To establish the relation between the loss in weight of solids when fully immersed (i) in tap water and (ii) in strongly salty water and the weight of water displaced by it by taking at least two different solids.
Have you ever felt lighter while swimming? Observe the pictures given below.
Figure 1(a) An object is submerged in a fluid. Figure 1(b) An object is removed from the fluid.
Imagine a scenario where you find yourself immersed or submerged in water, as illustrated in Figure 1(a).
In this context, when your body (object) is submerged in water (fluid), the weight of the object (Wobj) is directed downward due to gravitational force. Simultaneously, the fluid exerts an upward buoyant force (Fb) on the object. This buoyant force, also known as upthrust, is a result of the fluid's upward pressure on the immersed object.
The weight of an object is essentially the force due to gravitational pull. When immersed in a fluid, the object displaces the fluid. The interaction between the buoyant force and the gravitational force (weight) of an object might result in two different outcomes:
The interaction between gravitational and buoyant forces determines the behavior of the object when it is submerged in the fluid.
Now considering the scenario depicted in Figure 1(b), upon the removal of the object from the fluid, it is replaced by a volume of fluid that has the weight denoted as Wfl. In this specific context, the buoyant force equals the weight of the displaced fluid.
It is explained by Archimedes’ Principle.
When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
Buoyant force (Fb)= ρVg
ρ = density of fluid
V = volume of fluid displaced
g = acceleration due to gravity
It is evident that the buoyant force is directly proportional to the density of the fluid. For example, in situations (Fig 2(a) and Fig 2 (b)) where the density of saltwater exceeds that of pure water, the buoyant force in saltwater proportionally increases.
Figure 2(a) Measuring the weight of the object when it is immersed in water
(b) Measuring the weight of the object when it is immersed in salt water
When an object is submerged in a fluid, it experiences a reduction in weight compared to its weight in the air. This is attributed to the buoyant force applied by the fluid. This phenomenon is commonly referred to as the loss of weight, and the measured weight of the object in the fluid is termed its apparent weight.
Apparent weight of an object = Actual weight of an object – Buoyant force
When an object is immersed in a fluid, it displaces an amount of fluid equal to its volume.
Furthermore, the loss in the weight of the object is related to the buoyant force, which is equivalent to the weight of the displaced water. The complex connection between changes in fluid density and buoyant forces highlights how they directly impact the apparent weight of the object.
For example, in situations (Fig 2(a) and Fig 2 (b)) where the density of saltwater exceeds that of pure water, the loss in weight of the object is larger in salt water. The apparent weight of the object is larger in tap water.
Why do some materials sink, and others float on water?
Observe Figure 3, which shows how a cork floats on the surface of water, whereas an iron nail quickly sinks under the same circumstances. What is the reason behind this?
Figure 3: An iron nail sinks and cork floats when placed on the surface of the water
This phenomenon highlights a general principle: denser objects tend to sink in liquids, while objects with a lower density than the liquid will float.