Why objects sink

Engage Do a demonstration to show that the wax is heavier than the clay but that the wax floats and the clay sinks. Materials for the demonstration 1 tea light candle Clay Clear plastic container Water Large balance Teacher preparation Use a small enough piece of clay so that you are sure that the candle weighs more than the clay.

Procedure Place a piece of clay that weighs less than a tea light candle on one end of a balance. Remove the candle from its metal container and place the candle on the other end of the balance.

Expected results Even though the candle weighs more than the clay, the candle floats and the clay sinks. Explore Have students compare the density of water, wax, and clay.

Question to investigate Why does a heavier candle float and a lighter piece of clay sink? Materials for each group 2 tea light candles in their metal containers Clay Water in cup Small balance Tape Dropper Procedure Compare the density of wax and water Roll two pieces of tape and stick them to the center of the pan at each end of the balance.

Attach each tea light candle to the tape so that each candle is in the center of the pan. Use the wick to pull one candle out of its container. Expected results The water has a greater mass than an equal volume of wax. Ask students: Which weighs more, wax or an equal volume of water? Water weighs more than an equal volume of wax. Which is more dense, wax or water?

Water is more dense. Compare the density of clay and water Make sure you have one piece of tape in the center of each pan on the balance. Fill one container with clay and place it on the tape so that it is in the center of the pan. Place an empty container on the tape at the opposite end of the balance.

Slowly and carefully add water to the empty container until it is full. Expected results The clay has a greater mass than an equal volume of water. Ask students: Which weighs more, the clay or an equal volume of water? The clay weighs more than an equal volume of water. Which is more dense, clay or water?

Clay is more dense. 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.

If you were to try to push the beach ball down and displace more water, the water would push back with a force greater than the weight of the beach ball. The push of the water keeps the beach ball afloat. Buoyancy is the upward force we need from the water to stay afloat.

Buoyant forces are why we feel so much lighter when we are in a swimming pool. Our bodies are mostly water, so our density is fairly close to that of water. Because of this, an average person needs only a little bit extra buoyancy to float.

A life jacket provides this extra lift. Changing Density You can change the density of a substance by heating it, cooling it, or by adding something to it. 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? Babble Dabble Do has a fabulous ship building activity to demonstrate how displacement helps objects float. Can you make a superhero float?

We made out superhero arm bands and a raft,. Rainy Day Mum has a lovely activity using boats made from pie tins. Try a p irate themed sink and float investigation. Find out how many coins it takes to sink a foil boat. At this level it is appropriate for students to experiment with a variety of objects to see if they float or sink in water. They should be encouraged to identify common characteristics of objects that float and those that sink.

Students can try submerging a ball in a bucket of water in order to feel the upward push of the water on the ball. A more quantitative approach could involve measuring the weight of different objects to see whether that affects how they float or sink in water.

Students could experiment with different sized objects with the same weight to see how this influences their floating. Encourage students to experiment with little sealable containers such as empty film containers to see if they float, and to what level, in water. 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.