Raju’s class was waiting for science class to begin as they were going to learn about finding the density of objects that float on water, like wood or cork. The teacher entered the classroom and said: “This experiment is slightly different than the previous one. Last time we saw a solid that was sinking in water.”
She continued “Students by now it is clear to us that if the mass of the water displaced is less than the mass of the object, the object will sink. So we can now say that the objects that float on water will displace more mass of water than the mass of the object when immersed in water.”
Students understood this as they had conducted the experiment themselves.
Raju asked “If the object floats on water how will we get the volume of that object? It will not displace the water correctly as it will not immerse entirely.”
The teacher commended Raju and asked everyone to pay attention to the present experiment. The class had to find the density of a wooden block and a lead ball. Teacher weighed the wooden block while the students noted the mass of the wooden block.
“Let us find the mass and volume of the lead ball using the previous method,” said the teacher.
She then tied the lead ball to the wooden block and immersed the block in the measuring jar. The wooden block sank along with the lead ball. The students watched as this happened.
Mira asked now displaced water is equal to volume of the lead ball and wooden block together.
Rohan quickly asked if subtracting the volume of the ball from the volume of the block and lead ball would give them the volume of the wooden block.
Teacher said that he was right. The students measured the volume and noted the same. “Students you now know how to calculate the density of the wooden block.”
Adding to her statement Raju said “so mass divided by the volume will give us the density of the wooden block.”
There was a query from Anthony who said that his answer was in the form of a fraction. This meant that density is less than density of water which is 1.
“Now do you realise why wood floats on water? You can conduct similar experiments with cork or a thermacol block. Remember to take the mass in grams and volume in cubic centimetre,” said the teacher as the class ended.
(Writer is a mechanical engineer and runs a hands-on science activity centre at Margao)