Float or sink
Two forces decide.
Some float, some sink
You place some objects on water. Some float. Others fall to the bottom.
Gravity pulls all objects down. Why do not all of them fall to the bottom?
What this lesson covers
The idea
In a liquid, gravity pulls an object down while the buoyant force pushes it up; if the gravitational force is greater the object sinks, and if the two are equal it floats.
Some float, some sink
You place some objects on water. Some float. Others fall to the bottom.
Gravity pulls all objects down. Why do not all of them fall to the bottom?
- The liquid pushes up on them
- Gravity stops working on them
- They are too small
Compare the forces
Pick an object. Put it in the liquid and compare the two arrows.
Gravity against upthrust
In the liquid, gravity pulls an object down and the buoyant force pushes it up. The stone had more gravity than upthrust and sank. The other two had equal forces and floated.
In a liquid, gravity pulls the object down and the buoyant force pushes it up. If the gravitational force is greater, the object sinks. If the two forces are equal, it floats.
- Gravity and buoyant force | Result
- Gravity is greater | Sinks
- Both are equal | Floats
Notes
In a liquid, if gravity is greater than the buoyant force the object sinks. If they are equal it floats.
Check yourself
The gravitational force on an object in a liquid is greater than the buoyant force. The object …
The gravitational force and the buoyant force are equal. The object …
Which two forces act on an object in a liquid?
The pumice rock floats on water. What can you say about the two forces on it?
- sinks — correct. Yes!
- floats. It floats when the two forces are equal.
- rises to the top. It sinks when gravity is greater.
- sinks. It sinks only when gravity is greater.
- floats — correct. Yes!
- flies out of the liquid. With equal forces it floats.
- Friction and magnetic force. These are not the two forces that decide floating and sinking.
- Two downward forces. The buoyant force acts upward.
- Gravity downward and the buoyant force upward — correct. Yes!
- They are equal — correct. Yes!
- Gravity is greater. If gravity were greater, it would sink.
- There is no buoyant force. The liquid applies a buoyant force on every object in it.