Why Do Ships Float
This infographic demonstrates in an easy to follow manner why ships (no matter how big they are) float. This is a great resource for teaching.
Transcribed –
WHY DO SHIPS FLOAT?
You may think that if you put two things in water, the heavier one will sink and the lighter one will float, but that’s not quite true.
A one-pound lead weight will sink, but cruise ships float, even though some of them weigh around 100,000 tons. So if weight isn’t what makes things sink or float, what does?
Helpful Terms to know
Density:
Density refers to how much matter (how much “stuff”) there is in a certain amount of space
Displacement:
Any object that’s placed in water moves some of the water out of the way The amount of water that it moves is how much it “displaces”
Every material has its own density
- LEAD has an average density of 11.34 g/cm3
- BALSA WOOD has an average density of 0.16 g/cm3
So one cubic centimeter of lead Is about 100 times denser than one cubic centimeter of balsa wood
So long as an object’s density is less than water (1 g/cm3), It will float
Buoyancy:
Gravity pulls down on an object placed In water
“Buoyancy” is how strongly the water pushes up on the object
Buoyancy is related to displacement – the more water an object displaces, the more buoyant It Is
If the object is more dense than water, it will sink
Do the Math Will it Float?
If the density of water is 1 g/cm3, which of the following materials-given their average densities-would float in water?
An apple 0.7 g/cm3
Solid chalk: 2.5 g/cm3
REMEMBER:
An object’s density must be less than water for it to float
- Asphalt: 2.24 g/cm3
- Solid cork: 0.25 g/cm3
- Gelatin 1.29 g/cm3
- Oak charcoal: 0.58 g/cm3
The Importance of Average Density
An object’s density must be less than water to float, but cruise ships are usually made out of high-strength steel (density averaging 8 cm3)
If cruise ships are made out of a denser material than water, why do they float?
Cruise ships are mostly empty space Some of this empty space is taken up by people, but a lot of it only holds air
Cruise ships would sink If they were big blocks of solid steel, but they aren’t.
To understand why a cruise ship floats, you need to know its average density To figure out something’s average density, you take its total mass (the weight of everything Inside It) and divide by its volume (how much space It takes up)
p=m
Because a cruise ship takes up a lot of space, Its average density is still less than water, even though the cruise ship is very heavy
Try It Out For Yourself
To get a better understanding of the relationship between an object’s volume (how much space it takes up) and its density, try the following experiment. You will need:
PART 1
- A large bowl or container
- Permanent marker
- Metric ruler
- Water
- Roll of aluminium foil
- Weights (washers, coins, etc.)
- Scissors
HOW TO PERFORM THE EXPERIMENT:
1 Fill the bowl two thirds of the way with water
2 Measure a 25 cm square of aluminium foil and cut it out
3 Mark the four corners of aluminium foil with the permanent marker
4 Pull the four corners of the square together so that they are touching
This will help you remember where the top of the ball of foll is
5 Crumple the square into a rough ball about 6 cm across
See how far the ball sinks into the water. Write down how much of the ball is below the surface of the water
Is It 10%? 20%? etc.
Set the ball of foil in the water with the marked corners facing up. This should help keep the ball from Immediately filling with water and sinking
Remove the ball from the water, dry it off, and crumple it to about 5 cm across
Place it back into the water and see how far it sinks
Continue this (crumpling the ball smaller each time) until the ball sinks completely
Remember to keep track of how big the ball is and how much It sinks each time
QUESTIONS TO THINK ABOUT:
If you weigh the aluminum foil, can you figure out the density of the ball at each stage in the experiment? The volume of a sphere is 4/3 times pl (about 3.14) times the radius cubed
A circle’s radius is half its diameter So at the beginning of the experiment, the ball should have a volume of about 113 cubic centimeters
How big was the ball when it sunk completely?
Why did the ball sink more as it grew smaller?
How big was the ball when it had almost sunk all the way, but not quite?
What do you think Its density was at that point?
Why Displacement Matters
The amount of water a ship moves out of the way is called “displacement”
If the amount of water a ship displaces Is more than its weight, it will float.
The Allure of the Seas, for example, the biggest cruise ship in the world, displaces around 100,000 tons Because the cruise ship weighs less than the amount of water It can displace, It floats
The more weight a ship takes on, the more it sinks
As a ship sinks, It displaces more water
So long as Its buoyancy (the force pushing up on the ship) is equal to the ship’s weight (the force of gravity pulling the ship down), the ship will keep floating. If the ship’s weight Increases, the ship has to sink to displace more water If the ship ever weighs more than the amount of water It can displace, It will sink
Try It Out For Yourself
To better understand how displacement works, try the following experiment. Here is what you’ll need to
carry out the experiment:
PART 2
- Water
- A large bowl or container
- Some square pieces of aluminum foil (about 25 cm on each side)
- Weights (washers, coins, etc.)
HOW TO PERFORM THE EXPERIMENT:
Fill the container about two thirds of the way with water
Take the squares of aluminum and fold them into various shapes Try a box, bowl, or boat shape Begin filling the aluminum “boats” with washers
See which shapes are able to hold the most washers before sinking
QUESTIONS TO THINK ABOUT:
Which shapes were able to hold the most washers before sinking?
Why are some shapes better at holding washers than others?
Which shapes had the lowest average density?
While buoyancy may seem a little strange at first, if you remember how density and displacement work, you’ll see it does make sense after all. Big ships like cruise ships are able to float because they’re mostly hollow. All that empty space inside makes them lighter than water, which is why they float.