Archimedes' Principle and Apparent Weight
A body wholly or partly immersed in a static fluid experiences an upward buoyant force equal to the weight of the fluid displaced by the immersed portion of the body.
Why this shows up in the exam
Density measurement by apparent weight · Design of submarines and balloons · Scale-reading changes during immersion
Learn the idea
Buoyant force equals the weight of displaced fluid and changes a body's measured support force. Pressure is greater on a submerged body's lower surface than on its upper surface. The net upward pressure force is exactly the weight of fluid that would occupy the displaced region.
🧠 Memory hook: Buoyancy weighs the displaced fluid, not the whole container.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- F_b = rho_f V_displaced g — Archimedean buoyancy in a fluid of density rho_f under uniform effective gravity
- W_apparent = mg - F_b — support reading for a stationary immersed body when upward buoyancy opposes weight
- F_support,on fluid = F_b — reaction added to a vessel scale when an externally supported body is immersed without touching it
How to approach it
- 1Draw all forces on the body and on any scale separately
- 2Find only the actually displaced fluid volume
- 3Apply equilibrium or acceleration with a consistent sign convention
Common slip-ups that cost marks
- •Using total body volume for partial immersion
- •Forgetting the reaction on the liquid or container
- •Calling apparent weight a reduction in the body's true gravitational weight
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A wire 1 m long and cross-sectional area 2 mm^2 extends by 1 mm under a 200 N load. Find Young modulus.
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