IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# To Measure the Force of Buoyancy

## Objective

To measure the force of interaction between a liquid and a body partially immersed in it.

## Introduction

Archimedes principle is routinely stated as: ``When a body is partially or fully immersed in a liquid, the liquid applies a force on the body in upward direction and this force is equal to the weight of the displaced liquid''. This force is often called up thrust or buoyancy.

You can measure this force using a weighing balance. The present experiment not only shows that there is such a force but also can be used to discuss Newton’s third law.

## Apparatus

Weighing balance, a vessel containing water

## Procedure

Take a vessel half filled with water and put it on a weighing balance. The vessel should be wide enough (6 inch or more in diameter) so that you can put your hand freely in it without touching the walls. Read the scale. Ask the student, what will happen to the scale reading if I put my closed fist in the water. Most likely the students will reply that there will be no change. It is increased. As you gradually dip the first, the reading also increases gradually.

Why did the scale reading increase? Scale reading increases only when someone pushes the pan or the contents on the pan down. When I dip my fist in water, my hand exerts a force on the water in the downward direction. This increases the force on the pan and the reading increases.

Now use Newton's third law. If the hand has exerted a force on water in the downward direction, water must have exerted force on the solid in the upward direction. You can dip any solid in place of your hand and measure the buoyancy.

## Discussion

The main aim of this demo is to demonstrate that a solid dipped in a liquid exerts downward force on the liquid. Though Archimedes principle is taught in great detail in schools but this aspect of downward force is seldom emphasized. This demo is going to enlighten the students further on Archimedes principle and Newton’s third law.

Measuring vessels are available where there are graduations on the wall in milliliters. If you can get such a vessel, and dip a solid in water placed on the vessel, you can also get the volume of water displaced. You can check that if the volume displaced in 100 ml the weighing scale reading increases by 100 gram. Thus the buoyancy force is equal to the weight of the liquid displaced.