Foundational Physics

Short Answer: An object feels lighter underwater because the water provides an upward buoyant force, so less force is needed to hold it up. A spring scale supporting the same metal ball reads less underwater than in air, even though the ball’s mass and gravitational weight are essentially unchanged.

Why Does the Spring Scale Show a Smaller Number?

Imagine a solid metal ball hanging from a spring scale. In air, the spring stretches to support almost all of the ball’s gravitational weight. Now lower the ball completely into a tank of water, without letting it touch the bottom or sides. The spring stretches less and the scale reading drops.

The ball has not lost mass. Earth has not stopped pulling on it. The water now supplies part of the upward support through buoyancy, so the spring supplies less.

Did the Ball Actually Lose Weight?

No. In ordinary physics, the ball’s true weight is the gravitational force acting on it, and that is essentially unchanged when you submerge the ball.

What changes is the upward force that the string or scale needs to provide. This smaller supporting force is often called the ball’s apparent weight. That phrase describes what the support measures; it does not mean that gravity became weaker.

How Can the Water Help Hold Up the Ball?

Water pushes upward on the ball with a buoyant force. When the ball is motionless and fully submerged, the upward pull of the spring plus the upward buoyant force balances the ball’s downward weight.

That is why the scale can register less than it did in air. The scale measures the tension in the string, not the ball’s gravitational force directly. The water provides the difference.

Our separate article, Why Does Buoyant Force Exist?, explains how underwater pressure produces that upward force. Here the question is what the supporting scale reads.

What Would the Scale Read in Air and in Water?

Suppose the ball’s true weight is about 4 newtons and the water exerts a buoyant force of 2 newtons while it is fully submerged and stationary. The underwater spring scale would read about 2 newtons instead of 4.

The exact numbers depend on the ball’s volume and the fluid. The essential point is that the weight stays approximately 4 newtons; the water contributes 2 newtons of upward support, leaving only 2 newtons for the string to supply.

For a fully submerged rigid ball in water of nearly constant density, going even deeper does not substantially change this buoyant force or the scale reading.

Does Feeling Lighter Mean the Ball Would Float?

Not necessarily. A metal ball can still sink if you remove the string because its weight can exceed the upward buoyant force.

While you hold it stationary with the scale, the string provides the remaining upward force. Release it, and that extra support disappears. If its weight is greater than buoyancy, the ball accelerates downward, even though it had a smaller apparent weight underwater.

Buoyancy reduces the support you need; it does not guarantee floating.

Why Might It Feel Lighter Even Without a Scale?

The same force balance applies when you lift a rock in a pool. Your hands do not need to pull upward as hard as they would in air, because the water is helping support the rock.

You are sensing the force required to hold or raise it, not a change in the rock’s mass. When you lift it out of the water, the buoyant support rapidly decreases and it feels heavier again.

The Key Idea to Remember

An object feels lighter underwater because buoyancy reduces the supporting force needed to hold it. Its true gravitational weight remains nearly the same; the spring scale measures a smaller tension, often called apparent weight.

When This Breaks

The simple example assumes the ball is held motionless, completely surrounded by water, and does not touch the tank. If it rests on the bottom, the bottom also supplies a support force, so a hanging-scale interpretation no longer applies directly.

During upward or downward acceleration, the spring-scale reading also reflects the net force needed to accelerate the ball. Water motion and drag can change the measured force. A compressible object, partial submersion, or a fluid with changing density can change the buoyant force. The small buoyancy of air can also matter in precision measurements, but is neglected in the introductory comparison.

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