Floating+and+Sinking


 * Key terms for Floating and Sinking

Force: a push or a pull. Buoyancy: the upward force on a body immersed or partly immersed in a fluid Density: amount of mass per unit mass of an object (i.e. how ‘heavy for its size’ an object is). For example the density of water is 1 kg per litre. Pressure: the amount of force applied per unit area. At a given pressure, twice the area will experience twice the force. Activities to do at home: To Explore Upthrust forces from water ** • a netball • a garbage can full of water (this activity can also work with a medium-sized plastic ball in a bucket of water, but the effect is not as impressive). 1. Attempt to push a netball under water contained in a garbage bin. You will be able to feel, quite tangibly, the upthrust force which depends on the amount of water being pushed aside (displaced). 2. Draw a diagram representing the forces at work here. 3. What if the ball was full of a dense material like steel, rather than air. Imagine it almost submerged. How would the upthrust from the water compare with that for the netball in the same situation? Why would the netball pop up, and the steel ball sink, in this situation?
 * Underwater Netball **
 * Key idea:** The upthrust increases as more water is pushed aside.
 * You will need: **
 * Method **


 * Bricks Supported By Water **

• one or two bricks • a long cord to tie to the brick/s • a garbage can full of water. 1. Lower one or two bricks, suspended by a long cord, into the water. Predict what you will feel before trying it. 2. Write a statement that captures what the last two activities tell us about why things float.
 * Key idea:** Water exerts an upthrust force that opposes the force of gravity and effectively lessens the weight of objects.
 * You will need: **
 * Method: **
 * Explanatory note:** As the bricks enter the water, the weight felt in the rope becomes much less because now the upthrust from the water is helping support the bricks.


 * Measuring Upthrust **

• an elastic band • plasticine • a bucket full of water. 1. Attach a lump of plasticine to an elastic band. The stretch of the elastic band is a measure of the weight. If the plasticine were heavier the band would stretch more. 2. Predict what will happen to the stretch of the band when you lower the plasticine into water. Try it. Now explain in terms of a diagram of the forces. Explain it in words. 3. You might like to try this with a spring balance. What could you measure?
 * Key idea:** Water exerts an upthrust force that opposes the force of gravity and effectively lessens the weight of objects.
 * You will need: **
 * Method: **


 * Cartesian Diver

** • a water dropper (glass is best, or weight the plastic dropper slightly with plasticine) • a plastic juice bottle with a wide neck and a screw top. You may notice the water level changing. **
 * Key idea:** Water pressure can affect weight and therefore upthrust.
 * You will need: **
 * Method: **
 * 1. **** Partly fill a small glass or plastic dropper with water so that it just floats. **
 * 2. **** Put it in the top of a plastic bottle full of water and screw the lid on. The dropper will sink if the container (with the top on) is squeezed. **
 * 3. **** Can you work out what is happening when you squeeze the bottle? **
 * Look carefully at the dropper as you squeeze.
 * 4. **** Vary the conditions: the amount of water in the bottle, the amount in the dropper, try weighting the dropper, loosen the lid, etcetera. **
 * 5. **** What makes a difference, and why? **
 * Explanatory note:** Squeezing the bottle increases the pressure and drives more water into the dropper as the air is compressed. The added weight increases the density to the point where the dropper sinks.