1. Upthrust, Floating & Sinking
HIGHER PHYSICS
Upthrust, Floating & Sinking

2. UPTHRUST
Any object placed in a fluid receives an upward force, due to the pressure at that depth.
Fupthrust
Fweight h
surface area, A
liquid density, ρ
i.e. F = pA & p = ρgh
So, Fupthrust = ρghA
Resultant force on object,
Fresultant = Fweight - Fupthrust

3. Example – 1kg iron mass (see Virtual ‘H’ Physics)
mass, m = 1 kg, Fweight = mg = 1 x 9.8 = 9.8 N
area of base, A = m2,
depth, h = 2 cm = 0.02 m
Fupthrust = ρghA = 1000 x 9.8 x 0.02 x
= 0.49 N
Fresultant = Fweight – Fupthrust = 9.8 – 0.49 = 9.31 N
** N.B. up thrust increases as depth increases
for fully submerged objects, the upthrust is constant
Fupthrust = ρghA, where h = height of object

4. Fupthrust < F weight
FLOATING & SINKING
For an object to float its upthrust must be equal to its weight.
Fupthrust
Fweight
i.e. Fupthrust = Fweight
(so, Fresultant = 0 N)
If the object gets heavier, it will sink deeper to increase its upthrust.
If it becomes too heavy,
Fupthrust < F weight
and the object will sink.

5. Question