ballast

Everyday

1. Heavy material, such as stones, sand or water, put low in a ship or balloon to keep it steady. tree →
2. The stones laid under railway tracks to hold them in place. tree →
3. Something that gives steadiness or balance to a person or plan. tree →

In the lessons

In science lessons about floating and balance, ballast shows how weight low down keeps a boat upright and how submarines take in water to sink. Hot-air balloons drop sandbags of ballast to rise. Railway workers lay ballast under tracks. Phrases: "ballast tanks", "drop ballast", "take on ballast".

Examples

• The ship carried rocks as ballast to stop it rolling over. tree →
• The submarine filled its ballast tanks with water to dive. tree →
• The balloonist threw out a bag of sand ballast and the balloon rose. tree →
• Grey ballast stones lie under the railway sleepers. tree →
• We added coins to the bottom of our model boat as ballast. tree →
• Weight low down acts as ballast and keeps the boat steady. tree →

Note

Usually uncountable: "some ballast". Throwing out ballast makes a balloon lighter so it rises; taking on ballast makes a ship or submarine heavier so it sits lower. Do not confuse with "ballet" or "balance".