Some things float. Some things sink. Some are in between. Why?

Often heavier things sink and lighter ones float. Can mass be the reason?

**Question:** Why do some things float and others sink?

**Materials:**

Big bucket of water

Small jar with lid

Scale

**Procedure:**

Put the lid on the jar tightly

Measure the jar’s height and diameter in centimeters

Mass the jar in grams

Float the jar in the bucket of water

Take the jar out of the water

Pour about 1.5 cm water into the jar

Put the lid on tightly

Mass the jar and water

Float the jar in the water

Describe how well the jar floats

Add another 1.5 cm water to the jar, mass it and float it

Continue to do this until the jar sinks

**Observations:**

Jar measurements:

Height:

Diameter ( biggest distance across)

Masses:

Empty jar:

How well it floated

Jar with 1.5 cm water:

How well it floated

Jar with 3 cm water:

How well it floated

Jar with 4.5 cm water

How well it floated

Jar with 6 cm water:

How well it floated

Jar with 7.5 cm water:

How well it floated

Jar with 9 cm water:

How well it floated

**Analysis:**

Calculate the volume of the jar (volume = πrrh where π is 3.14, r is half the diameter and squared, h is height)

Density is how much stuff is in a certain space. Calculate the density of the empty jar by dividing its mass by its volume. The units will be grams/cc or cubic centimeter.

Calculate the density of the jar with each addition of water.

There is another way you can use if your jar is not very square like mine with the top and bottom tapering. Fill your jar to the very brim with water and put the lid on tightly. Mass it. Subtract the mass of the empty jar. Water is supposed to have 1 gram equal to 1 cc so the answer should be close to the volume of the jar.

**Conclusions:**

Water has a density of 1 g/1 cc.

Compare the density of each water level of jar to the density of water to how well the jar floated.

Does density show when something will float or sink? Why do you think so?

A ship is made of iron, a very heavy metal but floats. Why does the ship float?

**What I Found Out:**

My jar had a diameter of 6.8 cm so the radius was 3.4 cm and a height of 12 cm. This gave it a volume of 435.6 cc.

When I filled it with water and massed it, the mass was 736.5 g. The empty jar was 251.7 g so the volume by that method was 484.8 g or cc.

Since my jar had a mass of 251.7 g, the empty jar had a density of 0.6 g/cc. The jar floated on top of the water.

I did not measure the water very carefully but the mass was the important thing.

After adding the first amount of water, the mass went up to 332.9 g and the density went to 0.8 g/cc. The jar still floated sideways and high.

The second amount of water increased the mass to 406.1 g raising the density to 0.9 g/cc. The jar still floated but with the bottom angled down into the water.

The third amount of water increased the mass to 443.6 g raising the density to 1.0 g/cc. The jar now floated upright in the water with most of the jar under water.

The fourth amount of water increased the mass to 482.4 g raising the density to 1.1 g/cc. The jar still floated but was much lower in the water.

The fifth amount of water increased the mass to 546.6 g raising the density to 1.3 g/cc.. The jar floated but only the lid was still above water.

The sixth amount of water increased the mass to 595.4 g raising the density to 1.4 g/cc. Now only the very top of the lid was above the water.

The seventh amount of water had a mass of 631.7 g raising the density to 1.5 g/cc. The jar sank immediately to the bottom of the bucket.

If I used the volume from filling the jar with water, the densities were less. They were: empty 0.5 g/cc; 1st 0.7 g/cc; 2nd 0.8 g/cc; 3rd 0.9 g/cc; 4th 1.0 g/cc; 5th 1.1 g/cc; 6th 1.2 g/cc; and 7th 1.3 g/cc.

When the jar floated each time, the water level outside was a little higher than on the inside. This difference increased a little each time.

The jar barely floated once the density of the jar was the same as the density of water. The density didn’t need to be much more than that of water before the jar sank.

For the jar density does seem to match to how well the jar floated. This might have been more striking if I had a more precise volume for my jar. I think density determines whether the jar sinks or floats because the jar floated with the air part above water and the full part below until the density got too much and it sank.

Ships work the same way. They have big rooms filled with air to make the ship’s density less than the density of water. This makes them work just like the jar.