Hello, and thank you kindly for considering my question. Background: Medical needles come in gauges such that a smaller gauge represents a larger needle. Scenario: Doctor Jack says "18 gauge needles are best for this procedure", but Doctor Jane says "27 gauge needles are best because they are smaller". Parameters: 10 mL fluid being injected over 100 seconds. Inner diameter of 18 and 27 gauge needles respectively: 0.838 mm, 0.210mm. For simplicity, assume that the needle is in a vein. Questions: 1) How much faster (velocity) does the fluid exit a 27 vs. 18 gauge needle? 2) How much 'harder' does the fluid hit the wall of the vein with 27 vs. 18 gauge? I have answered question #1 using basic mathematics (volume of a cylinder): 18.131 cm/s vs. 288.716 cm/s, or roughly 16 times faster. I "dumbed down" question 2 because I'm not sure if I should be asking it in terms of pressure, kinetic energy, or something else. My gut instinct is that it isn't too much different from a car hitting a brick wall (in the sense that 2x velocity = 4x energy: 1/2 mv^2, if memory serves), so at about 16 times the velocity, the force(energy) of the fluid hitting the wall should be roughly 256 times more in the 27 versus 18 gauge (...I think?) Thanks so much for your time and consideration!