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Exploring why some small things are heavy and some big things are light.
Have you ever wondered why a tiny, heavy pebble sinks to the bottom of a lake, but a giant wooden log can float right on top? It’s not about how big an object is—it’s about a secret called density!
Everything around you is made of 'stuff' called matter. Imagine you have two identical boxes. In the first box, you pack toy cars as tight as you can. In the second box, you only put toy cars. Which box is more 'crowded'? The first box is like an object with high density. Density is a measure of how much 'stuff' (mass) is squeezed into a certain amount of space (volume). If the 'stuff' is packed together very tightly, the object is dense. If the 'stuff' is spread out with lots of empty space, the object has low density.
Quick Check
If you take a piece of bread and squish it into a tiny, hard ball, does the density of the bread increase or decrease?
Answer
It increases because the 'stuff' is now packed more tightly into a smaller space.
Let’s look at a metal ball and a foam ball that are exactly the same size. They take up the same amount of space, but they feel very different. The metal ball feels heavy because its tiny particles are packed together like people in a crowded elevator. The foam ball feels light because its particles are spread out with lots of air in between, like people scattered across a huge park. Even though they are the same size, the metal ball has more mass in that space, making it more dense.
Imagine two identical suitcases: 1. Suitcase A is filled with heavy books packed edge-to-edge. 2. Suitcase B is filled with fluffy pillows.
Because the books in Suitcase A are packed tighter than the fluff in Suitcase B, Suitcase A has a higher density and will be much harder to lift!
Quick Check
If two objects are the same size, but one is much heavier, which one is more dense?
Answer
The heavier object is more dense.
Density is like a superpower that tells an object where to go in water. Water has its own density. If an object is more dense than water, it will sink to the bottom. If an object is less dense than water, it will float on the surface. This is why a heavy cruise ship made of steel can float—it is designed with lots of air inside to make its total density lower than the water! A solid steel bolt, however, is very dense and will sink instantly.
Did you know some bowling balls float and others sink? 1. A -pound bowling ball has a lot of air mixed into the plastic, making its density less than water. It floats! 2. A -pound bowling ball is the same size but is packed with more heavy material. Its density is greater than water. It sinks!
An egg usually sinks in plain water because the egg is more dense. But if you add lots of salt to the water, you make the water 'thicker' and more dense. 1. Plain water density < Egg density = Egg sinks. 2. Salty water density > Egg density = Egg floats! By changing the water, you changed which item was more dense.
What is the best definition of density for a 3rd grader?
Why does a small marble sink while a giant beach ball floats?
If you have a foam ball and a metal ball of the exact same size, the foam ball is more dense.
Review Tomorrow
Tomorrow morning, look at your breakfast. Can you find one thing that would sink in milk and one thing that would float? Why?
Practice Activity
Fill a sink with water and find 5 small objects. Predict which ones are 'tightly packed' (dense) and will sink before you drop them in!