Topic 2A (fluids, upthrust, stokes law)

an object floats in water, this means _____

weight of object = upthrust

(so w = u = mg )

state the equation for upthrust

U = ρ(f)v(o)g

p = density of fluid

v = volume of object

Explain what happens to a submarine as it moves into a region of lower density seawater

- lower density means lower upthrust

- thus upthrust is now less than weight

- so there is resultant force downwards, so submarine accelerates downwards and sinks

- new trailer is more streamlined (aerodynamic)

- so there is less turbulent flow (or more laminar flow)

- so there is less drag (or air resistance)

- therefore less energy/work is needed to move new trailer at same speed

(work = force X distance moved in force direction )

- When the wax expands, the volume of fluid displaced increases.

- therefore, the upthrust increases

(Upthrust = weight of fluid displaced, so more volume of fluid displaced means more weight of liquid displaced.)

- thus upthrust > weight of wax and there is RF upwards

NOTE: this question is submerged case Q., for a floated case the sequence would be different

U = W + D

( OR → U - W - D = 0 )

NOTE:
- terminal velocity mentioned, so RF is 0 and upward force = downward forces

- wax moving upwards so drag must be downward (drag always opposes motion)