times 0.05^1 times 30.3 = 0.059 m^3/s = 59 m^2/s After tripod fails, the thrust force is imparted onto the mass di nozzle This gives acceleration to mass given by a = T/M_n = 1800 N/10 kg = 1800 m/s^2 The firefighter was at a