What makes float

Generally the more of the outside of an object that is touching the water the more buoyant it is. Water pushes back up against objects so the more surface area an object has the more water pushes back against it helping it to float. When an object floats, it pushes water out of the way displacement. Have you ever noticed that when you climb into a bath the water level rises? This easy activity demonstrates how reducing the density of a heavy object allows it to float.

Place the bubble wrapped ball on the surface of the water, you should find it now floats. Although the bubble wrap makes the ball weigh a little more, it also displaces extra water making the ball more buoyant. The pockets of air in the bubble wrap mean that the ball and bubble wrap together are less dense than the water, which means the ball floats!

Can you think of a different way to make the ball float? What if you made it a boat with plasticine? If you filled one bag with a kg of feathers and another with a kg of lead you would see that the feathers take up much more room, even though both bags have the same mass. This because feathers are less dense, they have less mass per volume. If you made a copper cube and an aluminum cube of the same volume and placed one in each hand, you would be able to feel that the copper cube would be heavier.

Copper has more mass per volume than aluminum. How can one substance have more mass per volume than another? There are a few possibilities:. Any one or a combination of these explanations could be the reason why one substance has a higher density than another. In the case of copper and aluminum, their atoms are arranged similarly, but copper atoms are smaller and have more mass than aluminum atoms, giving it a higher density. Density, Sinking and Floating Why do some things float, while others sink?

You might expect heavier objects to sink and lighter ones to float, but sometimes the opposite is true. The relative densities of an object and the liquid it is placed in determine whether that object will sink or float.

You can really see relative densities at work when you look at a heavy object floating and a lighter one sinking. For example, imagine putting a small piece of clay and a large, heavy wax candle in a tub of water. Sinking and floating applies to liquids too. For example, if you add vegetable oil to water, the oil floats on top of the water because the oil has a lower density than the water. The water pushes upward against the object with a force buoyancy equal to the weight of water that is displaced.

When the ball is submerged in the water, it displaces its volume in water. However, the weight of the ball is more like 55 N. A beach ball may have the same volume as a bowling ball, but it has a much smaller mass. When you a beach ball in a tub of water, it displaces the mass of water equal to its own mass—about 0. Molecules are very, very tiny. You can only see them with a special microscope.

In some objects, like a rock, molecules are squished tightly together. In others, like wood, the molecules are more spread out.

How closely molecules are crowded together in a space is called density. Objects like coins, rocks, and marbles are more dense than water. They will sink. They can then experiment by putting different amounts of sand into the containers to see how the change in weight affects the level of their floating.

The students could also model being an engineer by designing a submarine out of a plastic drink bottle to retrieve a sunken treasure. Introducing more sand into the bottle will increase the weight of the bottle and so increase its weight force resulting in it floating lower in the water. Scientists are curious and students can model being a scientist by exploring things that can affect whether an object floats such as its shape, weight or the amount or type of liquid used to float it.

This investigations are a perfect opportunity for students to develop experimenting and reporting skills. This story could provide opportunities for students to raise further questions about floating and sinking.

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