Fluid Velocity (College Board AP® Physics 1: Algebra-Based)

Fluid velocity

• Newton's laws can be applied to particles moving within a fluid

  • The net velocity of a region of fluid will remain constant unless acted on by a net force

• When applied to fluids, Newton's first law states:

The net velocity of a region of fluid will remain constant unless acted on by a net force

• When applied to fluids, Newton's second law states:

The net force acting on a region of fluid in a particular direction is equal to the rate of change of momentum of the region of fluid in that direction

• When applied to fluids, Newton's third law states:

If a force is applied on a fluid by an object, that fluid applies an equal force in the opposite direction on the object

• Consider a system where an ideal fluid is contained in a tube:

  • If a force is applied to one end of the tube, the particles of the fluid experience a net force and undergo a change in velocity

  • The fluid is incompressible, so the neighbouring particles experience a force from the first particles and also move, ensuring density is constant

  • This 'knock-on' effect occurs until the fluid at the far end also experiences a change in velocity due to forces from neighbouring particles

• The macroscopic force is transferred through the fluid via microscopic internal interactions between fluid particles

Ideal fluid transferring force

• One example of this transfer of force is liquid being squirted out of a syringe

  • The plunger exerts a force on one end of a column of fluid

  • This force is transferred to the other end of the fluid

  • This results in the region of fluid at the other end changing momentum and accelerating out of the syringe

Force transfer in a fluid in a syringe