The upward push
Pressure is greater at greater depth. The combined pressure forces on an immersed object produce an upward buoyant force equal to the weight of the displaced fluid.
Buoyancy & displacement
Connect displaced fluid to floating and sinking.
Related class notes
Open PDF →Selected pages previewed; the PDF contains the complete set.
Floating is a balance
A floating object at rest displaces fluid whose weight equals its own weight. Adding cargo makes a boat sit lower and displace more water.
Overall density matters
A sealed object less dense overall than the surrounding fluid can float partly submerged. A hollow hull includes enclosed air in the volume that excludes water. A denser object sinks in the simplified model.
Balance weight and buoyancy
Change the object or surrounding liquid and show the force arrows.
Change the mass of a sealed 100 mL object, then compare the surrounding fluids. Predict whether it floats and how much is submerged.
Sealed object, fixed volume 100 mL, no absorption or surface-tension effects. At equal density it can remain submerged; in real settings small differences matter. Diagram dimensions are schematic.
Investigate
Keep the object the same and compare water with another liquid. Predict how the fraction submerged changes.
Make the connection
When the object floats at rest, compare its weight with the buoyant force. Explain why a boat sits lower when loaded.
A sealed 100 mL object has mass 60 g in water at 1 g/mL.
Reveal the reasoning
It must displace 60 g, or 60 mL, of water to balance its weight. About 60% of its volume is submerged in this model.
Try it yourself
A sealed 100 mL object has mass 60 g and floats in water of density 1 g/mL. How much is submerged?
Give me a hint
A floating object displaces a weight of water equal to its own weight.
Compare with a worked response
It displaces 60 g of water, which occupies 60 mL. In the ideal model, 60% of the object’s volume is submerged.
Investigate & explain
Predict what happens when mass is added to a sealed floating model with fixed outer volume. Use the interactive to compare object density, fluid density and fraction submerged.
Why does a floating object still have weight?
Compare your reasoning
Gravity still acts on a floating object. When it is at rest, the upward buoyant force balances its downward weight. Add mass and it sinks farther until it displaces enough fluid to balance the new weight, if it can remain afloat.
Common question: Gravity does not act on a floating object.
Gravity pulls downward; the upward buoyant force balances its weight when it floats at rest.
Put the ideas to work
Choose an answer, then use the feedback to refine your reasoning.