Exploring Forces · Lesson 6 of 7
Buoyant Force, Floating, and Sinking
“Understand the upward force exerted by liquids and use it with gravity to explain why objects float or sink.”
• Describe buoyant force as an upward force exerted by a liquid. • Compare the downward gravitational force and upward buoyant force on an immersed object. • Explain when an object sinks or floats using the balance of these forces. • State the idea of Archimedes' Principle at the level used in the chapter. • Relate liquid density and object structure to floating behaviour.
Water Pushes Upward
Push a tightly closed empty bottle below the surface of water and you feel the water pushing back upward. If you release the bottle, it rises toward the surface. This shows that liquids exert an upward force on objects placed in them.
The upward force exerted by a liquid on an object placed in it is called buoyant force or upthrust.
Why Some Objects Sink and Others Float
An object in a liquid is acted on by at least two important forces in this discussion: gravity pulls downward, while the liquid pushes upward through buoyant force. What happens depends on how these forces compare.
| Comparison of forces | Result |
|---|---|
| Gravitational force greater than buoyant force | Object sinks |
| Buoyant force balances gravitational force | Object floats |
One factor affecting buoyant force is the density of the liquid. The chapter introduces this connection without developing density in detail, because density is studied later.
Problem
A tightly closed empty bottle is pushed under water and released. Why does it move upward?
- 1.Water exerts an upward buoyant force on the bottle.
- 2.Gravity acts downward on the bottle.
- 3.For the bottle in this situation, the upward effect is sufficient for it to move toward the surface.
Archimedes' Principle
Archimedes discovered a deeper relationship between an immersed object and the liquid around it. When an object is partly or fully immersed, the upward force on it is equal to the weight of the liquid displaced by the object.
When an object is fully or partially immersed in a liquid, it experiences an upward force equal to the weight of the liquid displaced by it.
If the weight of displaced liquid is smaller than the object's weight, the object sinks. If the displaced liquid's weight becomes equal to the object's weight, the object can float.
Floating at Different Depths
Objects of the same size and shape can float at different depths if they are made from different materials. An object that must displace more liquid before the upward force balances its weight will sit deeper in the liquid.
Pumice can float on water because it contains many trapped air pockets. Its porous structure makes its overall density lower than that of water.
A floating object is still pulled downward by gravity. It remains floating because the upward buoyant force balances that downward pull.
Quiz
What is buoyant force?
If gravitational force on an object is greater than buoyant force, the object will
What does Archimedes' Principle relate buoyant force to?
Why can an object feel lighter while submerged in water?
Why can pumice float on water?
Practice Problems
- Describe what you feel when you push a tightly closed empty bottle underwater and explain why.
- Name the two opposite forces discussed for an object immersed in a liquid.
- Explain the condition for floating in terms of gravitational force and buoyant force.
- State Archimedes' Principle in your own words.
- Why can two objects of the same size float at different depths?
- Explain why pumice can float even though it is a rock.
Key Takeaways
• Liquids exert an upward buoyant force, also called upthrust, on immersed objects. • Gravity acts downward while buoyant force acts upward. • An object sinks when its downward gravitational force is greater than the buoyant force. • An object can float when buoyant force balances its weight. • Archimedes' Principle links buoyant force to the weight of displaced liquid.