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A lead ball falls through water more slowly than through air mainly because of the viscous force (drag force) acting on the ball in water. Viscosity is the property of a fluid that resists the motion of an object moving through it. Water has a much higher viscosity compared to air, creating a stronger opposing force as the ball moves.
When an object falls through a fluid, three forces act on it:
Gravitational force (weight): Pulling the object downward.
Buoyant force: Upward force due to fluid displacement, reducing effective weight.
Viscous force (drag): A resistive force caused by friction between the object’s surface and fluid molecules.
In air, the viscosity and density are both low, so the ball experiences less resistance and quickly accelerates close to free fall conditions. In water:
The viscous force increases because water molecules are denser and exert more friction.
The buoyant force is also greater, reducing the effective downward force.
Eventually, the ball reaches a lower terminal velocity, meaning it moves at a slower constant speed compared to air.
The other options are incorrect:
The value of g is less in water: Gravitational acceleration (9.8 m/s²) is constant and unaffected by the medium.
Density of air is less than water: This is true but not the main reason; resistance (viscosity) is the key factor.
Surface tension of water: This affects only very small, light objects, not a heavy lead ball.
This principle is part of fluid dynamics and explains why:
Objects fall slower in water than air.
Parachutes work due to increased air resistance.
Oil flows more slowly than water due to higher viscosity.
Thus, the correct answer is “Of the viscous force in water”, as it slows down the motion of the falling lead ball by opposing its movement more strongly than air.
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