Floating and sinking
This Topic is About...
- I will find out how the mass and shape of an object can affect whether it floats or sinks.
- I will use force diagrams to explain the effects of forces.
- I will make predictions and test whether they are correct.
- I will plan fair tests and identify the independent, dependent, and control variables.
- I will practise fair testing and look for patterns in my results.
- I will take careful and accurate measurements.
- I will record my observations and results in tables.
- I will describe patterns in results and spot any that don’t fit.
- I will use my results to make conclusions about floating and sinking.
- I will suggest ways to improve a boat design.
- I will present results in a scatter graph.
You’re going to be a forces and floating expert!
Key Words
- displaced
- upthrust
Tap to Learn the Meanings!
- displaced: When an object pushes water out of the way as it floats or sinks.
- upthrust: The upward force from water or air that pushes against a floating or sinking object.
Great work learning about forces in water!
Forces Acting on Objects When They Float or Sink 🌊
When an object is on water, there are two main forces acting on it: the weight due to gravity and the upthrust force. Try pushing a ball down into the water — you’ll feel the water pushing back up on it. This upward push is called upthrust. The upthrust force makes the water move, or become displaced.
Archimedes’ Discovery 💡
Along ago, a scientist named Archimedes discovered the idea of displacement. While taking a bath, he noticed that the water level rose when he got in. He realised that the amount of water pushed out of the way was equal to the part of his body under the water. Excited by his discovery, he ran through the streets shouting “Eureka!”, which means “I have found it!” in Greek.
How Do We Know Whether an Object Will Float or Sink? ⚖️
We can measure an object’s mass in kilograms and compare it with the mass of the water it will displace. If the object’s mass is less than or equal to the mass of the displaced water, the object floats. If the object’s mass is greater, it sinks.
Understanding Force Diagrams ⚙️
In a force diagram, the floating ball has arrows showing gravity pulling down and upthrust pushing up. The arrows are the same length, meaning the two forces are balanced and the ball floats. If the gravity arrow were shorter than the upthrust arrow, the ball would still float. But if the gravity arrow is longer, the ball sinks.
Why Big Ships Can Float 🚢
Even though a container ship is very heavy, its shape allows it to displace a huge mass of water. This displaced water provides enough upthrust to keep the ship floating on the surface instead of sinking.
Think Like a Scientist 1
Question: How does mass affect floating and sinking?
You will need: A large container half-full of water, a measuring scale, a small plastic container, sand (or rice), and a spoon.
Method:
- Draw a table with three columns to record your predictions and results. Write these headings: Amount of sand, Prediction (float or sink), and Result (float or sink).
- Based on what you know about floating and sinking, predict whether the plastic container will float or sink. Write your prediction in the table.
- Test your prediction. Record your result in the table.
- Measure 10 g of sand. Put the sand into the small container. Predict whether the container will float or sink. Write your prediction in the table.
- Test your prediction again and record your result in the table.
- Keep adding another 10 g of sand each time and repeat steps 4 and 5 until the container sinks.
Observe carefully and think about how adding more mass changes whether the container floats or sinks.
How Shape Affects Floating and Sinking ⚓
Mass is not the only thing that affects floating and sinking. In the next practical activity, you will explore how the shape of an object changes whether it floats or sinks.
Think Like a Scientist 2
Question: How does the shape of an object affect floating and sinking?
You will need: A large container half-full of water, ten marbles, an aluminium foil dish, and some aluminium foil.
Method:
- From what you know about floating and sinking, predict whether a marble will float or sink in the water. Test your prediction.
- Put a marble into the aluminium foil dish. Predict whether the marble will float or sink in the water now. Test your prediction.
- Add more marbles, one at a time, to the aluminium foil dish. Continue until the dish sinks.
- In your group, make different shaped “boats” out of aluminium foil. See how many marbles each boat can hold before it sinks.
Observe carefully! What do you notice about how the shape of the foil affects how many marbles it can hold?