Predict Whether an Object Will Float or Sink: Understanding Density

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  • 0:01 Buoyancy and Forces
  • 1:30 Will It Float or Sink?
  • 3:30 Example
  • 4:33 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to describe the forces that act on an object in water and complete calculations to figure out if an object will float or sink. A short quiz will follow.

Buoyancy and Forces

Some objects float. Some objects sink. If you jump into some water, you'll probably sink. But if you spread your body out and lay back, you'll likely float. If you throw a pool noodle into the pool, it will float. But a brick... not so much. But not all objects are obvious. How do we figure out if an object will float or sink?

To do that, we need to talk about buoyancy and forces. If an object is put into water, there are two forces acting on it: gravity and the buoyant force.

Gravity's always the same, whether you're inside the water or outside of water. The force of gravity is equal to the mass multiplied by the acceleration due to gravity, which is 9.8 on the surface of the Earth. So if an object is 10 kilograms in mass, the force of gravity will be approximately 98 newtons, whether it's in water or on dry land.

So whether or not an object floats has a lot more to do with the buoyant force. Buoyancy is the ability or tendency of an object to float in a fluid: a liquid or a gas. This happens because fluid pressure increases with depth. As you go underwater, diving down deeper, the molecules are closer together. Because of this, they hit you more. So the pressure gets bigger the deeper you go. If you put an object underwater, there will be more pressure on the bottom of the object than the top, because the bottom is deeper underwater. And this creates an upwards force.

Will It Float or Sink?

Okay, but how do we know if an object will float or sink? Well, according to Archimedes Principle, the buoyant force on a submerged object is equal to the weight of the liquid displaced by the object. Which means that the larger the amount of an object that is underwater, the stronger the buoyant force will be.

A really large, light object is probably going to float. This is because a large object will displace a lot of water, creating a big buoyant force. And a light object will have a small force of gravity. With a small force of gravity and a big buoyant force, the object will definitely float.

But a really small, heavy object is probably going to sink. This is because a small object will not displace much water and will have a much smaller buoyant force. And, being heavy, it will also have a large force of gravity.

The combination of size and mass is called density. Density is a measure of how compact the mass in a substance or object is. Or in other words, density is mass spread out over a volume. The more each meter cubed weighs, the more dense the substance is. Heavy, small objects are super dense. And large, light objects are not very dense at all.

It turns out that density is really useful for figuring out if an object will sink or float. An object that is more dense than water will sink. And an object that is less dense than water will float.

Water has a density of 1000 kilograms per meters cubed. So all we have to do to figure out if something will float or sink is take some measurements, calculate the object's density and compare it to 1000.

The density of an object or substance can be calculated from this equation: density, in kilograms per meter cubed, is equal to mass, in kilograms, divided by volume, in meters cubed. So if you weigh an object to get its mass, measure an object to calculate its volume (its size) and divide the mass by the volume, you will be able to answer the question.


Let's go through an example.

An object shaped like a cube is weighed and turns out to have a mass of 3 kilograms. You measure one side of the cube and it's 0.2 meters long. Will the object float or sink?

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