🌊 Buoyant Force (Upthrust)
🔎 What is Buoyant Force?
When an object is partially or completely immersed in a fluid (like water), the fluid exerts an upward force on it. This upward force is called buoyant force or upthrust.
🔬 Real-Life Examples of Upthrust
- 🎈 A balloon filled with helium rises in air.
- 🚤 A boat floats on water because of upthrust.
- 🏊♂️ A swimmer experiences less weight inside water due to upthrust.
⚖️ Factors Affecting Buoyant Force
- Volume of object submerged in fluid — more volume, more upthrust.
- Density of the fluid — higher density fluid gives greater upthrust.
- Acceleration due to gravity (g).
📘 Expression for Buoyant Force
Buoyant Force (FB) = ρ × g × V
where,
ρ = density of fluid (kg/m³)
g = acceleration due to gravity (m/s²)
V = volume of fluid displaced (m³)
⚖️ Conditions for Floating or Sinking
- If FB = W → Body floats (neutral position)
- If FB > W → Body rises or floats
- If FB < W → Body sinks
💡 Reason for Upthrust
The pressure in a fluid increases with depth. The bottom of a submerged object experiences more pressure than the top, resulting in a net upward force — **Upthrust**.
🎯 Interactive Puzzles on Buoyant Force
1. What is the other name of buoyant force?
Upthrust
2. Name one factor on which upthrust depends.
Density of fluid
3. Why does a piece of wood float in water but a stone sinks?
Wood is less dense and upthrust is greater than its weight. For stone, weight is more than upthrust.
4. What is the unit of buoyant force?
newton (N)
5. True or False: Buoyant force acts downwards.
False. It acts upwards.
6. If an object is fully submerged and still floats, what does it mean?
The upthrust is equal to the weight of the object.
7. A balloon rises in air. Which force helps it rise?
Upthrust of air
8. Formula for upthrust is?
Upthrust = ρ × g × V
