Question

What makes a plastic mug full of water appear lighter inside water?

Answer 1

Hint:The weight of any plastic mug which is filled with water up to its brim will be heavy. But if such a heavy object appears to be light in a medium which is water here, the explanation can be formulated through famous laws in Physics. There is a principle that explains how water filled objects appear lighter when placed in water.

Complete answer:
The given details state that a mug that has been filled completely with water experiences a loss of weight when placed inside the water. So our task is to find out how water can cause such a loss of weight to the full mug. When any object is immersed within water or any liquid, there is a force that gets exerted on the object that helps it to float. This force is termed as the buoyant force which is applied upward to the immersed object.

So since the weight of any object is downward and buoyant force acts upward, it means that the buoyant force is an opposing one. Now that we understood buoyant force, a principle exists that utilizes this force to explain the concept where water filled objects are lighter when placed in water. This principle is the Archimedes principle.

Archimedes principle explains that this buoyant force that is applied upward to the immersed object is always equal to the fluid weight the object displaces. Therefore we try to calculate the difference in the weight of the original immersed object and the buoyant force.

So what the buoyant force does is that when the object is under water, it will reduce the net force or ‘weight of water filled mug + buoyant force’. The net result is low because the magnitudes of forces will take different signs according to their directions, so downward weight will be positive while the upward buoyant force takes a negative value.

Therefore since the total force on the mug is less due to the buoyant force according to Archimedes principle, that mug full of water appears lighter within water.

Note:When an object is placed within water, the pressure at the bottom end is always larger than the pressure at the top end. Due to such a difference in the pressures there will be a force in the upward direction causing acceleration for the immersed object. As a result of such upward force exertion, any object that has density less than that of water tends to float.