Investigating Sink or Float - First Grade Science
Imagine you are standing beside a swimming pool on a warm summer afternoon. You toss a small, heavy iron nail into the water, and it instantly plunges straight to the deep bottom: it sinks! Next, you toss in a giant, heavy wooden log, and to your amazement, the colossal log bobs right on the surface: it floats! Why do some objects sink down to the bottom while other objects float on top of the water? In science, whether an object sinks or floats does not depend solely on how heavy it is; it depends on a property called density and an upward force called buoyancy.
Defining Sink and Float
Before testing objects, scientists establish clear definitions for what it means to sink or float.
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| SINKING VS. FLOATING |
| |
| FLOAT SINK |
| An object stays on top of the An object drops straight down |
| liquid or bobs at the surface. through the liquid to the bottom|
| |
| ~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~ |
| [OBJECT] \ / |
| ~~~~~~~~~~~~~~~~~~ \ OBJECT / |
| +---------+ |
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The Secret: Density
The primary reason an object sinks or floats is its density compared to the density of water. Density is a measure of how tightly packed the matter (mass) is inside a given amount of space (volume).
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| UNDERSTANDING DENSITY |
| |
| HIGH DENSITY (Sinks in Water) LOW DENSITY (Floats in Water) |
| Molecules are crammed together Molecules are spread out and |
| tightly; very heavy for its size. loosely packed; light for size. |
| |
| +-----------------+ +-----------------+ |
| | * * * * * * * * | | * * | |
| | * * * * * * * * | | * | |
| | * * * * * * * * | | * * | |
| +-----------------+ +-----------------+ |
| Examples: Rocks, steel, lead Examples: Cork, wood, foam|
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The Water Rule
Water itself has a specific density (1 gram per cubic centimeter): If an object is more dense than water, gravity pulls it down through the water, and it SINKS. If an object is less dense than water, the water pushes it up, and it FLOATS.
Buoyancy: The Water's Upward Push
When you step into a swimming pool or bathtub, you probably notice that your body feels lighter. That is because water pushes upward against any object placed in it! This upward lifting force exerted by liquids is called buoyancy (or buoyant force).
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| THE FORCES AT PLAY |
| |
| | |
| v GRAVITY (Pulls object downward) |
| +-----------+ |
| | OBJECT | |
| +-----------+ |
| ^ BUOYANT FORCE (Water pushes upward) |
| | |
| |
| If Gravity > Buoyant Force ======> OBJECT SINKS! |
| If Buoyant Force >= Gravity ======> OBJECT FLOATS! |
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Testing Common Objects: Sink or Float?
Scientists make predictions and then test everyday objects in a tub of clean water to record real observations.
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| SINK OR FLOAT EXPERIMENT RESULTS |
| |
| OBJECT PREDICTION OUTCOME WHY IT HAPPENED |
| Metal Coin Sink Sinks Metal is dense |
| Wooden Craft Stick Float Floats Wood is less dense |
| Plastic Toy Duck Float Floats Air trapped inside |
| Granite Pebble Sink Sinks Stone is very dense |
| Foam Sponge Float Floats Air pockets reduce d|
| Glass Marble Sink Sinks Glass is more dense |
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How Giant Steel Ships Float: Shape Matters!
If a solid steel block sinks like a stone, how can a gigantic cruise ship made of thousands of tons of steel float across the ocean? The secret is shape!
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| THE CLAY EXPERIMENT: SHAPE MATTERS |
| |
| CLAY SHAPED AS A SOLID BALL: CLAY SHAPED INTO A HOLLOW BOAT:|
| |
| ( O ) \___________/ |
| (Air inside!) |
| Takes up very little room; Spreads weight wide; traps air |
| cannot push enough water away. inside; displaces lots of water|
| RESULT: SINKS TO BOTTOM! RESULT: FLOATS HIGH & DRY! |
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When steel is shaped into a wide, hollow ship hull, it encloses a massive volume of air. Because air is super-lightweight, the overall average density of the ship (steel plus all that empty air) becomes much less dense than water, creating enormous buoyant force that keeps the ship afloat!
Chapter Review Exercises
Question 1: What does it mean for an object to sink, and what does it mean to float?
Question 2: What is density, and how does density determine whether an object will sink or float in water?
Question 3: What is buoyancy, and in which direction does the buoyant force push?
Question 4: Explain how you can take a lump of modeling clay that sinks as a ball and make it float without removing any clay.
Solutions and Explanations
Solution 1: An object sinks when it falls completely through a liquid and rests on the bottom; an object floats when it stays suspended on top of the liquid or bobs along the surface.
Solution 2: Density is the measure of how tightly packed the mass of an object is within its volume. Objects that are more dense than water sink, while objects that are less dense than water float.
Solution 3: Buoyancy is the upward lifting force that a fluid exerts against the weight of any submerged or floating object placed inside it.
Solution 4: You can mold the clay ball into the shape of a wide, hollow bowl or boat hull. By hollowing it out, the clay encloses air and occupies a much larger volume, pushing aside more water and lowering its overall density so the buoyant force supports it.