upthrust

Everyday

1. (science) The upward push that a liquid or gas gives to an object placed in it; it is why things float or feel lighter in water. tree →
2. (geography) A pushing upward of rock or land, for example when mountains slowly rise. tree →

In the lessons

Used in school science lessons about floating and sinking, forces and density. If the upthrust is equal to the object's weight, the object floats; if the weight is bigger, it sinks. Often drawn as an arrow pointing up on a force diagram. Phrases: «upthrust and weight», «the upthrust on the boat».

Examples

• The water gives the boat an upthrust. tree →
• A ball feels lighter in the pool because of upthrust. tree →
• Draw an arrow to show the upthrust on the cork. tree →
• If the upthrust equals the weight, the object floats. tree →
• A stone sinks because its weight is bigger than the upthrust. tree →
• Air gives a small upthrust to a balloon. tree →
• The mountains were formed by an upthrust of rock. tree →

Note

Mostly UK school science; in the USA it is often called «buoyant force» or «buoyancy». Measured in newtons (N) like other forces. In problems: weight minus upthrust gives the apparent weight in water (a 10 N stone with 3 N upthrust feels like 7 N).