Physics
Physics
Problem
A student completely fills a plastic syringe with water and seals the tip. When the plunger is pushed inward with increasing force, the plunger moves only a tiny distance, yet the force required rises sharply. Which of the following best explains this observation? a) Because compressing the trapped water obeys Boyle’s law (), its volume must decrease, so ever-larger pressure—and therefore force—is required as the plunger moves. b) Since the plunger travels only a small distance, the work relation implies the applied force must be large, so the required force rises sharply. c) Water’s nearly constant density resists significant volume change, so the applied force primarily increases the internal pressure rather than decreasing the volume. d) As the pressure rises it acts back on the entire face of the plunger, exerting a growing opposing force; increasingly more force is therefore needed to keep the plunger moving inward.Intuitive explanation
Water is essentially incompressible—its volume barely changes even under large pressure increases. When you try to compress the water in the sealed syringe, you're not actually reducing its volume meaningfully; instead, you're building up internal pressure very rapidly. The sharp force increase reflects how steeply pressure must rise to achieve even tiny volume changes, governed by the bulk modulus of water. This is why liquids resist compression so differently from gases.
Intuitive explanation
Water is essentially incompressible—its volume barely changes even under large pressure increases. When you try to compress the water in the sealed syringe, you're not actually reducing its volume meaningfully; instead, you're building up internal pressure very rapidly. The sharp force increase reflects how steeply pressure must rise to achieve even tiny volume changes, governed by the bulk modulus of water. This is why liquids resist compression so differently from gases.