# HIGHER PHYSICS - Clydebank High School

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 = gh Fupthrust

h Fweight So, Fupthrust = ghA Resultant force on object, Fresultant = Fweight - Fupthrust surface area, A liquid density, 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 = ghA = 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 = ghA,

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

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