The weight of a material differs in water compared to outside (in air) due to the effect of buoyancy, a principle explained by Archimedes. When an object is submerged in water, it experiences an upward buoyant force equal to the weight of the water it displaces. This reduces the apparent weight of the object, even though its actual mass remains the same.
- Weight in Air:
Outside of water, the weight of a material is simply its mass multiplied by the acceleration due to gravity (W = m × g). This is the true weight, unaffected by any external forces other than gravity. - Weight in Water:
When the material is submerged in water, it displaces a volume of water. The buoyant force acting on it is equal to the weight of the displaced water (Buoyant Force = ρ_water × V × g, where ρ_water is the density of water, V is the volume of the object, and g is gravity). The apparent weight in water is then the true weight minus the buoyant force:
Apparent Weight = True Weight - Buoyant Force. - Key Factors:
- Density of the Material: If the material is denser than water (e.g., steel), it sinks, but its apparent weight is still reduced by the buoyant force. If it’s less dense (e.g., wood), it floats, and the buoyant force can balance or exceed its weight.
- Volume: The larger the volume of the object, the more water it displaces, and the greater the buoyant force.
- Water Density: Saltwater is denser than freshwater, so the buoyant force is slightly higher in saltwater.
