Floating and sinking

This central idea is explored through:

Contrast between student’s point of view and science


Students have frequent experience with floating and sinking objects in the bathtub, in the pool, or on the beach. They form insights at an early age about these ideas and, equally important, the words used to describe them. any understanding of the forces involved in why they float or sink. These can be quite difficult to identify and understand. Students often believe that:

  • Heavy objects sink and light objects float regardless of their size, shape or the type of material from which they are made
  • a true floater must be completely above the surface of the liquid
  • all floating objects must contain some trapped air and that’s the only reason they float
  • somehow the amount of liquid the object floats, i.e. an object will float higher in a larger volume or deeper liquid.

Research: Biddulph & Osborne (1984), Mitchell & Keast (2004)

Scientific point of view

An object floats when the weight exerted on the object balances the upward force of the water on the object. The upward force of water increases with the volume of the object in the water; it is not affected by the depth of the water or the amount of water. If the downward force is greater than the upward force of the water on the object, the object will sink. If the opposite is true then the object will float – rising is the opposite of sinking.Read more: why do people put others down | Q&A Different objects float at different rates in water because when most objects are lowered into the water, the upward force of the water increases steadily until it balances with the weight of the object and then The object continues to float. At this level with two forces balancing. Many hollow (and generally air-filled) objects float because the hollows increase the volume of the object (and thus the upward thrust) to very little increase the downward force of gravity. However, an object does not necessarily have to contain air to float.

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Critical Teaching Ideas

  • In order for an object to float, it must be balanced with the upward force of the water surface on the object.
  • The amount and type of material that makes up an object affect the magnitude of the force of gravity on the object.
  • The volume of an object, which can often be altered by changing its shape, will affect the size of the force exerted on the object.

globeExplore the relationship between ideas of floating and sinking in Concept Development Maps: Laws of Motion Students will be familiar with the idea that objects have weight and that the size of the force of gravity is determined by the type of material and the extent of its use Students should know that when an object floats, water pushes the object upwards. For example, when you try to push a surfboard underwater, you can feel the water pushing the board up. Objects such as rocks that have sunk down are still being pushed upwards; it’s not as strong as the force of weight. Students should be encouraged to consider buoyancy as the result of the balance of the force of weight on an object and the upward force of water on the submerged part of the object. be instructed to realize that changing the shape of an object:

  • does not change the weight of the object
  • can change the volume of an object.

Read more: why is there static electricity in my headphones | Top Q&AResearch: Carr, Barker, Bell, Biddulph, Jones, Kirkwood, Pearson & Symington (1994)

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Teaching activities

Give an open problem for students to explore through the game

Line drawing shows a no. Floating collapsible film containerAt this level, students experiment with different objects to see if they are suitable for floating or sinking in water. They should be encouraged to identify the common characteristics of floating and sinking bodies. Students can try dipping a ball in a bucket of water to feel the force of the water on the ball. A more quantitative approach might involve measuring the weight of different objects to see if that affects how they float or sink in water. Students can experiment with different sized objects with the same weight to see how this affects their buoyancy. Encourage students to experiment with less sealable containers such as empty membrane containers to see if and to what extent they float in water. They can then experiment by adding different amounts of sand to the containers to see how weight changes affect their buoyancy.

Practice using and building on the perceived usefulness of a scientific model or idea

Students can also model themselves as an engineer by designing a submarine (from a plastic drink bottle) to retrieve a sunken treasure. Adding sand to the bottle will increase the weight of the bottle and hence its increased weight force resulting it to float lower in the water. When the force of weight is greater than the force of water on the ‘submarine’ (bottle), the ‘submarine’ will sink to recover the treasure.

Clarifying and reinforcing ideas for/by communicating to others

Ask students to change the shape of a plastic ball (or ‘Blu-tack’) to see if they can make it float in water. This could lead to a discussion about why all boats don’t have to be made of wood or inflatable plastic, and a ship made of heavy metal, concrete or fiberglass can. Floating on the water so successfully. by exploring things that can affect an object’s buoyancy, such as its shape, weight, or quantity or type of liquid used to make it float. This survey is the perfect opportunity for students to develop their laboratory and reporting skills.

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Practice using and building on the perceived usefulness of a scientific model or idea

An example of an interesting story designed to focus on floating and sinking concepts is ‘Who sank the boat?’ written by Pamela Allen. This story can provide an opportunity for students to further ask questions about floating and sinking. Research: Allen (1988) Read more: Why are Nissan Skylines illegal in the US?

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