ACT Science: Buoyancy and Archimedes—Floating and Sinking Made Simple

Buoyant force (upward force on a submerged object) equals the weight of the fluid displaced by that object. Formula: F_b=ρVg, where ρ=fluid density, V=volume displaced, g=gravity. An object sinks if its weight exceeds buoyant force; it floats if buoyant force ≥ weight. The key insight: it doesn't matter how heavy the object is; what matters is how much fluid it displaces. A heavy but large object (ship) floats because it displaces a huge volume of water. A small dense object (pebble) sinks because it displaces little water.

Intuition check: A hot air balloon floats because the balloon displaces more air than its own weight. A submarine floats or sinks by controlling how much water it displaces.

Scenario 1 (Will it float?): An object has mass 2 kg and volume 0.005 m³. Water has density 1000 kg/m³. Weight=2×10=20 N. Buoyant force=1000×0.005×10=50 N. Since 50>20, object floats. Scenario 2 (Sinking): An object has mass 5 kg and volume 0.002 m³. Weight=5×10=50 N. Buoyant force=1000×0.002×10=20 N. Since 20<50, object sinks. Scenario 3 (Neutral buoyancy): An object has mass 3 kg and volume 0.003 m³. Weight=3×10=30 N. Buoyant force=1000×0.003×10=30 N. Since forces equal, object suspends in fluid (neither sinks nor floats). The pattern: compare weight to buoyant force to predict motion.

In dense fluids (salt water, mercury), objects float more easily because buoyant force increases with fluid density.

Calculation 1: A wooden cube (mass 1 kg, volume 0.002 m³) is placed in water. Will it float? Weight=1×10=10 N. Buoyant force=1000×0.002×10=20 N. Since 20>10, it floats. Calculation 2: A steel ball (mass 8 kg, volume 0.001 m³) is in water. Will it sink? Weight=8×10=80 N. Buoyant force=1000×0.001×10=10 N. Since 10<80, it sinks. Calculation 3: An object has buoyant force 60 N in water and weight 40 N. Is it accelerating upward, downward, or suspended? Net force=60-40=20 N upward. It accelerates upward (floats to the surface). Complete these three daily until calculations are fast and automatic.

On test day, remember: F_b=ρVg, and compare to weight to determine motion.

Buoyancy questions appear in roughly 1-2 ACT Science passages. They test straightforward application of a single principle and formula. Once you understand Archimedes' principle and can calculate buoyant force, these questions become mechanical. Investing 15 minutes in this topic yields 1-2 guaranteed points because the concept is simple and the formula is provided.

Review this principle one day before the test. On test day, buoyancy questions will feel like easy points.