Short Answer

Upthrust is the upward force that a fluid, such as water or air, exerts on an object placed in it. This force acts opposite to the weight of the object and makes the object feel lighter when it is inside a fluid.

Upthrust helps explain why some objects float while others sink. If upthrust is greater than or equal to the object’s weight, the object floats. If upthrust is smaller, the object sinks. Upthrust is also known as buoyant force and is an important concept in fluid mechanics.

Detailed Explanation :

Upthrust

Upthrust is a natural force that acts on objects when they are fully or partially immersed in a fluid. A fluid may be a liquid like water or a gas like air. When an object is placed inside a fluid, the fluid pushes it upward. This upward push is called upthrust. Upthrust always acts opposite to gravity, which pulls objects downward.

The concept of upthrust is deeply connected to Archimedes’ principle. The principle states that the upthrust acting on an object is equal to the weight of the fluid displaced by the object. This explains why some objects float, why others sink, and why people feel lighter in water. Upthrust plays an important role in swimming, shipbuilding, submarine operations, hot air balloons, and many natural processes.

Meaning of upthrust

Upthrust can be defined as:

“The upward force exerted by a fluid on a body immersed in it.”

This upward force reduces the apparent weight of the object. That is why an object feels lighter when lifted inside water compared to lifting it in air. The reason behind upthrust is the difference in pressure that the fluid exerts at different depths.

Why upthrust acts on objects

To understand why upthrust exists, we need to understand fluid pressure.

• Fluids exert pressure in all directions.
• Pressure increases as we go deeper.
• The bottom surface of an immersed object is at a greater depth, so it experiences more pressure.
• The top surface experiences less pressure because it is at a smaller depth.

This pressure difference between the top and bottom surfaces produces an upward force, which we call upthrust.

Without this difference in pressure, no object would float in any fluid.

Relation between upthrust and floating or sinking

Upthrust determines whether an object will float, sink, or stay suspended. This depends on the comparison between the upthrust and the weight of the object.

1. Object floats

The object floats when:

Upthrust ≥ Weight of the object

Here, the fluid provides enough upward force to balance the object’s weight.

Example:
A wooden log floats easily in water because the upthrust acting on it is large enough to support its weight.

2. Object sinks

The object sinks when:

Upthrust < Weight of the object

In this case, the fluid cannot push the object up against gravity.

Example:
A piece of metal sinks in water because its weight is greater than the upthrust.

3. Object remains suspended

The object remains suspended when:

Upthrust = Weight of the object

The object neither rises nor sinks. It stays in equilibrium at a certain depth.

Example:
A fish can stay at a fixed depth by adjusting air inside its swim bladder to match the upthrust.

Factors affecting upthrust

Upthrust depends on several factors:

1. Volume of the object immersed
   Larger volume → more fluid displaced → greater upthrust.
2. Density of the fluid
   Denser fluids give more upthrust.
   Example: Floating is easier in seawater than in freshwater.
3. Shape of the object
   A wide or hollow shape displaces more fluid and increases upthrust.
4. Depth of immersion
   Upthrust remains the same at all depths, as long as the fluid density does not change.
5. Object’s density
   Low-density objects receive large upthrust relative to their weight and float easily.

Examples of upthrust in daily life

1. Floating boats and ships
   Ships float because they displace large amounts of water, creating strong upthrust.
2. Swimming
   Swimmers feel lighter in water because the upthrust reduces their apparent weight.
3. Icebergs floating in oceans
   Ice has lower density, so upthrust keeps it afloat.
4. Hot air balloons
   The upthrust provided by the air allows the balloon to rise.
5. A ball popping up in water
   When pushed underwater, the ball quickly rises due to strong upthrust.
6. Lift while walking in water
   Moving your body inside water feels easier because upthrust supports part of your weight.
7. Submarines diving and rising
   Submarines control upthrust by adjusting water in their tanks.

Importance of upthrust

Upthrust is important because it:

• Helps explain floating and sinking
• Allows ships and boats to be designed safely
• Helps swimmers stay afloat
• Helps scientists measure density of liquids using instruments like hydrometers
• Plays a key role in natural processes like iceberg movement
• Helps design balloons, blimps, and underwater vehicles

Without understanding upthrust, many technologies, including submarines and ships, would not function properly.

Conclusion

Upthrust is the upward force exerted by a fluid on an immersed object. It exists due to the pressure difference in the fluid at different depths. Floating, sinking, and suspension of objects depend on the balance between upthrust and weight. Upthrust plays an essential role in daily life, engineering, transportation, and natural processes. Understanding upthrust helps explain many practical activities such as swimming, ship travel, and balloon flight.