FLOWS AND JETS
Question. Imagine shooting water from a syringe by pushing the piston. With Newton's first law in mind (motion is steady if no force is applied), I ask: Does it take any force to move the piston with a constant speed—assuming a perfectly frictionless piston and perfectly nonviscous water? In other words, once I push the piston to give it some speed and let it go, will it continue at the same speed by inertia?
Answer. A force is required to push the piston with constant speed—even in a perfectly frictionless world. Newton's first law of steady motion by inertia doesn't apply here because some parts of fluid do accelerate—namely, those nearing the exit from the cylinder. Figure 5.2 shows this in more detail.
Figure 5.1. Will the piston move steadily by inertia if there is no viscos-
ity and no friction?