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Why does it take significant force to push a beach ball underwater?
The beach ball doesn't weight a lot, so it doesn't have a lot of buoyant force on it. However, as you push it down, it displaces more water, which increases the buoyant force on it.
For example:
If you pushed a large beach ball completely underwater, it would displace a volume of water that would weight about 294 newtons. If the beach ball weighs only 4 newtons, you need to push down with at least 290 newtons (about
65 pounds) of your weight to get the ball underwater!