HIGHER PHYSICS
Upthrust, Floating & Sinking
UPTHRUST
Any object placed in a fluid
receives an upward force,
due to the pressure at that
depth.
i.e. F = pA & p = ρgh
Fupthrust
h
Fweight
So, Fupthrust = ρghA
Resultant force on object,
Fresultant = Fweight - Fupthrust
surface area, A
liquid density, ρ
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 = 0.0025 m2,
depth, h = 2 cm = 0.02 m
Fupthrust = ρghA
= 1000 x 9.8 x 0.02 x 0.0025
= 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
FLOATING & SINKING
For an object to float its upthrust
must be equal to its weight.
Fupthrust
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.
Fweight
Question